| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | Karim A Nassar <Karim(dot)Nassar(at)NAU(dot)EDU>, Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)com>, Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: Delete query takes exorbitant amount of time |
| Date: | 2005-03-29 17:31:49 |
| Message-ID: | 3340.1112117509@sss.pgh.pa.us |
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| Lists: | pgsql-performance |
Simon Riggs <simon(at)2ndquadrant(dot)com> 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. 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.
regards, tom lane
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