Re: Slow Bitmap Index Scan - Mailing list pgsql-performance

Upon further analysis, this is - unsurprisingly - taking place when we have multiple prefixed search terms in a
ts_querygoing against a tsvector index.
 

We have roughly 30 million rows in the table, and the search column is basically a concatenation of a location's name
(think"Walmart #123456") and its street address.
 

We use these searches mostly for autocompleting of a location search.  So the search for that record above might be
"Walmart123", which we change to be to_tsquery('walmart:* &123:*').  We prefix both terms to correct for misspellings
orlazy typing.
 

Is it unrealistic to think that we could have sub-1000ms searches against that size of a table?

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 ?

    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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