Re: index on large table

Tom Lane wrote:
> >> "explain select * from events order by oid limit 10 offset 1000000"
> >> NOTICE: QUERY PLAN:
> >> Limit (cost=424863.54..424863.54 rows=10 width=130)
> >> -> Sort (cost=424863.54..424863.54 rows=1025245 width=130)
> >> -> Seq Scan on events (cost=0.00..35645.45 rows=1025245 width=130)
> >>
> >> Bummer. This is very slow again, sequential scan again. Why the index is
> >> not used for this query? Use of index would make it very fast!
>
> Not necessarily. Using the index for this would require fetching
> 1000000+10 values in the indexscan (and throwing away all but 10).
>
> The planner is counting on its fingers and guessing that the sort
> is faster. It might or might not be right about that (have you
> compared timings?) but certainly the index method won't be
> instantaneous.

This question is being asked a lot. I hope my new FAQ item 4.8 wording
helps, but it will take time for people to read the new version. I will
add it to 7.2.X CVS.

--
Bruce Momjian | http://candle.pha.pa.us
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