Re: [PATCHES] Minimum selectivity estimate for LIKE 'prefix%'

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.