Re: Incorrect assumptions with low LIMITs
От | Jeff Davis |
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Тема | Re: Incorrect assumptions with low LIMITs |
Дата | |
Msg-id | 1332206363.3803.27.camel@sussancws0025 обсуждение исходный текст |
Ответ на | Re: Incorrect assumptions with low LIMITs (Simon Riggs <simon@2ndQuadrant.com>) |
Ответы |
Re: Incorrect assumptions with low LIMITs
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Список | pgsql-hackers |
On Sat, 2012-03-17 at 12:48 +0000, Simon Riggs wrote: > The problems are as I described them > > (1) no account made for sparsity, and other factors leading to an > overestimate of rows (N) > > (2) inappropriate assumption of the effect of LIMIT m, which causes a > costly SeqScan to appear better than an IndexScan for low m/N, when in > fact that is seldom the case. > > Overestimating N in (1) inverts the problem, so that an overestimate > isn't the safe thing at all. I think the actual problem has more to do with risk. The planner doesn't know how uniform the distribution of the table is, which introduces risk for the table scan. I would tend to agree that for low selectivity fraction and a very low limit (e.g. 1-3 in your example) and a large table, it doesn't seem like a good risk to use a table scan. I don't know how that should be modeled or implemented though. Regards,Jeff Davis
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