Re: [SQL] indexes

Remigiusz Sokolowski <rems(at)gdansk(dot)sprint(dot)pl> writes:
> NOTICE: QUERY PLAN:
> Unique (cost=77.02 size=0 width=0)
> -> Sort (cost=77.02 size=0 width=0)
> -> Nested Loop (cost=77.02 size=1 width=28)
> -> Nested Loop (cost=74.97 size=1 width=12)
> -> Seq Scan on b1 (cost=72.97 size=1 width=8)
> -> Index Scan on e2 (cost=2.00 size=1 width=4)
> -> Index Scan on e1 (cost=2.05 size=1304 width=16)
>

I think the real problem here is that the optimizer thinks your tables
are small (notice the size=1 estimates in the inner loop). Have you
done a VACUUM lately? You need that to update the statistics that the
optimizer uses.

Unless you are dealing with very small tables, you don't want to see
nested-loop joins (that means scanning the lower table once for each
tuple in the upper table!). You want to see merge joins or hash joins.

Vadim's suggestion of a better-adapted index was a good one, but I
wonder whether the speedup you saw wasn't just a side effect from
CREATE INDEX having updated the optimizer's stats, so that it stopped
using nested loops...

regards, tom lane