I wrote:
> Arjen van der Meijden <acmmailing@vulcanus.its.tudelft.nl> writes:
>> SELECT COUNT(*) FROM
>> data_main AS dm,
>> postcodes AS p
>> WHERE dm.range BETWEEN p.range_from AND p.range_till
> Planner error ... because it doesn't have any good way to estimate the
> number of matching rows, it thinks that way is a bit more expensive than
> data_main as the outside, but in reality it seems a good deal cheaper:
BTW, it would get the right answer if it had recognized the WHERE clause
as a range restriction --- it still doesn't know exactly what fraction
of rows will match, but its default estimate is a great deal tighter for
"WHERE x > something AND x < somethingelse" than it is for two unrelated
inequality constraints. Enough tighter that it would have gone for the
correct plan.
The problem is that it doesn't recognize the WHERE as a range constraint
on dm.range. I thought for a moment that this might be a
recently-introduced bug, but actually the code is operating as designed:
clauselist_selectivity says
* See if it looks like a restriction clause with a pseudoconstant
* on one side. (Anything more complicated than that might not
* behave in the simple way we are expecting.)
"Pseudoconstant" in this context means "a constant, parameter symbol, or
non-volatile functions of these" ... so comparisons against values from
another table don't qualify. It seems like we're missing a bet though.
Can anyone suggest a more general rule? Do we need for example to
consider whether the relation membership is the same in two clauses
that might be opposite sides of a range restriction? It seems like
a.x > b.y AND a.x < b.z
probably can be treated as a range restriction on a.x for this purpose,
but I'm much less sure that the same is true of
a.x > b.y AND a.x < c.z
Thoughts?
regards, tom lane
