Tom Lane-2 wrote
> PG doesn't try to estimate inequalities exactly, because it usually
> doesn't make enough of a difference to matter. Currently we don't
> even bother to distinguish say ">" from ">=" for estimation purposes,
> though certainly we would need to in order to deal with zero-width ranges
> with any great amount of precision.
Thank you for your answer!
I'm sorry, but after looking into documentation and sources (scalarineqsel
function in selfuncs.c, clauselist_selectivity and addRangeClause functions
in
clausesel.c) and experimenting a little I've got an impression that
PostgreSQL
actually bothers to distinguish ">" from ">=" for estimation purposes
sometimes (probably, when MCV is used), but in my example it uses histogram
and indeed doesn't distinguish them.
My simple test (using MCVs) is below:
-----
CREATE TABLE t2(n int);
INSERT INTO t2(n) VALUES (0),(0),(0),(0),(1),(1),(1),(1),(2),(2),(2),(2);
ANALYZE t2;
EXPLAIN SELECT * FROM t2 WHERE n > 0 AND n < 2
-- rows=4
EXPLAIN SELECT * FROM t2 WHERE n > 0 AND n < 2
-- rows=12
------
Looking further, I found ineq_histogram_selectivity function in selfuncs.c,
and this fragment seems relevant:
-----
/*
* We have values[i-1] <= constant <= values[i].
*
* Convert the constant and the two nearest bin boundary
* values to a uniform comparison scale, and do a linear
* interpolation within this bin.
*/
<skip>
binfrac = (val - low) / (high - low);
-----
And now I'm stuck. Can ">" operators can be distinguished from ">="
operators at this point?
-----
WBR, Yaroslav Schekin.
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