Re: Slow Bitmap Index Scan

On 11/28/18, 2:18 PM, "Justin Pryzby" <pryzby@telsasoft.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’sAWS (same RAM, CPU).  After the move, we’re finding that certain queries that we run against a GIN full-text
indexhave some occasionally very slow executions and I’m struggling to figure out what to do about it.   I would be
verygrateful 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
thereplica, 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=100loops=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..20.604rows=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..40.682rows=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
ison for these instances.  I would hope that Amazon's already done that...
 

    Justin



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