Thread: redundent index?
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
one if i were to delete the second one. its a heavily updated table, so
axing the second one would be a bonus for performance, am i missing
something? Thanks in advance,
Robert Treat
--
Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL
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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On Wed, 29 Oct 2003 10:17:24 -0500, Rod Taylor <rbt@rbt.ca> wrote: >On Wed, 2003-10-29 at 09:03, Robert Treat wrote: >> 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 Yes, the actual cost may be a little higher. But the cost estimation might be significantly higher, so there can be border cases where the planner chooses a sequential scan over a multi-column index scan while a single-column index would correctly be recognized as being faster ... Servus Manfred