Question: We have a lot of
SQL's which are very non-performant after performing dbms_stats with
oracles default job (version 10). The explain plan shows the optimal
access path, but oracle executes a full table scan (but only if the statistics
of our performance index exists).
1. If we don't have statistics for the
index, oracle will use the index. The statement returns in 100 ms.
2. If we use the sql without line 'AND (mv_dispo.dispo_belwert_in +
mv_dispo.dispo_belwert_out) != 0', oracle will use the index and returns in
100ms.
3. If we use the hint /*+ INDEX(mv_dispo I_MVDISPO_VGBIS_VGAB_BEST_ZEIT) */,
oracle will use the index and returns in 100ms.
4. If current CBO statistics (automatically with the dbms_stats
oracle default settings), the statement returns in 2-3 minutes and take a
full table scan. Oracle uses a different access strategy!?
Why does oracle shows a optimal explain plan
and executes the SQL with different access path? I don't want to use an
index hint, so what kind of solution could we take?
Answer: This is a very
interesting problem! The CBO is a proprietary hunk of extremely
sophisticated software, and Oracle does not release details of it's internal
machinations. In general, the execution plan with match the fetch, but sometimes
the explain plan does not match the real-time execution. Some possible
explanations may include:
- An "older" execution plan is cached in the library cache
- Dynamic sampling is altering the execution plan
- A bind variable influence runtime execution by using query
rewrite (via setting cursor_sharing=force of cursor_sharing=similar).
- Your materialized view re-write has a bug
The only way to know for sure is to run a detailed 10046 (TKPROF) dump and open
an SR with MetaLink.
>> If we don't have statistics for the index, oracle will use the index.
That's a good clue! How selective is this index? Are you creating a histogram on
the index?
>> What kind of solution could we take?
Me, I would force it with a hint, unless you want to explore it with Oracle
technical support (and you may find that you have just hit a bug!).
If you don't want to use hints, your only other option is to gerrymander your
CBO stats to get the desired results. Don't assume that it's not a bug, either.
>> If we have performing statistics (automatically with oracle default settings)
I would start by adding
histograms to the questionable column's:
Have you done your system-level SQL tuning first? Check for optimal system-wide
settings for (optimizer_mode, optimizer_index_caching, optimizer_index_cost_adj).
You need to optimize all
SQL to your overall workload before tuning outliers:
