21c

How to Convert LONG to Clob PART 2

Published on: by Mustafa Comment

Hi,

I just want to write a small post today. Some time ago, I shared a way to convert long to clob in a query directly: https://mustafakalayci.me/2020/04/06/how-to-convert-a-long-column-to-clob-in-a-query/ Even if this method works, it works really slow and anything slow annoys me!

So, I want to share another method which is much much faster than Sys_dburigen function but THIS IS UNDOCUMENTED and UNSUPPORTED! It is used by internally by oracle so it works fine but you must be careful. Since, no one uses LONG column in their application (right? you don’t! please don’t!) this kind of code is handy for mostly admins because for backward compatibility data dictionary has some LONG data type columns.

What we are going to use is a LIBRARY called UTL_XML_LIB . I almost never create objects under SYS user, so I will create this code under a privileged admin user. first we must grant privileges on UTL_XML_LIB library and then create an EXTERNAL procedure in plsql.

now create external procedure:

“kuxLong2Clob” is a function which returns void (basically it is a procedure) written in C by Oracle. this procedure takes a select statement for your LONG column and rowid. procedure returns CLOB data as an out parameter. I created my_long2clob procedure but to be able to call it via SQL statement, it must be a function, so:

to test it, I won’t create a new table with a LONG column but instead I will use one of in the data dictionary (which I use these kind of codes against). Let’s use SYS.TRIGGER$ table which is the base table for DBA_TRIGGERS view. ACTION# column is a LONG data type. of course we must first grant select privilege on SYS.TRIGGER$ to mustafa user.

I suppressed the output but here it is. really fast long to clob conversion in SQL.

as a CON, it will only work against tables not VIEWs! you cannot use it as “f_my_long2cloc(rowid, ‘DBA_TRIGGERS’, ‘TRIGGER_BODY’)”. Since it requires ROWID, you must be referring underlying table and its rowid.

beyond that, if you want to get LONG as VARCHAR2, similarly you can use “kpdbLong2Varchar2” procedure in DBMS_PDB_LIB library. as UTL_XML_LIB it is very fast. only difference is, it returns VARCHAR2 and trims the overflow part. Where is it in use? Check how DBA_VIEWS shows TEXT_VC column data 😉

I hope this helps to fight against LONG columns in your codes.

Wish you all healthy, happy days.

Zero Data Loss DISASTER Database for Standard Edition 2

Published on: by Mustafa Comment

Hi,

As you know Data Guard is an Enterprise Edition option and only accepted method to protect your data against a “DISASTER” (DR) is data guard. How about Standard Edition 2(SE2)? Is this possible? Well I did it so, king of yes 🙂 with some risk of course.

Warining: I don’t take any responsibility for wrong settings. Wrong actions can damage your database. Please be careful.

if you google Oracle SE2 Data guard or Manual Log Shipping, you will find many blog post and documents about it. Even youtube has videos about this but almost all of them (at least all I’ve seen and read) are not “Zero Data Loss”. Steps are quite easy:

  1. you have a production database based on SE2
  2. you use another server as your DR server. (it could be on the same server if you use duplicate snapshot method)
  3. copy all your backup
  4. restore database and keep it in MOUNT mode
  5. copy your archivelog ever X minutes to DR server
  6. recover database using rman or sqlplus.

And that is it. This is a valid method. Only GAP is in this, you are transferring your archive logs but not redo logs. This means, you will certainly loose some data in case of a disaster because changes in redo logs will be lost (if you cannot reach out the server). So, what can we do? basic answer is by copying redo logs too but how? Redo logs are changing constantly. Even if you copy redo logs with in a 1 minute interval, you are still be able to lost 1 minute data.

To achieve this, you must force database to write your both redo logs and archive logs onto DR server. Not copying files in an interval, force database to write it there. Probably, you already understood what I did. to use this kind of method you should (must) have a strong, reliable, stable, fast network connection between your production and disaster servers. Because, we will send redo logs and archivelogs instantly to DR server.

I am using 19.15 databases on Oracle Linux 8 (virtual box). Same things can be done in other operating systems too. Logic will be same.

Here are the steps that I am about to do:

  1. On the DR server, create a folder to share over network via NFS.
  2. On the PROD server, Mount this shared folder on PROD server.
  3. On the PROD server, Add an archive log destination to this shared folder.
  4. On the PROD server, Add redo log members for each group to this shared folder.
  5. On the DR server, create a copy of prod database via rman (not duplicate)
  6. On the DR server, catalog archive logs, which resides on shared folder and recover database.
  7. In a disaster situation, copy redo logs to their original location with original name and recover database.

that’s it. Let me demonstrate those steps.

I created 2 virtual machines. They have both Oracle DB Software and one of them (PROD) has a database and I want to use other virtual machine as DR.

  1. Oracle home is under /u01/app/oracle/product/19c/dbhome_1
  2. I (always) use OMF (Oracle Managed Files).
  3. data files are under /u01/app/oracle/oradata
  4. recovery area is under /u01/app/oracle/fast_recovery_area
  5. SID of database is cdb
  6. PROD server ip is 192.168.56.101
  7. DR server ip is       192.168.56.104

1- On the DR server, create a folder to share over network via NFS

As the oracle user (Oracle db software owner user), create necessary directories.

this is the folder that I will share over the network and mount it (map it) on the production server.

to share over network, as root user:

2- On the PROD server, Mount this shared folder on PROD server

as oracle user, create a directory to mount shared directory on DR. /u01/app/oracle/fast_recovery_area/DR_LOGS will be used for mount point. directories in it is for OMF.

as root user:

at this point if I put a file into /u01/app/oracle/fast_recovery_area/DR_LOGS directory on PROD server, it will be written to /u01/PROD_LOGS on DR server

3- On the PROD server, Add an archive log destination to the shared folder

By default for OMF, if there is no value for log_archive_dest_n parameters, then Oracle internally use log_archive_dest_1 parameter as LOCATION=USE_DB_RECOVERY_FILE_DEST. that way, archivelogs are written under /u01/app/oracle/fast_recovery_area directory with sub directory structure of course (/u01/app/oracle/fast_recovery_area(<SID>/archivelog/YYYY_MM_DD).

but! if you assign a value to any of those parameters, then it will just use that location to create archivelogs. so I will set 2 log_archive_dest_n parameter, one location for PROD server and one for shared folder.

by setting these 2 parameters (log_archive_dest 1 and 2) we make sure that oracle will create 2 copies at the same time to 2 different location and one of them is actually out DR server. As you see there is one more parameter which is very important. log_archive_min_succeed_dest, by default is 1 and if you don’t set this as 2, in an access problem situation (network might have issues or DR server could be shutdown), Oracle will just write the first location and pass the inaccessible location. this will cause not writing necessary archive logs to DR server. if we want “zero data loss” DR db then oracle must write archive log to dr server. (also you can use MANDATORY clause).

4- On the PROD server, Add redo log members for each group to the shared folder

as the archive logs, we need to do the same things for redo log too.

as log_archive_dest_n parameter, if we set any of db_create_online_log_dest_n parameter, we need to set all three of them. 2 locations are on PROD server and 1 location is on DR server. this is not a mandatory step actually but it is a good practice. Because, we can just add redo log members (last 3 command above) to the shared location and this is enough but if one day you need to create another log group and forget to add a log member to shared folder then, you could be exposed to data loss! so set it any way.

then, add 1 member for each log group to shared folder. I use standard naming here because I need to know the groups of files by their name (we will use this later).

lastly, I create pfile to move to DR server, so we will create our DR database.

5- On the DR server, create a copy of prod database via rman

on PROD server, database “cdb” is up and running. Let’s start:

I don’t want to go all the details about standard copy backups and restore a database so, I won’t put outputs here.

On PROD server, take a backup of database via RMAN and then copy everything to DR server via scp.

from shell:

copy all backups, archivelogs etc and also pfile.

then go to DR server and create necessary directories & restore your database:

these directories will be needed for database. restore database:

at this point we have a copy prod database on DR in MOUNT mode.

somet very important thing to do

DO THIS ON DR SERVER ONLY! we are deleting any copied redo logs because during recovery mode, there must not be any redo log!

6- On the DR server, catalog archive logs, which resides on shared folder and recover database

from now on, you can create a recover.sql script and call it ever n minute from cron to recover database regularly. content of recover.sql is:

we must catalog newly arrived archivelogs first and then recover database. I want to remind that /u01/PROD_LOGS directory is the one that we shared over network and map it on to prod server.

every recovery command will end up with an ERROR:

this is an expected behavior. since we don’t apply redo logs, Oracle is looking for the next archive log file in the sequence but couldn’t find it. our recovery is now working. after archive log with sequence 18 is arrived, recover command will raise the same error above for sequence 19.

7- In a disaster situation, copy redo logs to their original location with original name and recover database

from now on, our archive and redo log files are stored on DR server too. if there is a disaster situation, you must open the database with applying all logs including redo logs. this is the only moment that we will apply redo logs.

these are the redo log locations on PROD:

/u01/app/oracle/oradata/CDB/onlinelog
/u01/app/oracle/fast_recovery_area/CDB/onlinelog
/u01/app/oracle/fast_recovery_area/DR_LOGS/CDB/onlinelog (this one is the network directory on DR server)

actual directory that redo files are written is /u01/PROD_LOGS on DR server. Those directories must be existed on DR server too (we already created them in scripts above). We must copy redo logs under /u01/PROD_LOGS to actual locations now. I deliberately named redo logs as redoN.log so that I can identify which redo file is belong to which redo group. a very simple sql script and shell script will copy redo logs to original locations:

calling put_redo_logs.sh file will run an sql script which will produce copy shell commands.

After that, you can run recover.sql one more time and this time RMAN will not raise any error and you will see “Recovery Completed” message. now you can open your database and use it.

Now, why did I delete copied redo log files via scp command on DR server? if you recover a redo log in mount mode, control file will raise some flags and all recover commands will start from the sequence of applied redo log, this will cause problems after some time because archivelogs will be deleted eventually because of retention policy. This also means, if you apply a redo log during recovery, to have a healthy copy of database, you must restore both control file & database all over again.

Pros

obviously, you have a DR db with full consistent data.

Cons

This approach contains high risk because if database cannot write archived or redo log to shared folder then production database will hang! You might have some issues about network or DR server might be restarted or shutdown. You might loose disk drives on DR server which shared folder resides on etc etc…

I will write another post about those situations and make some tests. if loosing some data for a small time then, still refreshable pluggable database would be a better option. I will also write a post about it.

if you don’t have a reliable network, this option could be catastrophic! So, try it carefully. I hope this helps in some way.

Wish you all healthy, happy days.

Indexes with NOSEGMENT a.k.a Virtual Index

Published on: by Mustafa 2 Comments

Hi,

Today, an old trainee has contacted with my on linkedin and we talk about some performance issues about a query. During the conversation I suggest to use “virtual indexes” to simulate the index so I wanted to write about virtual indexes and their benefits in this post.

if you don’t work on a large database (or a small database server) then, you probably don’t get bored while creating an index but if you do then, index creation can be a pain. Let’s say you examined a query and decided to create an index based on some columns could get increase the performance but will it? if your table is a large one then, creating an index could take serious amount of time. During that time, your server will have extra work load and if that index does not work as you assumed then all those time and work load will be for nothing! Also, you will have generated too many archivelogs. Those archivelogs could increase your recovery time in case of a failure.

So what can we do? You can use “virtual index”. Virtual index is an index without a segment which means this index has no data, does not index anything at all! So, what good come out of it? it’s creation time almost instantaneous and no work load on your server, no archivelog generation, no performance impact in any way. So, why do we create it? Because you can check your execution plan and see the cost and other metric values so that you can decide whether this index should be created or not!

This will save you from wasting too much time and give an initial idea.

Here is an example:

I have 72.544 rows in my dba_objects view. By using this view, I created a table:

I created a BIG table which contains 1.1 Billion rows in it. Those rows are copies of dba_objects view and it takes 35 minutes to create it in parallel (I enabled parallel dml and ddl operations to achieve that). Table size is around 185GB.

Now, Let’s create indexes on it:

Unique index took almost 18 minutes to complete and non unique took 10 minutes (no parallel). I must say that, my laptop is a monster with 10th gen i9 (8core), 64GB ram and nvme ssds. those timings could be double or triple on a regular db server. As I said, after all this time, if indexes aren’t useful then you drop them.

Instead of that, we can use NOSEGMENT clause while creating the index (virtual index)

As you see, it is fast because it just created an index definition into oracle data dictionary. Literally, there is no such index. check dba_indexes:

How Oracle can use an index if it is not actually created? Answer is STATISTICS. To use virtual index and get a consistent result from it, your statistics must be up to date. Of course, result will not be certain but close enough to give us an idea. By default Oracle will not use virtual indexes in your execution plan so you must set a hidden parameter, then your explained plans will have virtual indexes:

after setting hidden parameter _use_nosegment_indexes optimizer will create an execution plan with considering virtual indexes and I can see that the cost of the query has decreased to 2 from 6.450.000. This saves so much time. after checking your execution plans, if you are ok with them you can actually create the index.

EDIT: There is 2 things that I forgot to mention. after creating virtual index run DBMS_STATS.GENERATE_STATS procedure to generate some statistics of your virtual indexes.

and the second thing is, this is can be used only in enterprise edition not standard edition. Standard edition has no deferred segment creation so you cannot create a data object without data segment.

To create this virtual index as a real one, you must drop it and re-create it without nosegment clause.

Time is the most valuable thing for us. Don’t waste your time carelessly.

Wish you all healthy, beautiful days.

3 Ways to Migrate a Non-CDB Database to a PDB

Published on: by Mustafa Comment

Hi Everyone,

Time is fast and everything is changing constantly. Multi-tenant architecture has come many years ago and starting by 21c, it is the only option! So,  old “non-cdb” databases must be migrated/converted to a PDB (Pluggable database). I must admin that I was being lazy for many years and I didn’t work much to learn new things and now I am literally trying to learn everything. One of the first goal was about multi-tenant architecture and now, since I feel that I am confident enough about it, I will move my clients’ databases to multi-tenant architecture. Also, I must say, I loved multi-tenant architecture.

I won’t be going details about what multi-tenant or a pdb is in this post. I will cover only Migration / Conversion.

So, for a long time I am testing about migrating noncdb databases to pdb and I want to explain 3 main methods to do that.

  • Plugging in a noncdb as pdb
  • Creating a pdb from noncdb over a database link (dblink)
  • Autoupgrade

Methods like export import can be used of course but I don’t think that can be count as a “migrate”. So, I won’t be talk about it in this post. Let’s start then.

Environment

For all 3 methods same environment is used: Oracle Linux 8.5 (virtual box), Oracle Database 19.14 Enterprise Edition. (I will add changes on 21c in my post)

Noncdb Database SID        : orcl
Multi-tenant Database SID: cdb

Both databases are in same home (could be in different homes, that would change nothing).

I always use OMF so I won’t be using FILE_PATH_CONVERT parameter in my examples. if you don’t use OMF (I don’t know why you do that) then you must use file_path_convert and point necessary directories.

I put around 4.5 GB of data in orcl (noncdb) database because amount of data will affect the completion time of the process.

By the way, I will suppress some outputs to keep it short and emphasis real results.

Pre Steps

Steps that I am about to show you should (must to me) do whatever method you are using to migrate your database. You must always check if noncdb database is applicable as a pdb. To do that, you will use simple pdb plug in method basically. Of course in this case, we cannot “unplug” our noncdb database because it is not a pdb, so what?

As you know, you can plug in and unplug a PDB to/from a container database in multi-tenant architecture. this is the coolest option I think. As if you are plugging in a pdb, you can plug in a noncdb as a PDB. While plugging in a pdb, we need an XML file that defines properties and necessary information about the pdb like compatibility, nls info, data file locations etc. To plug in a noncdb, we will need that file as well. Oracle already embedded a package to achieve that. DBMS_PDB. Describe procedure in this package will create XML file that we need.

if you are going to use PLUG IN method then put your source database (non-cdb database) into transactionally consistent state which means open it in READ ONLY mode. Reason is XML file contains some SCN information too but if db is up and running scn will move. When you tried to plug in database, you will get an error about data file scn number and config file (XML file) does not match. Other 2 methods doesn’t require that! I personally prefer this in db link method too. I don’t want any process keep doing something while converting a noncdb to pdb.

for plug in method:

I connect to “orcl” database and create necessary xml file:

Now we have an xml file that defines our noncdb database.

Then, check if this is a compatible database with our container database. Connect to CDB database and check the xml file:

DBMS_PDB has a function called “check_plug_compatibility”. this function reads XML file and returns true or false. if true then you can plug this noncdb as pdb (maybe not so fast). if result is false then you must check what is wrong? even if result of this function is true you must check “violations”. every CDB database has a data dictionary view called PDB_PLUG_IN_VIOLATIONS. After running “check_plug_compatibility” function, errors/warnings/infos will be written to this view and you must check them before plugging in the database.

if you see any ERROR in this view you must not proceed until solving the problem.  You might see various problems here like different database parameters like open_cursors could be 500 on noncdb but 300 on cdb. remember, cdb parameter values will override previous parameters.  (I will explain “PDB plugged in is a non-CDB, requires noncdb_to_pldb.sql be run” warning later)

Some important points are DB OPTIONS. Your databases might have different options. Let’s say your noncdb database has SPATIAL option installed but it is not installed on CDB. if so, then you must install SPATIAL option before plug in the database. how to install options on a database? You can check my previous post about it:  https://mustafakalayci.me/2022/03/03/how-to-install-oracle-database-options-like-spatial-label-security-etc-after-db-creation/

As a rule, your non-cdb database might have equal or less than compatible parameter value. if this is the situation you will see a row in PDB_PLUG_IN_VIOLATIONS.

if you resolve a problem on PDB_PLUG_IN_VIOLATIONS, it will still be there. even so, after solving a “warning” problem (like installing an option) will be listed as PENDING not resolved. it requires open the pluggable database.

As I said, these steps should be mandatory to you before migrating non-cdb to pdb. after checking violations, you can proceed to plug in operation.

1- Plugging in a Noncdb as PDB

At this point we should have checked the xml file and solved the violation problems. Please remember you should have created the XML file when noncdb database in READ ONLY mode!

Now, we have XML file so we can plug in noncdb as a pdb. Before do that, close source database (noncdb).

then connect to CDB and create a pluggable database via plug in:

I don’t want to go to detail of create PDB via plug in but as a summary; you can use a different PDB name. instead of orcl, you can use orcl_pdb for example. Also, default mode for plugging in a pdb is COPY which means it will copy the datafiles that orcl.xml pointed out (they are under /u01/app/oracle/oradata/orcl directory currently). PLUG IN method will copy them under /u01/app/oracle/cdb directory (wherever db_create_file_dest is).  this way, noncdb database (orcl) will be still available (it is in shutdown state).

it takes 10 seconds to complete on my laptop. as I said before it has 4.5GB extra dummy data in this database. total size is around 7.5GB. 10 seconds is the time of copying datafiles to a new location. if you use MOVE instead of COPY method it will take much less time.

as you see it only takes 0.68 seconds. size of data is not matter now because data files are MOVEd to another location and this is a very fast operation. even if you have 1TB of data it will be completed in seconds. Also, if you are satisfied where datafiles reside currently, you can use NOCOPY option too.

if you use same with noncdb database then you must sure that noncdb is closed because there will be 2 services with same name.

you can use this method even if your databases are on different servers. just create XML file on noncdb database, then move datafiles and XML file to new server, modify XML file because file locations might be changed and wherever you copy the data files, change locations in XML file to new location and then run create pluggable database command.

Always check PDB_PLUG_IN_VIOLATIONS view after creating PDB because there might be new issues.

Now we have a new pdb called orcl:

NOTE: Another amazing option we have is we can create a PDB from a lower version noncdb. for example, if you have a 12.2 noncdb database, just install 19c and follow the same steps. Cool right? there is only one thing to do! before calling noncdb_to_pdb.sql file run dbupgrade -c “ORCL” command on shell. dbupgrade is kind of a shortcut for catupgrd.sql file. this will upgrade your newly created pluggable database first.

as you see it is in MOUNTED mode which means it is closed but we CANNOT open it yet on 19c database. For 19c, we must connect to newly created pdb and run noncdb_to_pdb.sql script which is under $ORACLE_HOME/rdbms/admin

it will take a while. as I said, don’t open pdb. set your container as new pdb then run noncdb_to_pdb.sql file. question mark means ORACLE_HOME in sqlplus. so I basically called @/u01/app/oracle/product/19c/dbhome_1/rdbms/admin/noncdb_to_pdb.sql

21c UPDATE: After plugging in (create pluggable database) command, you don’t need to run dbupgrade (if noncdb is a lower db version) or noncdb_to_pdb.sql scripts. when you run ALTER PLUGGABLE DATABASE OPEN command, 21c will automatically run them but open command, of course, will take some time to complete.

That is it.

2- Create PDB from Non-CDB via DBLINK

This step is much easier for an Admin but takes much more time to complete. Good thing about this method is your source DB can stay open during the operation and when pdb creation is completed, it will apply archivelog of noncdb database. So, PDB will be transactionally consistent at the time of END of the pdb creation.

Do the PRE STEPs first as described above. create XML file and check for the violations. if everything is OK then, you can proceed.

This method just requires a database link (dblink) from CDB to NONCDB. Dblink must be created at the cdb$root container. First create a user on noncdb database which will be use to connect in dblink.

Remember, this user must have resource, connect and create pluggable database privileges. after that create database link on CDB:

from now on we will create a pdb from a remote link:

it completed in 10 seconds. if source database was bigger it would take much more time. Since non-cdb has no PDB name, we use NON$CDB.

as I explained in previous methods, for 19c, if you created pdb from a lower version run dbupgrade command. after that, you must set current container as new pdb and run noncdb_to_pdb.sql file.

if this is a 21c you can simply open pdb. 21c will do the job but open command will take some time.

I am not sure if this is supported (I did not checked yet but test it of course). I use this method to create a PDB from a noncdb which is on a different server and different OS and it worked!

3- Autoupgrade

New King of the upgrades/convertions/migrations 🙂 Autoupgrade is an amazing tool and support many options now. Even, I have just read that version 2 is released while I was writing this post on Mike Dietrich’s blog ( https://mikedietrichde.com/2022/03/25/autoupgrade-2-0-has-been-released-and-got-reuploaded/ )

You can use autoupgrade to convert a noncdb database to pdb. Autoupgrade will take care of everything even pre steps and post steps. It will even run the fix scripts.

I still would like to do my “Pre STEPS” to check the operation.

autoupgrade is a jar file and requires java to run but no worries, everything you need is in the ORACLE_HOME. Before take an action, always download the latest version of autoupgrade.jar file from https://support.oracle.com/epmos/faces/DocumentDisplay?id=2485457.1 it is a single jar file and put it under $ORACLE_HOME/rdbms/admin for 19c and above.

autoupgrade has different modes and I won’t detail all of them here. What we use here is ANALYZE and DEPLOY modes. ANALYZE mode will run all the steps just skipping the actual operations and will show you a result. After that you can run it in DEPLOY mode which will do the actual job. autoupgrade requires a config file to identify what to do and necessary info. here is my sample:

it is quite self explanatory. source is upg1.sid which is noncdb and upg1.target_cdb is cdb which autoupgrade understands this is a noncdb to pdb conversion. you can give a new name for your pdb (my_pdb in my example). define a start time, set if a timezone upgrade will be proceed etc. I saved this file under /home/oracle/noncdb_to_pdb.cfg.

EDIT: I forgot to mention about something. By default, autoupgrade will remove your noncdb! it is creating PDB with MOVE option so datafiles of noncdb are gone. if you want to prevent this, use KEEP_SOURCE_PDB=yes in your config file like:

upg1.keed_source_pdb=yes

Before Running Autoupgrade : This is an important note. it might be solved already because autoupgrade team is really fast but I had some problems while doing that because of glogin.sql file. for details: https://mustafakalayci.me/2022/03/20/666/ thanks to Mike Dietrich and Joseph Errede, they take care of it really fast.

just run autoupgrade in analyze mode first:

then check the result and run it in DEPLOY mode. Deploy mode will start a CLI (command line interface) so you can watch the steps by typing “lsj” command:

When operation is over it will automatically exit the cli and will print the log file locations. as you see in my example, it already started noncdb_to_pdb.sql file. Yes, all of those steps are automatic in autoupgrade.

here is the end:

here is the html file screen shot:

that’s it.

How to install Oracle Database Options like Spatial, Label Security etc After DB Creation

Published on: by Mustafa Comment

Hi,

yes, I am aware that title is a little bit long and too informative but I believe it is necessary because terms are complicated according to me.

When you say “option” it could be many thinks like “advanced compression” option in EE or Spatial option or Partitioning option and not all of these are the same “option”. they have different backgrounds and structures. some times they called “components” too but in the documentation they always mentioned as options. I want to write about “options” like Spatial, Jserver, Label Security, Data Vault, Olap… Do you see where I am going to? These are the “options” in dbca utility that you choose while installing the database (only if you choose “custom database” not general purposes or dw as db type) but also I will talk about partitioning and RAT (real application testing) option because their namespaces intersect in some “options” (ok, I will stop putting double quotes around the option).

I didn’t care about options much until now and I wanted to learn how to install/uninstall after db creation or what options do we have. Started to dig the documentations and I realized that it is a little bit more complicated than I thought. What I focused was the options on dbca as I said:

here they are. So instead searching docs and google first, I use a small trick and check the option “Generate Script” at the last page of dbca and I looked for the scripts. fortunately, Oracle puts every options creation into a script so it is really easy to find them but before talking about the scripts I want to talk about the option types.

Shared Objects & DLLs

Some options are made of just simple scripts but some of them are deeper! For example, if you want to install Spatial option (to use geographic coordinates in your db) you just need to run an sql script but for OLAP, you need to “link” some objects into Oracle kernel and then run some scripts. What does it mean? some options are not just tables/packages etc, they need some special C functions in oracle kernel but not everything is in the kernel. Shared Objects in Linux and (mostly known) DLL files in windows are widely used to add or remove functionalities to your application. In the end Oracle is an application and uses those dynamic libraries. I am not a software engineer so this part is a little bit above my paygrade but I will try to explain. In Linux if you want to add a functionality to your application via shared objects (dlls in windows os) you must “link” them to your application by “make” command. For example, to use OLAP it must be linked to your oracle binaries in Linux, as such necessary DLLs must be available in windows. Remember this operation is not per database but Oracle Home. When you do this, all current and future databases in this Oracle Home will be or will not be able to use these options.

Of course, it is not that easy 🙂 some options must be linked to Oracle binaries but they don’t need to run an sql script. So, if we group the options by their installation types, we have 3 different types:

  1. SQL Scripts (for ex: spatial)
  2. Linking Binaries (for ex: unified auditing)
  3. Linking Binaries & SQL Scripts (for ex: OLAP)

To install Spatial you just need to run $ORACLE_HOME/md/admin/mdinst.sql script as a sysdba user. That is it. This script will create MDSYS user and necessary objects to use spatial option.

To enable “unified audit” option in database you must just link some binaries but you don’t need to know everything of course. you can check this post of mine: http://mustafakalayci.me/2021/09/08/enable-unified-audit-on-linux-and-windows/

spoiler:
cd $ORACLE_HOME/rdbms/lib
make -f ins_rdbms.mk uniaud_on ioracle

and To install OLAP, first you must link OLAP module first and then then run $ORACLE_HOME/olap/admin/olap.sql script as sysdba.

Linking Binaries & chopt

for linking operation Oracle provides as a simple tool called chopt short for Change Option. Instead of running “make” command you can use chopt and disable and enable “some” options from Oracle Kernel. it is basically just calling “make” command itself. It does support very few option:

as you see, you can enable/disable OLAP, Advanced Analytics, Partitioning and RAT.  I want to repeat again, when you use chopt, you are linking/unlinking some binaries. So, this will add or remove some components from Oracle kernel and currently available and future databases will be or will not be able to use these options. usege of chopt is very easy:

if you run it on Windows, it will just rename a DLL (to enable option, it will rename it to correct name and to disable, rename it to something else)

chopt has different options from version to version. for example in old version there a module called DM (data mining) but in 19c it is under the Advanced Analytics so if you want to enable/disable data mining you must use chopt enable/disable oaa command. Also, you can see that “partitioning” is an option too. if you don’t have a partitioning license you can disable partitioning option so that non of developers create a partitioned table and caused (maybe) thousands of dollars cost.

Only use the options you need! Nothing more. Consider that unnecessary options will increase db upgrade timings too. Also, as in partitioning example, some options are licensed options which means you have to pay to use them. if you don’t have the license then disable it.

As far as I understand, there is a historical development in this. for example, in old versions, Spatial was an option that needed to be linked too. sdo_on and sdo_off was used to enable/disable the Spatial.

as you see, when you try to turn it off now, warns us about “sdo_off is disable”. it is always on anymore.

here is some examples to enable/disable options in Oracle kernel but some of them (like Label security) is deprecated.

Product/Component Enable Switch Disable Switch
Automated Storage Management asm_on asm_off
Oracle Data Mining dm_on dm_off
Database Vault dv_on dv_off
Oracle OLAP olap_on olap_off
Oracle Label Security lbac_on lbac_off
Oracle Partitioning part_on part_off
Real Application Cluster rac_on rac_off
Real Application Testing rat_on rat_off

Also, please consider that if you disable an options like OLAP using chopt (or make command) it will be disabled in dbca too. you won’t be able to choose it:

WARNING: before use chopt or make commands, always close all services in the oracle home first (database, listener etc). for windows servers, also stop the related Oracle Services in Windows Services (like OracleServiceOrcl…)

Dbca Options

Options that you can install into a database (might required binary linking). you saw them at the screenshot above. Every each of them has some unique features (I won’t go detail the purposes of the options) and some of them depends to others. So, basic hierarchy is like this:

  1. Oracle JVM
    1. OLAP
    2. Multimedia
  2. Oracle Text
  3. Spatial
  4. APEX
  5. Label Security
    1. Database Vault
  6. Extension for .NET

to be able to install Multimedia you must have Oracle JVM first or you can install APEX or Label security without installing anything else. What are the scripts to install? you should always check the documentation or support to achieve this but as an example I wanted to write them in 19.13 version. DO NOT run them directly! Always check the documentation first, scripts might be changed! those samples are taken from dbca generated scripts.

  1. Oracle JVM
    $ORACLE_HOME/javavm/install/initjvm.sql;
    $ORACLE_HOME/xdk/admin/initxml.sql;
    $ORACLE_HOME/xdk/admin/xmlja.sql;
    $ORACLE_HOME/rdbms/admin/catjava.sql;
    connect “SYS”/”&&sysPassword” as SYSDBA
    $ORACLE_HOME/rdbms/admin/catxdbj.sql;
  2. OLAP
    $ORACLE_HOME/olap/admin/olap.sql SYSAUX TEMP;
  3. Multimedia
    $ORACLE_HOME/ord/im/admin/iminst.sql;
  4. Oracle Text
    $ORACLE_HOME/ctx/admin/catctx.sql Xbkfsdcdf1ggh_123 SYSAUX TEMP LOCK;
    alter user CTXSYS account unlock identified by “CTXSYS”;
    connect “CTXSYS”/”CTXSYS”
    $ORACLE_HOME/ctx/admin/defaults/dr0defin.sql “AMERICAN”;
    connect “SYS”/”&&sysPassword” as SYSDBA
    alter user CTXSYS password expire account lock;
    $ORACLE_HOME/rdbms/admin/dbmsxdbt.sql;
  5. Spatial
    $ORACLE_HOME/md/admin/mdinst.sql;
  6. Apex
    $ORACLE_HOME/apex/catapx.sql Xbkfsdcdf1ggh_123 SYSAUX SYSAUX TEMP /i/ NONE;
  7. Label Security
    $ORACLE_HOME/rdbms/admin/catols.sql;
  8. Database Vault
    $ORACLE_HOME/rdbms/admin/catmac.sql SYSAUX TEMP;
  9. Extension for .NET
    $ORACLE_HOME/rdbms/admin/dbmsclr.plb (in windows only)

How to Install Dbca Options

Well, I pointed out the scripts and you can run them (for the same version, and always check the documentations first) but of course it is not that simple 🙂

This is very important! Installation method depends on whether you are using a Container database or Non Container database! if you are not using container database (old design which is not supported anymore by starting 21c), you can run the scripts by connecting a sysdba user but for a container database it is more complicated.

for a container database, most of those options must be run in cdb$root and then all the pdbs but this is a hard job and opens for mistakes. instead of this, use catcon.pl perl file to automate this. spatial sample creation script taken from dbca generated scripts:

it seems complicated but mostly paths makes it longer. simply call a perl script and as a parameter it passes the original script of the necessary option. Always use catcon.pl to install/uninstall an option in a container database.

How to Check Which Options are Installed

This is (as always) not that simple 🙂 Some options are “registered options” so you can list them via dba_registry data dictionary view:

These rows do not match to an option exactly. OLAP option is listed as 2 rows, “Oracle OLAP API” and “OLAP Analytic Workspace”. Also, some options are installed by default like “Oracle Workspace Manager”.

is that all? of course not. these are registered options (components) but also there are options in Oracle kernel like unified auditing. it is not depend on an sql script. for all options in the kernel use V$OPTION view:

I just listed some of the rows because it is a long list.

Uninstall an Option

Uninstalling and option is not that easy because some of the options create specific objects in different schemas (like sys) and to uninstall it either you must find every object that used by the option or use some scripts again that either Oracle Support provide or exist under the oracle home directory.

To Uninstall Spatial, for instance, you can simply just drop MDSYS schema but there might be some tables which uses spatial data types, indexes etc. you must find and drop them first. even if it seems easy there is always a catch. So, be careful and always follow support documents about it.

In Conclusion

So, it is a deep topic and probably there are much more. I said this before but again; remove all unused options from your database. They will be a burden to your system. So far these are what I understand and learn… I hope, these are useful and helpful to you while understanding the db options.

As always, wish you healthy days, NO WAR in anywhere, all life in this planet matters.

Tables with Memoptimize for Write (fast ingest)

Published on: by Mustafa Comment

Hi There,

this is the second part of this post: http://mustafakalayci.me/2022/02/15/tables-with-memoptimize-for-read/

Oracle adda new feature called memoptimized tables for mostly devices like IoTs. Fast lookup and fast ingest tables. I have already mentioned about fast lookup (memoptimize for read) tables in my previous post and now I want to write about Fast Ingest tables. as Fast ingest tables, those tables are optimized for inserting new data. Especially small devices (like any IoTs) are tend to be send data more than processing or querying data and we have billions of them.

for the Fast Lookup tables (in my previous post), plsql is not an option but fortunately we can use fast ingest tables in plsql. My tests will be based on plsql codes. as fast lookup tables, you will set table as fast ingest table (memoptimize for write) and while inserting you need to use a special hint “MEMOPTIMIZE_WRITE”. this way Oracle will understand this is a fast ingest table and inserts will be completed accordingly.

How it is work? your insert statements (either row by row or bulk) will be stacked in LARGE_POOL (a memory component in database) and after passing a threshold, data will be inserted (as if a bulk insert). this provides significant performance increment but as always, nothing is perfect and there is always a “but”. So, this will increase your write speed “but” your data will not be immediate anymore! you might not see your data just after inserting and committing it because it will be cached in large pool and after a time will be inserted to the table. Secondly, you might loose some data! this is a big “but”! since data is in large pool for a certain amount of time, any thing that can go wrong like an instance crash, might cause loose some of the data. you must be careful while using this feature.

So let’s test and see the difference. To demonstrate this, I will shutdown and startup my database to remove everything in the memory and then connect as “mustafa” user.

ok, before start my test, I would like to check my large pool size first. large_pool parameter is 0 as usual because I am using ASMM (sga_target is set) so I need to check dynamic components view:

sizes are in megabytes. as you see I have 96MB assigned large pool.

Let’s create two tables, one for memoptimize for write (fast ingest) and one for standard heap table.

now, I will do inserts row by rows and commit at the end:

as you see I use MEMOPTIMIZE_WRITE hint in the first plsql code for test_fast_ingest table. of course timing takes our attention:

fast ingest : 25.44 seconds
standard heap: 25.54 seconds

huh! is there a problem? well actually not, let’s check large pool size again:

as you see, large pool has reached it’s limits. it is now 768MB. as I said, memoptimize for write uses large pool, since it was not enough to do the job we waited larges pool’s growth operation. let’s run the same script again:

here it is. fast ingest table is more than two times faster than standard heap table. this is a major performance increment. Let’s try commit row by row:

this is a hard job to do and even for this job, fast ingest table is doing a great job.  for row by row commit, fast ingest time increased 2.25 times (approximately) and heap table time increased 2.6 times. if you have many devices sending many insert statements then fast ingest table will be able to handle it. even so, increasing large pool size would increase the performance.

I have already mentioned about some cons like inserts are not immediate (even if they are committed) but also there is one more cons! lets truncate the table and make inserts. please remember that ID column is PRIMARY KEY.

as you see, I inserted three rows and they all have same ID, so there must be a unique constraint violations right? well, in my tests it didn’t probably because data is not inserted yet. it is gathered in large pool and then will be inserted but it is not my session doing the inserts so I won’t get any unique constraint violation for this action. you must be careful while using this.

fast ingest tables have more satisfying results than fast lookup tables for me. we can increase the performance at least 2 times here! And it works with bulk inserts too:

created a base table (tmp_base) with 1M rows and use it to insert the data. as a result, fast ingest table is 4.2 times faster than heap table.

I expect memoptimize tables (both read and write) will be developed much more and become more popular. Just careful using fast ingest because it has significant cons. agan; you might loose data even if you committed them!

wish you all healthy days.

Tables with Memoptimize for Read (fast lookup)

Published on: by Mustafa Comment

Hi,

I would like to share my initial thoughts about memoptimized for read (fast lookup) tables in this post. fast lookup tables came with 18c and they are here to provide faster access to a table if you are using primary key with a equality condition. this is basically single table hash cluster. instead of using primary key index, Oracle creates a hash index and searching on a hash index should be much more faster than a unique index.

first, create the test environment.

to use a fast lookup table you must set memoptimize_pool_size paramter to a number higher than 0. this is a static memory component in sga to store hash index of the table. this memory component is not resizable which means it won’t grow or become smaller with automatic shared memory management.

create a table for fast lookup:

segment creation must be immediate otherwise you will get an error! Your table must have a primary key as well, since whole structure is based on PK, it is logical to have one. I roughly inserted 146.000 rows into table.

EDIT: I also tried those tests with 14 million rows too but results are similar.

this is not enough! you must use dbms_memoptimize package to use this table as fast lookup table.

let’s see what is happening now:

I skipped first execution statistics! you will see a small consistent gets at the first execution but consecutive executions will be like above.

as you see there are new execution plan steps “TABLE ACCESS BY INDEX ROWID READ OPTIM” and “INDEX UNIQUE SCAN READ OPTIM”, so these steps tell us, an hash index is used to retrieve data like as in key-value pairs. there is no “consistent gets”. this is amazing. almost nothing has read for this query and that should make this query so much faster.  Is it? well, I couldn’t find it, not as much as I expected at least. Actually, I found different blog posts mentioning about fast lookup tables but non of them has made a performance test.

So, I want to test the speed but there is a problem. for now, there are many limitations on fast lookup tables. one of them is “you cannot use it via plsql”. this means I can’t create a simple plsql procedure and run a query for 100000 times and compare timings. to do a test, I wrote a small python code.

Warning: I am no python developer, I just now python to solve my small problems, this code that I share below probably has many bugs but I just use it for this test purposes.

here is the python code I use:

after changing code many times, I used this final code. this code creates 9 threads and every thread run “select * from tmp_optimize where id = :myid” query for 100.000 times and myid variable value is between 1 and 10000 in circular way in this loop. each threads print total execution time for 100.000 runs of the query. here is the result:

timing is changed between 16.0 and 16.1 seconds for this fast lookup tables. how about a normal table with normal primary key?

I used same python code just changed table name from “tmp_optimize” to “tmp_not_optimize” and result is like this:

it is between 17.1 and 17.2 seconds. there is %6 performance gain (give or take).

I am not sure whether %6 is a good gain or not because to use fast lookup table actively, we need to separate serious amount of memory area. Also, there are few limitations like, cannot be used with partitioned or compressed tables, cannot be used in plsql (not yet anyway). only equality conditions on primary keys can advantage of fast lookup.

During my tests I didn’t check wait events. I expect that less latch waits on fast lookup table since no consistent gets occur. Maybe there is something I missed and I will do more tests on these tables (I didn’t even test “memoptimize for write” (fast ingest) yet).

wish you all healthy days.

How to Restore Dropped PDB

Published on: by Mustafa Comment

Hi,

it seems easy, right? Well, not that much but not that hard either. if you drop a pdb in a cdb, you cannot restore it using current database because when you drop the pdb, all information about the pdb is lost. So, what is the solution? you need a point in time recovered duplicated database.

I have a cdb named CDB and had a pluggable database named PDB1. this database is a 21c database and installation of CDB is pretty standard.

db_create_file_dest     = /u01/app/oracle/oradata
db_recovery_file_dest = /u01/app/oracle/fast_recovery_area

Let’s say I dropped it by mistake and make a recover:

I created the pdb and create a table in it. I also get the time because it will be required while restoring pdb. now, let’s drop it by mistake:

I don’t add outputs for now. Okay, now I lost my pdb. to restore it, we must create a duplicate database, recovered until a specific time. step by step:

1- create a new pfile for duplicate database (let’s called out db name as “dup”)

2- create password file:

3- startup the instance

4- use rman and create duplicate database

you can get different error messages at this point but they all have enough information to solve the problem. if you get this error:

RMAN-05501: aborting duplication of target database
RMAN-05657: There are multiple database IDs present.

then you must set dbid before run the duplicate database command.

5- after duplicate database command completed (which will take for a while) your new database called “dup” will be ready and in mount mode.

at this point if you query pdbs (containers), a strange name will appear “_###_UNKNOWN_PDB_#_4” (4 is pdb number so it might  be different on your systems). after Opening your database with resetlogs (this is point in time recovery) it will be converted to correct name.

here it is. after this point we have 2 container databases on the server, cdb and dup. so, we can simply unplug and plug pdb1 into database CDB again.

I use “move” while plugging it into CDB database because I want its datafiles move to db_create_file_dest locations of CDB database.

that’s it. I found this solution is a little bit tiresome and I believe next releases will have easier solutions but still we can complete what we need in a few commands.

thanks for reading, I hope this helps.

wish you all healthy days.

CONTAINER_DATA for Common Users

Published on: by Mustafa Comment

Hello Everyone (if anyone reading this 🙂 ),

it has been a long year and I am glad it is over. As always, we hope the for “a time of period” like new year, birthday etc etc but hope is the only thing that we can hold on. I hope whole world get rid of this illness called corona, racism, wars (and again etc etc, list goes on)… Wish everyone a healthy, peaceful year.

In my personal blog, I mentioned vary of things and I realized that I didn’t mention about container database structure at all. Main reason for this is I was lazy on multi-tenant architecture and didn’t work on it, yeah, pretty much this was the reason 🙂

I wanted to start with common user’s container_data attribute (or property I am not sure). As you know, in multi-tenant architecture there two types of users COMMON and LOCAL. Common users are defined in the CDB (not in a pluggable database) and they are known by every current and future pdb. Local users are defined in a pluggable database and can access to only pluggable database they are defined. By saying “defined”, I mean “created”. So, local users don’t aware of other pluggable databases or container database (cdb) but common users are aware that they are in a CDB and can connect to PDBs on that CDB if they have privileges.

By default, newly created common users doesn’t see whole data in container data views or tables. what is “container data views”? As you noticed (even if you use non-cdb database 12c and above) there are some new columns on data dictionary and v$ views like CON_ID. in a multi-tenant database when a privileged user like SYS queried V$SESSION, user can see whole sessions in all containers (cdb$root and pdbs). Let’s see:

I will be running those queries on a 21.3 version. I have 3 PDBs on my CDB:

these are just dummy PDBs that I use for testing purpose on VM. Now Lets connect as SYS and query some views:

I connected as SYS and I am in the cdb$root container. I queried V$SESSION and V$DATAFILE views by grouping con_id column. I am able to see sessions on different PDBs, in my example, con_id 3 and 5 has 1 session each. V$DATAFILE query shows count of all data files per PDBs.

Now, let’s create a common user and give necessary privileges and query with that user:

I created C##MY_ADMIN user and grant DBA for all containers. So, this user can connect to any pdb and query any data dictionary view. for example, c##my_admin can connect to pdb1, pdb2 etc and query v$session in all of them and is able to see whole session for current pdb that user in it but problem is when c##my_admin connected to root (cdb$root), he/she won’t be able to see whole sessions in CDB (including PDB sessions). just the ones in cdb$root:

as you see, only sessions in con_id 1 which is cdb$root are listed. By the way, con_id 0 is CDB itself and only background processes are in there and they are listed in v$session all the time.

again, v$datafile lists only files in current container which is cdb$root. Please consider that c##my_admin is a DBA in CDB so it can connect to any PDB and query their data. it has necessary privileges for that. It has just don’t have “privilege” to query PDBs’ data in the CDB$ROOT container.

That is what CONTAINER_DATA does. it allows a common user to query some or all pdbs data in cdb$root. you must add necessary pdbs to common user’s container_data property:

this alter must be done by a privileged user in cdb$root container. Here, I set container_data of c##my_admin user as PDB1, PDB2 (con_ids are 3 and 4) and CDB$ROOT.

there are few important keys here:

  1. this command must be run in cdb$root.
  2. CDB$ROOT must be in container_data list because it is the root and common user will query data in it so, it must be able to see it
  3. CONTAINER=CURRENT is mandatory because by default it is ALL and this command can be run only in cdb$root which is “current”

so, c##my_admin will be able to see whole sessions in v$session for cdb$root, pdb1 and pdb2 (not PROXY_PDB which its con_id is 5). Let’s see:

so, c##my_admin is able to query other pdbs data in cdb$root. There is no session in pdb2 by the way, that is why no session for con_id 4 in v$session.

instead of giving all container data view access you can specified necessary view as this:

by setting container_data to default it will reset everything. now, I just add container_data info just for V_$session view (which is based view for v$session, you can not grant on v$session) and query v$session and datafile again with c##my_admin:

here it is. we can see session rows for other pdbs in v$session but not for v$datafile.

if you want to set all pdbs at once:

you can use ALL clause for pdb list. remember when you use SET as “ALTER USER … SET CONTAINER_DATA” it will overwrite previous container_data property. if you want to add a new PDB into container_data list then you can use ADD instead of SET:

You can query who has access to what using CDB_CONTAINER_DATA data dictionary view.

I hope this will helpful, any comments is appreciated.

Thanks for reading, wish you healthy days.

How to Demonize Apex V Function!

Published on: by Mustafa 2 Comments

Hi,

Answer of the question in the subject is “Using too much V in SQL statements”. Let’s see why and what should you do.

First things first, I am NOT a APEX developer. I am a DBA and Database Developer but some of my clients are using APEX and I am dealing with many sql functions that uses V function in apex. Since the day 1, I did not like this V function because developers using it in very different ways and it returns VARCHAR2 but it uses for all kind of compares without any conversion functions (this is not on the V function of course).

So, what is the problem here? At my clients’ codes, V function is used heavily in sql statements. Some queries that I dealt with lead me to investigate V function because even if the query has no performance issue, still, it takes too much to complete. When I checked the V function, I find out that it is not DETERMINISTIC (which shouldn’t be because of the logic of DETERMINISTIC but I will talk about this later). Let’s do some tests but before I must explain a few things. I hacked the code of V a little bit and add some logging codes to demonstrate how it is worked and performance issues about it but I won’t explain how I do it (probably many of you can do it easily) and share any source code of it. whenever a V function is called my logging code will write the parameter name of v function into “my_log” table.

I added my log codes, set an apex session in my sql developer session and run these selects:

I delete log table first then call a select statement with four V function in select list run over DUAL table and then check the log table, as you see it is called 4 times. So same data is read 4 times and 4 plsql function call is occurred. this caused too much context switch between SQL and PLSQL and this can reduce your performance dramatically.  Especially, Where clause is extremely dangerous, why? Because V function might run for all rows in the table which will cause a disaster.

as you can see,  this query called  V function 79.647 times why? because there are 79.645 rows in the TMP table. Since I use V in Where clause, condition ( object_id = v(‘OBJECT_ID’) ) executed for every rows in the table (79.645 rows) and only 1 of the rows meets the condition so select list runs 1 times and there are 2 more V function (same with the one in where clause) so totally V called 79.647 times. I hope you see the problem here. in your application code if you use V function in your sql statements, it might be called too much!

How about an index? yep that will help:

so it run only 3 times at this execution because this time search operation (where condition) is run on an index not table so V function is called 1 times in where clause and 2 times in select list. this means you should have indexes at least for your condition columns.

is that enough? Let me show you a pseudo code on a database:

so many same V function call in select statement. creating and index will help you about where clause how about select list? since all items in select list cause a function call (even with same parameters) let’s test performance:

calling this query for 10.000 times (or you can thing this as calling sql 1 times for 10.000 rows) takes 3.2 seconds (20 same V function). if we change the code to this:

this query takes only 0.5 seconds. timings might significant but think about 1000 different queries that run on your database 1000 times per day and calculate spend of time for just calling V function. it is a lot! your significant amount of db time might be wasted.

Let’s improve the code a little bit more:

even better. if you increase number of call, you will see that first 2 plsql code timings will increase linearly but the last code (code just above) will give you almost same performance. Remember, V is a function not a variable. Use variables instead of using V if you can.

Even if these might help your system, main problem is still there: V is not DETERMINISTIC. you might change your codes as I suggested above or create more indexes but many developers also use V in VIEWS which is another and for me a bigger disaster. it is harder to modify the code you cannot use bind variables etc… What would happen if V was deterministic? I modified V and made it deterministic and let’s see the result with first test case without index:

pay attention that there is no index but V is just called 2 times (1 for where clause and 1 for select list, event if select list has 2 copy). it was 79.647 without DETERMINISTIC if you remember the first result.

So making V as deterministic will solve almost every kind of problem but only problem is what I looking for here is not deterministic. Why? Let me explain what deterministic is first. if a function always returns same value with same parameters then this function is a deterministic function like TO_CHAR or SUBSTR. this allow oracle to know that if calls with same parameters are repeated, it can be cached and used first calculation result for on going calls. This way, if same function with same parameters are called in a SQL statement, that function is called only 1 times but this is not V. Result of V is not deterministic because application might change the value of a V item and this means V call with same parameters can return different results because it is just return a context variable value.

So, it shouldn’t be deterministic but it should be something that acting like deterministic for an sql statement. I don’t think any apex developer wants to see a change of V values during a select statement. I think they assume values of V won’t change during the sql statement. So, there should be something like DETERMINISTIC_FOR_CALL in Oracle database. we should be able to define plsql objects as “deterministic” while they are used in a sql statement. Their result should be calculated once and used during the sql call. I think this is also necessary for any plsql object that we can call on SQL. because current structure of plsql is kind of violating the consistency. A select statement returns rows as they were when the sql call started. even if the rows are changed during the sql call, result of the statement won’t be affected from those changes but PLSQL breaks that. when a function calculated for every row, a select statement inside the plsql object can return different results because it will be affected by on going changes. This is a database option. Database should provide us this functionality, Apex team has nothing to do about it.

By the way, I am really frustrated about data type conversions about V. since V returns VARCHAR2, many developer does not care about type conversions. if you compare a V value with a number then you should use “TO_NUMBER(V(…))” for example or at least (as I search for it) NV function (number result of V). this is another issue to me.

For now, I suggest to avoid using V in a SQL statement as much as possible and use bind variable if you can.  Let’s say you have some SQL with so many V call with same parameter, what can you do?  you can “force” oracle to cache them:

there are better way to write this sql but I just want to demonstrate a way. as you see this query takes 3.26 seconds to complete because of too many V calls. you can rewrite your query and cache the V results first and then use them as this:

I am adding a NO_MERGE hint to prevent optimizer to merge queries before query run. as you see this version returns in 0.19 seconds 15 times faster becuase V functions are called only 2 times (2 different V item is selected OBJECT_ID and USER_NAME).

Edit:

Another and maybe better solution for this problem is using your own V function and create it as deterministic:

this is faster than previous join method. only thing that you should change V calls with MY_V.

I hope, this helps you on your systems. Now after those tests, I must connect with my clients and explain them to change their coding to gain significant db time.

wish you all healthy days.