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Re: [PATCHES] Minimum selectivity estimate for LIKE 'prefix%'
- From: Peter Eisentraut <peter_e(at)gmx(dot)net>
- To: pgsql-hackers(at)postgresql(dot)org
- Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
- Subject: Re: [PATCHES] Minimum selectivity estimate for LIKE 'prefix%'
- Date: Mon, 31 Mar 2008 12:02:47 +0200
- Message-id: <200803311202.47718.peter_e@gmx.net> <text/plain>
Am Donnerstag, 6. März 2008 schrieb Tom Lane: > What I propose doing about this is a small variant on Peter's original > suggestion: compute the estimated selectivity for > col = 'prefix' > and clamp the result of prefix_selectivity to be at least that. OK, first results with this patch are in: The selectivity estimations are adjusted nicely, but the cost calculation doesn't change at all. Before: Index Scan using foo_idx_3 on foo foo (cost=0.01..6.03 rows=1 width=8) After: Index Scan using foo_idx_3 on foo foo (cost=0.01..6.03 rows=627 width=8) How is that possible? Btw., the corresponding query plan for the LIKE 'constant' case is: Index Scan using foo_idx_3 on foo foo (cost=0.00..2527.84 rows=627 width=8) This is what we had hoped to get in the "after" case.
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