Hi there,
I'm running a simple query with 2 inner joins (say A, B and C). Each of
the join columns has indexes. If I run queries that join just A and B,
or just B and C, postgres uses indexes. But if I run "A join B join C"
then the "B join C" part starts using a sequential scan and I can't
figure out why.
Here's the query, which basically retrieves all meta-data for all
messages in a given forum. The relationship is pretty simple. Forums
contain threads, which contain messages, which each have associated
meta-data:
SELECT message.message_id, message_meta_data.value
FROM thread
JOIN message USING (thread_id)
JOIN message_meta_data ON (message.message_id=message_meta_data.message_id)
WHERE thread.forum_id=123;
Explaining:
Hash Join (cost=337.93..1267.54 rows=180 width=35)
Hash Cond: ("outer".message_id = "inner".message_id)
-> Seq Scan on message_meta_data (cost=0.00..739.19 rows=37719 width=30)
-> Hash (cost=337.79..337.79 rows=57 width=13)
-> Nested Loop (cost=0.00..337.79 rows=57 width=13)
->
Index Scan using thread_forum_id_idx on thread (cost=0.00..41.61
rows=13 width=4)
Index Cond: (forum_id = 6)
->
Index Scan using message_thread_id_idx on message (cost=0.00..22.72
rows=5 width=17)
Index Cond: ("outer".thread_id = message.thread_id)
As you can see, the message and message_meta_data tables use a Seq
Scan. The only way I can think of forcing it to use the Index Scan in
all cases would be to use two separate nested queries: The outer query
would retrieve the list of messages in the forum, and the inner query
would retrieve the list of metadata for an individual message.
Obviously I want to avoid having to do that if possible.
Any ideas?
Many thanks if you can help.
James
