> The members table contains about 500k rows.  It has an index on
> (group_id, member_id) and on (member_id, group_id).

Yes, bad stats are causing it to pick a poor plan, but you're giving it
too many options (which doesn't help) and using space up unnecessarily.

Keep (group_id, member_id)
Remove (member_id, group_id)
Add (member_id)

An index on just member_id is actually going to perform better than
member_id, group_id since it has a smaller footprint on the disk.

Anytime where both group_id and member_id are in the query, the
(group_id, member_id) index will likely be used.

Rod Taylor <rbt [at] rbt [dot] ca>

Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL
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