PG Bug reporting form <noreply@postgresql.org> writes:
> Planner does not pick unique btree index and goes for seq scan but unsafe
> hash index works.
I couldn't reproduce this from the given instructions.
Looking at the code, it looks like it might be possible to explain it
with a combination of (1) old_snapshot_threshold being enabled (not -1),
and (2) something holding back global xmin, such as a long-running
transaction. (Maybe you have an uncommitted prepared transaction?)
In that situation a newly-created index won't be used until all older
transactions have gone away. But the existence of an unlogged index
(hash index) disables the snapshot threshold feature for the associated
table.
There may be some additional condition needed to cause it, because
I still couldn't reproduce the behavior with those two conditions
set up.
IMO old_snapshot_threshold is a complete kluge and you should not
have it turned on unless you desperately need it. It has a lot of
poorly-documented drawbacks, including this one.
regards, tom lane