Re: Postgres not using indexes

I force postgresql to use indexes instead of sequential scans by setting
enable_seqscan = off in postgresql.conf and it helps in a lot of cases.
Probably not the best practice, but it does improve a lot of the queries
we will execute on a regular basis. It forces the planner to prefer
indexes. I've also noticed that limit behavior which is sort of puzzling
to me.

From: pgsql-bugs-owner(at)postgresql(dot)org
[mailto:pgsql-bugs-owner(at)postgresql(dot)org] On Behalf Of Lawrence Cohan
Sent: Wednesday, March 30, 2011 10:01 AM
To: pgsql-bugs(at)postgresql(dot)org
Subject: [BUGS] Postgres not using indexes

We have a huge performance issues in Postgres that surfaced due to
existing indexes not being used like in the example below in both 8.35
and 9.0 versions.

Client_Orders table with and int ID as PK which is the order_id and
indexed - about 155,000 rows

Order_Items table with and int ID primary key and INDEX on Order_id
(int) matching the ID in the above client_orders table. - about 33
million rows

A query like below takes almost ten minutes to complete however the
result set is 33768 rows and Total query runtime: 427539 ms.!!! This is
due to the fact that the index on Order_Items it is NOT used and a
sequence scan is done instead but this is obviously not acceptable from
performance point of view. If I add a LIMIT 1000 for instance then the
index is used and query returns results in no time as expected but as
soon as I go higher in the limit to a few thousands then the index on
Order_Items.Order_id is no longer used - why??? Is there any way to
force Postgres to use the existing indexes instead of table seq scan
which is deadly?

select oi.id from order_items oi INNER JOIN client_orders co ON
oi.order_id = co.id

Regards,

Nenea Nelu.

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