Intermediate SQL Color Coded SQL, UNIX and Database Essays

1Jun/127

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.

23Sep/113

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:

Note
-----
   - SQL plan baseline "SQL_PLAN_01yu884fpund494ecae5c" used FOR this statement

But can you find it globally for ALL SQLs in the shared pool ?

18Jun/116

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!

10Apr/117

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.

29Jan/115

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:

su - <regular user>

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).

define BECOME_USER=&1

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
6Oct/103

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:

BEHAVIORSQL ProfileSPM Baseline
“Fire” for object in a different schemaYESYES
“Fire” for object with a different DATAYESYES
“Fire” for object with a different STRUCTUREYESYES
“Fire” when indexes are differentYESMAYBE, will “fire” if the same execution plan is still produced
“Fire” when indexes are missingYESNO
“Fire” when TABLE is replaced by VIEWYESNO
“Fire” when TABLE is replaced by MVIEWYESNO

And, if you do not believe me, read on …

3Oct/1011

What is the difference between SQL Profile and SPM Baseline ?

SQL Profiles

Note
-----
   - SQL profile "SYS_SQLPROF_012ad8267d9c0000" used FOR this statement

and SPM Baselines

Note
-----
   - SQL plan baseline "SQL_PLAN_01yu884fpund494ecae5c" used FOR this statement

are both relatively new features of ORACLE Optimizer with Profiles first appearing in version 10 and SPM Baselines in version 11.

Both SQL Profiles and SPM Baselines are designed to deal with the same problem: Optimizer may sometimes produce a very inefficient execution plan, and they are both doing it by essentially abandoning the idea that “all SQLs are created equal”. Instead, another idea is put forward: “Some SQLs are special and deserve individual treatment”.

12Sep/1030

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:

Note
-----
   - SQL profile "SYS_SQLPROF_012ad8267d9c0000" used FOR this statement

This seems to mean that:

  1. An external ‘helper’ object was used during evaluation of your SQL statement
  2. 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 ?

9Jul/103

ORACLE 11g XML alert logs. Surprise: They are actually more convenient to work with

I have to admit, I was initially very skeptical about ORACLE 11g Automated Diagnostic Repository or ADR.

Why do I need to use “special” tool to access database alert log ? (and not vi) Who decided to move trace directories to some weird location ?? (why not keep them where they’ve always been?) Why store alert records as XML ??!! (it is meant for humans, you know …)

These things have “inconvenience” and “pain in the a%%.” written all over them … And, of course, the usual ORACLE attitude: “we decided to change it, so suck it up and learn how it works” did not help much …

Old habits die hard, but lately, I’ve started to come around using ADR. It turns out that with a bit of adrci knowledge and a few tweaks in the environment, working with alert logs from a regular UNIX shell becomes not only manageable but actually (gulp!) much more convenient …

18Jun/1016

ORACLE 11g SQL Plan Management: The Dark Side of SPM. Part 4

With SQL Plan Management being relatively new, it is inevitable that many people run into problems using it.

Most of those problems are caused by our (relative) ignorance: SPM does change the way how ORACLE runs SQL statements and it simply takes time to get used to how it works. Yet, some of the consequences of using SPM are truly bizarre and will surprise many people (including, probably, some ORACLE developers).

In this post I will describe the 3 scenarios where we have recently run into problems using SPM and you can judge for yourself …