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Re: Postgres optimizer choosing wrong index

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Tom Lane wrote:
Jack Orenstein <jack.orenstein@xxxxxxx> writes:
Tom Lane wrote:
If you plug in a value that *does* occur in the table it should probably
choose the more-relevant index consistently.

Unfortunately, it matters a lot at runtime. The dh value is not very selective, as shown by the statistics above.

A dh value that does not occur in the index is *perfectly* selective.
I'm not sure what your problem is but this example isn't illustrating
anything wrong that I can see.

I see your point.

I may have simplified too far. Our application runs a number of different queries. All our WHERE clauses restrict dh and fh. For a given pair of (dh, fh) values, the initial query should come up empty and then insert this pair, and then there is further processing (SELECT, UPDATE). Something is causing a huge number of index row reads (according to pg_stat_user_indexes) but only in tables that have been vacuumed.

Jack

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