Thread: Re: default cardinality with non-existent value

Igor Kustov <iakustov@yandex.ru> writes:
> I noticed that the optimizer expects one row when there is a
> condition on the field for which optimizer does not know the value
> based on statistics,

Actually, in this example it's probably estimating a selectivity
fraction of exactly zero (plus or minus roundoff error), but later
that gets clamped to the minimum allowed rowcount estimate of one row.
See var_eq_const's handling of the its-not-any-of-the-MCVs case:


https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/utils/adt/selfuncs.c;h=a96b1b9c0bc69e30865221c5e24e37c594f16d21;hb=HEAD#l413

The clamp-to-one-row bit is done by clamp_row_est:


https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/optimizer/path/costsize.c;h=60b0fcfb6be542552903f00643de82ac1e91cb80;hb=HEAD#l5348

            regards, tom lane