pgsql: Improve estimation of IN/NOT IN by assuming array elements are d

Improve estimation of IN/NOT IN by assuming array elements are distinct.

In constructs such as "x IN (1,2,3,4)" and "x <> ALL(ARRAY[1,2,3,4])",
we formerly always used a general-purpose assumption that the probability
of success is independent for each comparison of "x" to an array element.
But in real-world usage of these constructs, that's a pretty poor
assumption; it's much saner to assume that the array elements are distinct
and so the match probabilities are disjoint. Apply that assumption if the
operator appears to behave as equality (for ANY) or inequality (for ALL).
But fall back to the normal independent-probabilities calculation if this
yields an impossible result, ie probability > 1 or < 0. We could protect
ourselves against bad estimates even more by explicitly checking for equal
array elements, but that is expensive and doesn't seem worthwhile: doing
it would amount to optimizing for poorly-written queries at the expense
of well-written ones.

Daniele Varrazzo and Tom Lane, after a suggestion by Ants Aasma

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/66a7e6bae98592d1d98d9ef589753f0e953c5828

Modified Files
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src/backend/utils/adt/selfuncs.c | 74 ++++++++++++++++++++++++++++++++++++--
1 files changed, 71 insertions(+), 3 deletions(-)