Re: How Reliable is Explain Plan in 9.2

Yes, explain plan will become increasingly unreliable as the cbo takes more and more factors and current conditions current into account. Some of the factors that can change the outcome of a parse from session to session are:

for Oracle 8
- different session parameters (db_file_multiblock_read_count, hash_multiblock_io_count, sort_area_size, hash_area_size)

for Oracle 9i additionally
- you can let Oracle dynamically set the sort_area_size and other memory parameters so you have a moving target now - bind variable peeking - the first parse determines the plan for all following sql depending on its bind value - system statistics in effect at the time of parse. If the system statistics get changed, existing plans do not get invalidated, but if you do an explain the cbo will use the current values - dynamic sampling where the optimizer tries to improve on its estimates by sampling predicate values at the time of parsing.

from comments I heard, it will get "worse" (as far as explain differing from reality is concerned) with Oracle 10. The optimizer will try and learn from past executions of a sql and modify the plan if appropriate.

At 08:35 AM 3/5/2003 -0800, you wrote:
>Just had a fellow tell me that explain plan in completely unreliable in 9.2
>and getting accurate results requires direct SGA access on executing SQL
(he
>is working in a RAC environment). They are running Precise, a good
product,
>but this sounds like something a sales person told him. I can only recall
>that occasionally the plan executed is not the plan you see in explain
plan.
>Anyone know the truth of this issue?

Wolfgang Breitling
Centrex Consulting Corporation
http://www.centrexcc.com

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