Re: Seq Scan vs Index on Identical Tables in Two Different Databases

On Wed, Jul 17, 2013 at 07:50:06PM +0000, Ellen Rothman wrote:
- I have the same table definition in two different databases on the same computer. When I explain a simple query in both of them, one database uses a sequence scan and the other uses an index scan. If I try to run the Seq Scan version without the where clause restricting the value of uniqueid, it uses all of the memory on my computer and never completes.
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- How can I get the Seq Scan version to use an index scan?
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- Explain results - good version:
- "GroupAggregate (cost=0.00..173.78 rows=1 width=15)"
- " -> Index Scan using pubcoop_ext_idx1 on pubcoop_ext (cost=0.00..173.77 rows=1 width=15)"
- " Index Cond: (uniqueid < '000000009'::bpchar)"
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- Explain results - problem version:
- "HashAggregate (cost=13540397.84..13540398.51 rows=67 width=18)"
- " -> Seq Scan on pubcoop_ext (cost=0.00..13360259.50 rows=36027667 width=18)"
- " Filter: (uniqueid < '000000009'::bpchar)"

(Assuming that your postgresql.conf is the same across both systems and that
you've run vanilla analyze against each table... )

I ran into a similar problem before and it revolved around the somewhat random nature of
a vaccum analyze. To solve the problem i increased the statistics_target for the table
on the box that was performing poorly and ran analyze.

I believe that worked because basically the default_statistics_taget of 100 wasn't
catching enough info about that record range to make an index appealing to the optimizer
on the new box where the old box it was.