Re: Slow Bitmap Index Scan

On 11/28/18, 2:18 PM, "Justin Pryzby" <pryzby(at)telsasoft(dot)com> wrote:

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 ?

If I reduce the number of search terms in , I get this:

SELECT ls.location AS locationId FROM location_search ls WHERE ls.client = 83 AND search_field_tsvector @@ to_tsquery('9000:*'::text) AND ls.favorite = TRUE LIMIT 100

Limit (cost=13203.99..13627.40 rows=100 width=8) (actual time=66.568..66.759 rows=100 loops=1)
Buffers: shared hit=1975
-> Bitmap Heap Scan on location_search ls (cost=13203.99..13923.79 rows=170 width=8) (actual time=66.568..66.729 rows=100 loops=1)
Recheck Cond: ((search_field_tsvector @@ to_tsquery('9000:*'::text)) AND (client = 83))
Filter: favorite
Heap Blocks: exact=86
Buffers: shared hit=1975
-> BitmapAnd (cost=13203.99..13203.99 rows=170 width=0) (actual time=66.471..66.472 rows=0 loops=1)
Buffers: shared hit=1889
-> Bitmap Index Scan on location_search_tsvector_idx (cost=0.00..2235.02 rows=11570 width=0) (actual time=20.603..20.604 rows=29155 loops=1)
Index Cond: (search_field_tsvector @@ to_tsquery('9000:*'::text))
Buffers: shared hit=546
-> Bitmap Index Scan on location_search_client_idx (cost=0.00..10968.63 rows=442676 width=0) (actual time=40.682..40.682 rows=482415 loops=1)
Index Cond: (client = 83)
Buffers: shared hit=1343
Planning time: 0.181 ms
Execution time: 66.806 ms

I see almost no IO reads, and pg_stat_statements shows no cache misses.

Also, you have big ram - have you tried disabling KSM or THP ?
https://www.postgresql.org/message-id/20170718180152.GE17566%40telsasoft.com

Since this is Amazon RDS, we don't have any control over or access to the underlying OS. I know that huge_page support is on for these instances. I would hope that Amazon's already done that...

Justin

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