Thread: SELECT LIKE 'xxx%' doesn't use index scan

db =# explain SELECT seq FROM tab WHERE str like 'edt%';
QUERY PLAN
-----------------------------------------------------------
Seq Scan on tab (cost=0.00..6465.55 rows=1 width=4)
Filter: (str ~~ 'edt%'::text)
(2 rows)

db=# explain SELECT seq FROM tab WHERE str = 'edta';
QUERY PLAN
------------------------------------------------------------------------------
Index Scan using tab_pkey on tab (cost=0.00..8.30 rows=1 width=4)
Index Cond: (str = 'edta'::text)
(2 rows)



PostgreSQL 8.2.5 on i586-mandriva-linux-gnu, compiled by GCC
i586-mandriva-linux-gnu-gcc (GCC) 4.2.2 20070909 (prerelease)
(4.2.2-0.RC.1mdv2008.0)


Thank you.


Edward Liu

Hello

It's not bug. a LIKE 'some%' is different operation than a = str_const.

if you don't use C locale, then you have to use  varchar_pattern_ops.

please try:

  create index like_index on tab(str varchar_pattern_ops);

don't forget do ANALYZE tab before

Regards
Pavel Stehule

On 19/12/2007, dhliu <dh.liu@msa.hinet.net> wrote:
> db =# explain SELECT seq FROM tab WHERE str like 'edt%';
> QUERY PLAN
> -----------------------------------------------------------
> Seq Scan on tab (cost=0.00..6465.55 rows=1 width=4)
> Filter: (str ~~ 'edt%'::text)
> (2 rows)
>
> db=# explain SELECT seq FROM tab WHERE str = 'edta';
> QUERY PLAN
> ------------------------------------------------------------------------------
> Index Scan using tab_pkey on tab (cost=0.00..8.30 rows=1 width=4)
> Index Cond: (str = 'edta'::text)
> (2 rows)
>
>
>
> PostgreSQL 8.2.5 on i586-mandriva-linux-gnu, compiled by GCC
> i586-mandriva-linux-gnu-gcc (GCC) 4.2.2 20070909 (prerelease)
> (4.2.2-0.RC.1mdv2008.0)
>
>
> Thank you.
>
>
> Edward Liu
>
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