| From: | Justin Pryzby <pryzby(at)telsasoft(dot)com> |
|---|---|
| To: | Scott Rankin <srankin(at)motus(dot)com> |
| Cc: | "pgsql-performance(at)lists(dot)postgresql(dot)org" <pgsql-performance(at)lists(dot)postgresql(dot)org>, Eric Meaney <emeaney(at)motus(dot)com> |
| Subject: | Re: Slow Bitmap Index Scan |
| Date: | 2018-11-28 19:17:53 |
| Message-ID: | 20181128191753.GM30707@telsasoft.com |
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| Lists: | pgsql-performance |
On Wed, Nov 28, 2018 at 07:08:53PM +0000, Scott Rankin wrote:
> We recently moved our production database systems from a 9.4 running on a self-managed EC2 instance to 9.6.10 on Amazon’s AWS (same RAM, CPU). After the move, we’re finding that certain queries that we run against a GIN full-text index have some occasionally very slow executions and I’m struggling to figure out what to do about it. I would be very grateful for any ideas!
>
> The setup we have is a 32-core, 244 GB RAM primary with a same-sized read replica. The queries are running off the replica, but performance is roughly the same between the master and the replica.
>
> Here’s a query that’s performing badly:
Can you compare or show the explain(analyze,buffers) for a fast query instance
vs slow query instance ? Is it slower due to index access or heap? Due to
cache misses ?
Also, you have big ram - have you tried disabling KSM or THP ?
https://www.postgresql.org/message-id/20170718180152.GE17566%40telsasoft.com
Justin
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