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                              WORKLOG TASK
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TASK...........: Subquery optimization: Avoid recalculating subquery if external fields
		values found in subquery cache
CREATION DATE..: Wed, 25 Nov 2009, 22:25
SUPERVISOR.....: Monty
IMPLEMENTOR....: Sanja
COPIES TO......: 
CATEGORY.......: Server-BackLog
TASK ID........: 66 (http://askmonty.org/worklog/?tid=66)
VERSION........: Server-5.2
STATUS.........: Assigned
PRIORITY.......: 60
WORKED HOURS...: 0
ESTIMATE.......: 0 (hours remain)
ORIG. ESTIMATE.: 0

PROGRESS NOTES:



DESCRIPTION:

Collect all outer items/references (left part of the subquiery and outer
references inside the subquery) in key string. Compare the string (which
represents certain value set of the references) against values in hash table and
return cached result of subquery if the reference values combination has already
been used.

For example in the following subquery:
(L1, L2) IN (SELECT A, B FROM T WHERE T.F1>OTER_FIELD)
set of references to look into the subquery cache is (L1, L2, OTER_FIELD).

The subquery cache should be implemented as simple LRU connected to the subquery.

Size of the subquery cache (in number of results (but maybe in used memory
amount)) is limited by session variable (query parameter?).


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