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                              WORKLOG TASK
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TASK...........: Subquery optimization: Efficient NOT IN execution with NULLs
CREATION DATE..: Fri, 27 Nov 2009, 13:22
SUPERVISOR.....: Monty
IMPLEMENTOR....: 
COPIES TO......: 
CATEGORY.......: Server-RawIdeaBin
TASK ID........: 68 (http://askmonty.org/worklog/?tid=68)
VERSION........: Benchmarks-3.0
STATUS.........: Un-Assigned
PRIORITY.......: 60
WORKED HOURS...: 0
ESTIMATE.......: 0 (hours remain)
ORIG. ESTIMATE.: 0

PROGRESS NOTES:



DESCRIPTION:

The goal of this task is to implement efficient execution of NOT IN
subquery predicates of the form:
  <oe_1,...,oe_n> NOT IN <non_correlated subquery>
when either some oe_i, or some subqury result column contains NULLs.

The problem with such predicates is that it is possible to use index
lookups only when neither argument of the predicate contains NULLs.
If some argument contains a NULL, then due to NULL semantics, it
plays the role of a wildcard. If we were to use regular index lookups,
then we would get 'no match' for some outer tuple (thus the predicate
evaluates to FALSE), while the SQL semantics means 'partial match', and
the predicate should evaluate to NULL.

This task implements an efficient algorithm to compute such 'parial
matches', where a NULL matches any value.


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