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From: Jeff Janes <jeff.janes@gmail.com>
Date: Thu, 9 Jun 2016 09:37:50 -0700
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Subject: Re: Performance of LIKE/NOT LIKE when used in single query
To: Ed Felstein <efelstein@gmail.com>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
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On Tue, Jun 7, 2016 at 9:57 PM, Ed Felstein <efelstein@gmail.com> wrote:
> Hello,
> First time poster here.  Bear with me.
> Using PostgreSQL 9.5
> I have a situation where I have a LIKE and a NOT LIKE in the same query to
> identify strings in a varchar field.  Since I am using wildcards, I have
> created a GIN index on the field in question, which makes LIKE '%xxxx%'
> searches run very fast.  The problem is the NOT LIKE phrases, which (as
> would be expected) force a sequential scan.  Being that we're talking about
> millions of records, this is not desirable.
> Here's the question...
> Is there a way, using a single query, to emulate the process of running the
> LIKE part first, then running the NOT LIKE just on those results?

Just do it.  In my hands, the planner is smart enough to figure it out
for itself.

explain analyze select * from stuff where synonym like '%BAT%' and
synonym not like '%col not like%' ;

                                                               QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on stuff  (cost=16.10..63.08 rows=13 width=14)
(actual time=9.465..10.642 rows=23 loops=1)
   Recheck Cond: (synonym ~~ '%BAT%'::text)
   Rows Removed by Index Recheck: 76
   Filter: (synonym !~~ '%col not like%'::text)
   Heap Blocks: exact=57
   ->  Bitmap Index Scan on integrity_synonym_synonym_idx
(cost=0.00..16.10 rows=13 width=0) (actual time=8.847..8.847 rows=99
loops=1)
         Index Cond: (synonym ~~ '%BAT%'::text)
 Planning time: 18.261 ms
 Execution time: 10.932 ms


So it is using the index for the positive match, and filtering those
results for the negative match, just as you wanted.

Cheers,

Jeff


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