=?koi8-r?B?/cXLyc4g8dLP08zB1w==?= <ladayaros...@yandex.ru> writes:
> I've noticed strange row count estimations in BETWEEN predicate:
> EXPLAIN
> SELECT *
> FROM t1
> WHERE f1 = 42;
> -->> Bitmap Heap Scan on t1 (cost=203.07..28792.94 rows=10662 width=24)
> <skipped>
> EXPLAIN
> SELECT *
> FROM t1
> WHERE f1 BETWEEN 42 AND 42;
> -->> Index Scan using table1_field1_idx on t1 (cost=0.44..8.46 rows=1
> width=24) <skipped>
> Why row count estimations for two logically equivalent queries are so
> different?
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.
regards, tom lane
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