Re: index usage in not like

From: A. Kretschmer
Subject: Re: index usage in not like
Date: ,
Msg-id: 20100218121810.GG23676@a-kretschmer.de
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In response to: Re: index usage in not like  (Thom Brown)
Responses: Re: index usage in not like  (Kenneth Marshall)
Re: index usage in not like  (Thom Brown)
List: pgsql-performance

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index usage in not like  (AI Rumman, )
 Re: index usage in not like  (Thom Brown, )
  Re: index usage in not like  ("A. Kretschmer", )
   Re: index usage in not like  (Kenneth Marshall, )
    Re: index usage in not like  ("A. Kretschmer", )
   Re: index usage in not like  (Thom Brown, )
 Re: index usage in not like  (Thom Brown, )

In response to Thom Brown :
> On 18 February 2010 11:55, AI Rumman <> wrote:
> > "Not like" operation does not use index.
> >
> > select * from vtiger_contactscf where lower(cf_1253) not like
> > lower('Former%')
> >
> > I created index on lower(cf_1253).
> >
> > How can I ensure index usage in not like operation?
> > Anyone please help.
> >
>
> How many rows do you have in your table?  If there are relatively few,
> it probably guesses it to be cheaper to do a sequential scan and
> calculate lower values on-the-fly rather than bother with the index.

That's one reason, an other reason, i think, is, that a btree-index can't
search with an 'not like' - operator.



test=*# insert into words select 'fucking example' from generate_series(1,10000);
INSERT 0 10000
test=*# insert into words select 'abc' from generate_series(1,10);
INSERT 0 10
test=*# explain select * from words where lower(w)  like lower('a%') or lower(w)  like lower('b%');
                                     QUERY PLAN
-------------------------------------------------------------------------------------
 Bitmap Heap Scan on words  (cost=1538.75..6933.39 rows=55643 width=36)
   Recheck Cond: ((lower(w) ~~ 'a%'::text) OR (lower(w) ~~ 'b%'::text))
   Filter: ((lower(w) ~~ 'a%'::text) OR (lower(w) ~~ 'b%'::text))
   ->  BitmapOr  (cost=1538.75..1538.75 rows=57432 width=0)
         ->  Bitmap Index Scan on idx_words  (cost=0.00..1027.04 rows=39073 width=0)
               Index Cond: ((lower(w) ~>=~ 'a'::text) AND (lower(w) ~<~ 'b'::text))
         ->  Bitmap Index Scan on idx_words  (cost=0.00..483.90 rows=18359 width=0)
               Index Cond: ((lower(w) ~>=~ 'b'::text) AND (lower(w) ~<~ 'c'::text))
(8 rows)

test=*# explain select * from words where lower(w) not like lower('a%') or lower(w)  like lower('b%');
                            QUERY PLAN
-------------------------------------------------------------------
 Seq Scan on words  (cost=0.00..10624.48 rows=282609 width=36)
   Filter: ((lower(w) !~~ 'a%'::text) OR (lower(w) ~~ 'b%'::text))
(2 rows)


In other words: revert your where-condition from 'not like' to multiple 'like' conditions for all letters except 'f%'.


Andreas
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Andreas Kretschmer
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