Wednesday, March 13, 2013

Finding Duplicate SQL

The presence of duplicate SQL indicate that there are some SQL statements which doesn't have Bind variables. These duplicate SQL can raise performance issue because they will increase the number of hard parse in database.

ORACLE 10g introduced two new columns in v$sql view, which can help to identifing duplicate SQL more accurately. Those two new columns are:
  1. force_matching_signature 
  2. exact_matching_signature

exact_matching_signature - If two or more SQL has same value in this column, ORACLE assumes they are same after making some cosmetic adjustments (removing white space, uppercasing all keywords etc) to them. The is simmiler, when parameter cursor_sharing is set to EXACT.

force_matching_signature - the same value in this column (excluding 0) marks SQLs that ORACLE will consider they are same when it replaces all literals with binds (that is, if cursor_sharing=FORCE).

SELECT sql_text , count(1)
FROM v$sql
WHERE force_matching_signature > 0
  AND force_matching_signature <> exact_matching_signature
GROUP BY sql_text
HAVING count(1) > 10
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