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    Oracle statistics invalidating sql

    The default value of SHARED_POOL_RESERVED_MIN_ALLOC should be adequate for most systems.In the last week, there were couple of questions targetting a similar topic, library cache.The optimization is nothing but the steps that are needed to understand the query in a better way and prepare a plan to run the query aka, execution plan.This step is necessary to do for the first time but must not be repeated all the times when the query is executed.Statistics ---------------------------------------------------------- 982 recursive calls 0 db block gets 951 consistent gets 0 physical reads 0 redo size 106664 bytes sent via SQL*Net to client 7599 bytes received via SQL*Net from client 661 SQL*Net roundtrips to/from client 12 sorts (memory) 0 sorts (disk) 9900 rows processed SQL select * from t where n=:n; Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 155 consistent gets 0 physical reads 0 redo size 476 bytes sent via SQL*Net to client 350 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed select * from table(dbms_xplan.display_cursor('g2n32un6t1c55')); PLAN_TABLE_OUTPUT -------------------------------------------------------------------- SQL_ID g2n32un6t1c55, child number 0 ------------------------------------- select * from t where n=:n Plan hash value: 1601196873 -------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 42 (100)| | |* 1 | TABLE ACCESS FULL| T | 9900 | 531K| 42 (0)| | -------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("N"=: N) 18 rows selected., then you want the plan with an index range scan because it will provide optimal performance most of the time, while resulting in suboptimal performance for only 1% of executions.With the histogram in place, one day you will be unlucky enough to have Oracle hard parse the query with a bind variable value of 0, which will force everyone else to use an FTS (as was demonstrated above), which in turn will result an abysmal performances for 99% of executions (until the next hard parse when you might get lucky again).If the profile is being used, but the plan is not what you expected, there are a couple of likely culprits.

    There are three things that might put you at risk of unstable plans due to bind variable peeking. ” On an OLTP system, there is no way that you need histograms on a third of your tables (and I can hardly think of any DSS system where this amount of histograms can be justified).The first time is hard to avoid as oracle knows nothing about the query , so there is no way out othere than optimizing the query.This first time optimization is called Hard Parsing of the query.Here’s a bit of an email I sent with my thoughts on the issue during a conversation on the Oracle-L list.First, I think you need to convince yourself as to whether the profile is being used or not.

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