Re: Index on VARCHAR with text_pattern_ops inside PL/PGSQL procedure.

Hello

this is known problem of prepared statements. Prepared statement has
plan built without knowledge any values and should not be optimal.

try use dynamic query and statement EXECUTE INTO

Regards
Pavel Stehule



On 10/12/2007, Piotr Gasidło <quaker@barbara.eu.org> wrote:
> Hello,
>
> I've created table:
>
> quaker=> \d users
>                                  Table "public.users"
>    Column   |       Type        |                     Modifiers
>
> -----------+-------------------+----------------------------------------------------
>   id        | integer           | not null default
> nextval('users_id_seq'::regclass)
>   user_name | character varying | not null
>   extra     | integer           |
> Indexes:
>      "users_pkey" PRIMARY KEY, btree (id)
>      "users_user_name_unique_text_ops" UNIQUE, btree (user_name text_ops)
>      "users_user_name_unique_text_pattern_ops" btree (user_name
> text_pattern_ops)
>
> Filled with random data (100k records).
>
> I do simple queries using above indexes (asking for existing record).
>
> explain analyze select id from users where user_name = 'quaker';
>                                                                QUERY
> PLAN
>
---------------------------------------------------------------------------------------------------------------------------------------
>   Index Scan using users_user_name_unique_text_ops on users
> (cost=0.00..8.28 rows=1 width=4) (actual time=0.040..0.043 rows=1 loops=1)
>     Index Cond: ((user_name)::text = 'quaker'::text)
>   Total runtime: 0.084 ms
> (3 rows)
>
> explain analyze select id from users where user_name like 'quaker';
>                                                                    QUERY
> PLAN
>
-----------------------------------------------------------------------------------------------------------------------------------------------
>   Index Scan using users_user_name_unique_text_pattern_ops on users
> (cost=0.00..8.28 rows=1 width=4) (actual time=0.022..0.024 rows=1 loops=1)
>     Index Cond: ((user_name)::text ~=~ 'quaker'::text)
>     Filter: ((user_name)::text ~~ 'quaker'::text)
>   Total runtime: 0.050 ms
> (4 rows)
>
> Everything looks fine.
>
> Now, I've created PL/PGSQL function:
>
> create or replace function user_login(
>    _v_user_name varchar
> ) returns integer as $$
> declare
>    _i_user_id integer;
> begin
>    select id into _i_user_id from users where user_name = _v_user_name
> limit 1;
>    if found then
>      return _i_user_id;
>    end if;
>    return -1;
> end;
> $$ language plpgsql security definer;
>
> As shown above, I use "=" operator, which should use
> users_user_name_unique_text_ops index:
>
> explain analyze select user_login('quaker');
>                                       QUERY PLAN
>
> ------------------------------------------------------------------------------------
>   Result  (cost=0.00..0.01 rows=1 width=0) (actual time=0.320..0.322
> rows=1 loops=1)
>   Total runtime: 0.340 ms
> (2 rows)
>
>
> Some performance loss, but OK. Now I've changed "=" into "LIKE" to use
> users_user_name_unique_text_pattern_ops index and rerun query:
>
> explain analyze select user_login('quaker');
>
>                                        QUERY PLAN
>
> --------------------------------------------------------------------------------------
>   Result  (cost=0.00..0.01 rows=1 width=0) (actual time=41.606..41.608
> rows=1 loops=1)
>   Total runtime: 41.629 ms
> (2 rows)
>
> Second run give 61.061 ms. So no improvements.
>
> Why PL/PGSQL is unable to proper utilize
> users_user_name_unique_text_pattern_ops?
>
> quaker=> select version();
>                                                         version
>
>
----------------------------------------------------------------------------------------------------------------------
>   PostgreSQL 8.2.5 on i486-pc-linux-gnu, compiled by GCC cc (GCC) 4.1.3
> 20070831 (prerelease) (Ubuntu 4.1.2-16ubuntu1)
> (1 row)
>
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