Re: Query not using Index

Wei Shan <weishan(dot)ang(at)gmail(dot)com> wrote:

> Hi all,
>
> Please provide some advise on the following query not using the index:
> I have 2 questions:
>
> 1. Why does the optimizer chose not to use the index when it will run faster?

because of the estimated costs.:

Seq Scan on testdb auditrecor0_  (cost=0.00..18147465.00
Bitmap Heap Scan on testdb auditrecor0_  (cost=2291521.32..19046381.97

The estimated costs for the index-scan are higher.

> 2. How do I ensure the optimizer will use the index without setting
> enable_seqscan='off'

You have a dedicated tablespace for indexes, is this a SSD? You can try
to reduce the random_page_cost, from default 4 to maybe 2.(depends on
hardware) This would reduce the estimated costs for the Index-scan and
prefer the index-scan.

Regards, Andreas Kretschmer
--
Andreas Kretschmer
http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services