Tom Lane wrote:
"Jim C. Nasby" <[EMAIL PROTECTED]> writes:

On Wed, Apr 06, 2005 at 06:09:37PM -0400, Tom Lane wrote:

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

In a case like this, you could actually look at the  data in b and see
what the average range size is.

Not with the current statistics --- you'd need some kind of cross-column
statistics involving both y and z.  (That is, I doubt it would be
helpful to estimate the average range width by taking the difference of
independently-calculated mean values of y and z ...)  But yeah, in
principle it would be possible to make a non-default estimate.

regards, tom lane

Actually, I think he was saying do a nested loop, and for each item in the nested loop, re-evaluate if an index or a sequential scan is more efficient.

I don't think postgres re-plans once it has started, though you could
test this in a plpgsql function.


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