From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Mike Christensen <mike(at)kitchenpc(dot)com> |
Cc: | "pgsql-general(at)postgresql(dot)org" <pgsql-general(at)postgresql(dot)org> |
Subject: | Re: Indexing queries with bit masks |
Date: | 2010-04-30 15:08:08 |
Message-ID: | 24924.1272640088@sss.pgh.pa.us |
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Lists: | pgsql-general |
Mike Christensen <mike(at)kitchenpc(dot)com> writes:
> When a certain event happens, let's say event 4, I need to query for which
> users to notify. So I'll be doing something like:
> SELECT UserId FROM Users WHERE Subscriptions & 8;
> My question is say there's a million rows in the Users table. If I have an
> index on Subscriptions, will this index be used in the above query?
No. At least not with a standard btree index.
I'm not exactly sure that an index would be helpful at all --- it seems
like the selectivity of this condition won't be very good anyway, will
it? The more popular notifications will be subscribed to by a large
fraction of the user base. Maybe it'd be useful to index unpopular
notifications, but how often will you be searching for those?
regards, tom lane
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