The Hermit Hacker <scrappy@hub.org> writes:
> There is an index on all three conditions in the WHERE clause:
> Yet EXPLAIN shows:
> Aggregate (cost=2.05 rows=1 width=4)
-> Index Scan using referrer_link_counter_id on referrer_link (cost=2.05 rows=1 width=4)
> Why does EXPLAIN only show the use of one of the indices, why counter_id
> and why not all three?
Indexscans only know how to use one index at a time.
The optimizer picked the counter_id index out of the three available
choices because it thought that would be the cheapest (most selective)
alternative --- or, if the computed selectivities were all the same,
just because it happened to try that one first.
Do you have reason to think that one of the other indexes would have
been cheaper?
regards, tom lane