On Tue, 2005-03-29 at 12:31 -0500, Tom Lane wrote: > Simon Riggs <[EMAIL PROTECTED]> writes: > > That implies to me that LIMIT queries are not considered correctly in > > the M&L formula and thus we are more likely to calculate a too-high cost > > for using an index in those circumstances....and thus more likely to > > SeqScan for medium sized relations? > > You misunderstand how LIMIT is handled.
Huh? Well, not this time. (Though my error rate is admittedly high.) > The plan structure is > > LIMIT ... > regular plan ... > > and so the strategy is to plan and cost the regular plan as though it > would be carried out in full, and then take an appropriate fraction > of that at the LIMIT stage. To cost it as if it would be carried out in full and then not execute in full is the same thing as saying it overestimates the actual execution cost. Which can lead to selection of SeqScan plan when the IndexScan would have been cheaper, all things considered. ...it could work like this LIMIT .... regular plan (plan chosen knowing that LIMIT follows) so that the LIMIT would be considered in the M&L formula. Not that I am driven by how other systems work, but both DB2 and Oracle allow this form of optimization. There's not a huge benefit in sending LIMIT 1 through on the FK check queries unless they'd be taken into account in the planning. Anyway, I'm not saying I know how to do this yet/ever, just to say it is possible to use the information available to better effect. This looks like a TODO item to me? Thoughts? Best Regards, Simon Riggs ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])