Re: select distinct and index usage

> Still doing the sequential scan on the table, but at least it's
> avoiding the expensive disk merge sort. It still seems as if I ought
> to be able to coax it into using an index for this type of query,
> though- especially since it's using one on the other table. Is there
> perhaps some way to reformulate the index in such a way as to make it
> more useful to the planner?

You're asking postgres to examine EVERY visible row (hence the sequential scan makes good sense), and tell you what field values there are.

You may be able to make use of an index by rearranging your query to generate a series between your min & max values, testing whether each value is in your table.

You've got 4252 distinct values, but what is the range of max - min? Say it's 5000 values, you'd do 5000 lookups via an index, unless postgres thought that the number of index based lookups where going to be more expensive than reading the entire table.

Regards,
Stephen Denne.

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