What to do when your query is too slow?

First of all, you have to know why it is slow. What is the real cause of your problem. If the reason why is not known, suggesting to rewrite the query, or hinting the query, suggesting parallellization et cetera is not very productive. Once in a while you may get lucky. But even then you have to realize that if your problem seems "solved", but you don't know why, nothing guarantees you that the problem won't come back tomorrow. So the first step should always be to investigate the root cause.

The tools at your disposal are, among more:
- dbms_profiler
- explain plan
- SQL*Trace / tkprof
- statspack

Use dbms_profiler if you want to know where time is being spent in PL/SQL code. Statspack is a must if you are a dba and want to know what is going on in your entire database. For a single query or a small process, explain plan and SQL*Trace and tkprof are your tools.

explain plan

in SQL*Plus you have to type:

explain plan for ;
select * from table(dbms_xplan.display);


When you get error messages or a message complaining about an old version of plan_table, make sure you run the script utlxplan.sql.

The output you get here basically shows you what the cost based optimizer expects. It gives you an idea on why the cost based optimizer chooses an access path.

SQL*Trace/tkprof

For this you have to type in SQL*Plus:
- alter session set sql_trace true;
-
- disconnect (this step is important, because it ensures all cursors get closed, and "row source operation" is generated)
- identify your trace file in the server directory as specified in the parameter user_dump_dest
- on your operating system: tkprof a.txt sys=no sort=prsela exeela fchela

The file a.txt will now give you valuable information on what has actually happened. No predictions but the truth.
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