Re: why is this index not being used?

You can always try and force it by doing

 "set enable_seqscan=0"

I'd try explain analyze on the query with it on (=1) and off (=0) and
see why the planner likes seqscan better.

Gavin

mike@linkify.com wrote:

>BTW -- We vacuum nightly, and running vacuum analyze doesn't make a
>difference.
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>>Hey all,
>>
>>I'm using PostgreSQL 7.3.4.
>>
>>I have a query that isn't using a particular index, and I'm wondering
>>why.
>>
>>The query is:
>>select i.ItemID, d.Extension from ITEM i, SHARING s, DOCUMENT d where
>>i.ItemID = d.ItemID AND s.ItemID = i.DomainID AND s.UserIDOfSharee = 12
>>
>>Item's primary key is ItemID.
>>Document's primary key is ItemID.
>>Sharing's primary key is (ItemID, UserIDOfSharee).
>>Item has index item_ix_item_3_idx on (DomainID, ItemID).
>>Sharing has index sharing_ix_sharing_1_idx on (UserIDOfSharee, ItemID).
>>
>>Explain says:
>>Hash Join  (cost=25526.26..31797.78 rows=6105 width=23)
>> Hash Cond: ("outer".itemid = "inner".itemid)
>> ->  Seq Scan on document d  (cost=0.00..5629.14 rows=113214 width=11)
>> ->  Hash  (cost=25502.60..25502.60 rows=9465 width=12)
>>       ->  Merge Join  (cost=310.16..25502.60 rows=9465 width=12)
>>             Merge Cond: ("outer".domainid = "inner".itemid)
>>             ->  Index Scan using item_ix_item_3_idx on item i
>>(cost=0.00..24634.71 rows=175519 width=8)
>>             ->  Sort  (cost=310.16..310.47 rows=123 width=4)
>>                   Sort Key: s.itemid
>>                   ->  Index Scan using sharing_ix_sharing_1_idx on
>>sharing s  (cost=0.00..305.88 rows=123 width=4)
>>                         Index Cond: (useridofsharee = 12)
>>
>>
>>Why is there a Seq Scan on Document?  How can I get it to use
>>Document's primary key?
>>
>>
>>Thanks!
>>Mike
>>
>>
>>
>>
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