Re: Understanding histograms

"Tom Lane" <tgl@sss.pgh.pa.us> writes:

> Right.  As a matter of policy we never estimate less than one matching
> row; and I've seriously considered pushing that up to at least two rows
> except when we see that the query condition matches a unique constraint.
> You can get really bad join plans from overly-small estimates.

This is something that needs some serious thought though. In the case of
partitioned tables I've seen someone get badly messed up plans because they
had a couple hundred partitions each of which estimated to return 1 row. In
fact of course they all returned 0 rows except the correct partition. (This
was in a join so no constraint exclusion)

--
  Gregory Stark
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