Re: index duplicates primary key, but is used more?

jonathan vanasco <postgres(at)2xlp(dot)com> writes:
> What I noticed when checking stats earlier, is that although `idx_test_foo_id_asc` is the same as the PKEY... it was used about 10x more than the pkey.

> Does anyone know of this is just random (perhaps due to the name being sorted earlier) or there is some other reason that index would be selected ?

It's almost certainly just an artifact. The planner considers a table's
indexes in OID order. I don't recall offhand whether it would keep the
first or last of a series of identical-cost plans, but it'd be one or the
other of those behaviors; it would not continue to consider both indexes
once it noticed the plans were the same.

One thing that could favor a newer index is that it probably has somewhat
less bloat in it, resulting in a fractionally smaller cost estimate. This
doesn't make it better in any absolute sense; reindexing the older index
would reverse that preference, at least for a time.

regards, tom lane