On Wed, 2003-10-29 at 09:03, Robert Treat wrote: > I just noticed on one of my tables I have the following two indexes: > > Indexes: entity_watch_map_pkey primary key btree (entity_id, watch_id), > ewm_entity_id btree (entity_id), > > > I can't think of why the second index is there, as ISTM there is no > instance where the first index wouldn't be used in place of the second
The cost in evaluating the first index will be a little higher (more data to pull off disk due to second item), so there may be a few borderline cases that could switch to a sequential scan rather than an index scan.
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