RMOUG 2013 Training Days: SQL Top-N and Pagination pattern
I just returned from RMOUG 2013 training days in Denver, CO and I have to say that I’m impressed!
This was an incredibly well organized and well run event with lots of great speakers and great presentations. As a former board member of Tampa’s ORACLE users group, I know how hard it is to make an event like this happen, especially at this scale. Great job, @DbaKevlar and the rest!
Thanks to everyone who attended my presentation at RMOUG 2013 and if you did not have a chance to catch it, I’ll be presenting it again at IOUG Collaborate 2013.
Cheers!
Read inconsistent queries in ORACLE database ? Sure, why not!
Read consistency is one of the coolest features of ORACLE database.
In a nutshell, it means that your queries (at least, logically) do not pay attention to the outside world. I.e. your tables may be hit with thousands of changes per second, but your query will ignore all the hoopla and will always see only the data that existed when the query started.
IOUG Collaborate 2012: The art of database sharding
Thanks everyone, who attended my presentation about database sharding at IOUG Collaborate. That was a lot of fun!
Sharding is an awesome technique and I’ll blog separately on it, but for now see my presentation and white paper.
Cheers
How to find if your SQL is using SPM baseline
I’ve always wondered how many of my SQLs are NOT using baselines.
Of course, when you run DBMS_XPLAN.DISPLAY_CURSOR for the statement, it can put a nicely formatted note for you, something like:
-----
- SQL plan baseline "SQL_PLAN_01yu884fpund494ecae5c" used FOR this statement
But can you find it globally for ALL SQLs in the shared pool ?
How to add a hint to ORACLE query without touching its text
If you’ve been a DBA long enough, you’ve probably seen multiple cases where certain SQL queries just refuse to cooperate.
I.e. you KNOW that the query is supposed to use index IDX1, but ORACLE stubbornly decides to take IDX2. Rats!
The usual suspect here is bad statistics and thus the usual advise is: start re-collecting them. But while this might help, the success is far from guaranteed! Moreover, with a ‘real production’ data statistics collection could take hours (days?) and you already have people screaming about slow performance … In other words, you need to fix the problem NOW and waiting until stats collection maybe fixes the problem a few hours down the road is just not an option!
How to find SPM baseline by sql_id
When you start working with SQL Plan baselines, one of the annoying things that you might find is that the main “baseline” dictionary view dba_sql_plan_baselines does not have sql_id column.
How to become regular ORACLE user when you do NOT know the password
This is a fairly common situation for a DBA: you can always connect as SYSDBA, but sometimes you really need to connect as a regular database user (i.e. to have proper environment for your queries).
If you are root in UNIX, this would be extremely easy to do:
Unfortunately, ORACLE does not have an equivalent su command and, even if you are SYS, to connect to any regular database user you need to know its password (which could be a problem).
Still, there is a simple workaround that can be applied here. Essentially, you can save user’s old password, change it to some dummy value and quickly change it back after your connection is established.
While user password hashes are no longer exposed in DBA_USERS view (starting with 11g, I believe), they are still available in SYS.USER$.
Below is the script that does exactly that (full credit goes to Matt Parker who showed me this trick).
COLUMN user_pwd new_value SZ_PWD
SET termout off
SELECT password AS user_pwd FROM USER$ WHERE UPPER(name)='&BECOME_USER';
spool '/tmp/.&BECOME_USER..CHANGE'
prompt SET termout off
prompt CONNECT / AS sysdba
prompt ALTER USER &BECOME_USER IDENTIFIED BY VALUES '&SZ_PWD';;
prompt exit
spool off
ALTER USER &BECOME_USER IDENTIFIED BY TEMPORARY;
CONNECT &BECOME_USER/TEMPORARY
host sqlplus -S /nolog @/tmp/.&BECOME_USER..CHANGE
host rm /tmp/.&BECOME_USER..CHANGE
SET termout ON
SHOW USER
undef BECOME_USER
The troubling global nature of SQL Profiles and SPM Baselines
Did you know that SQL profiles and SPM baselines collected for SQL statements in one schema can apply to “the same” SQL statements in another schema ?
They do ! And, besides, objects that these SQLs apply to do NOT need to be exactly the same … And I’m not just talking about different data values or different number of records … The objects can have different structure! Or, they can actually be of different type, i.e. views instead of tables …
The only things that matter are: SQL text and object names.
This strange behavior is summarized in the table below:
| BEHAVIOR | SQL Profile | SPM Baseline |
|---|---|---|
| “Fire” for object in a different schema | YES | YES |
| “Fire” for object with a different DATA | YES | YES |
| “Fire” for object with a different STRUCTURE | YES | YES |
| “Fire” when indexes are different | YES | MAYBE, will “fire” if the same execution plan is still produced |
| “Fire” when indexes are missing | YES | NO |
| “Fire” when TABLE is replaced by VIEW | YES | NO |
| “Fire” when TABLE is replaced by MVIEW | YES | NO |
And, if you do not believe me, read on …
What are SQL Profiles and why do we need them ?
If you use DBMS_XPLAN package to analyze execution plans for your SQL statements (and you really should these days), you might have noticed that at times the following line might be displayed along with your execution plan:
-----
- SQL profile "SYS_SQLPROF_012ad8267d9c0000" used FOR this statement
This seems to mean that:
- An external ‘helper’ object was used during evaluation of your SQL statement
- This ‘helper’ object changed (or, at least, influenced) its execution plan
While having some extra help is nice, one has to wonder: what exactly is this “profile” object ? What does it do ? And, in a bigger picture, why do we need “external” help evaluating SQL statements at all ?
Can ORACLE 11g memory_target work with AIX large pages ?
One of my readers recently asked me if ORACLE 11g can use memory_target along with AIX large pages … (Thanks Randolf!)
At the first blush this seems to be impossible. The main reason, of course, is that memory_target and large pages are used for the purposes that are, in fact, completely opposite!
- The goal of 11g memory_target is to flow memory efficiently between ORACLE SGA and PGA to the place where it is most needed. This means that SGA may reduce its size from time to time, releasing memory to the operating system
- AIX large pages, on the other hand, are designed to never leave physical memory
I hope, you can see a contradiction here …