"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.
John =:->
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