Re: partial index on varchar-coloumn in 7.4.1

Hi,

Ah. Thanks. I wouldn't have thought of that. That's a solution I can
work with (if not a very elegant one).

Regard,
Martin.

Am 07.01.2004 um 16:29 schrieb Tom Lane:

> Martin Hampl <Martin(dot)Hampl(at)gmx(dot)de> writes:
>> Do partial indexes not work for varchar?
>
> Works for me:
>
> regression=# create table token(word varchar(30));
> CREATE TABLE
> regression=# CREATE INDEX word_idx on token (word) where not (word =
> 'the');
> CREATE INDEX
> regression=# explain select * from token where word = 'abc' and not
> (word = 'the');
> QUERY PLAN
> -----------------------------------------------------------------------
> -
> Index Scan using word_idx on token (cost=0.00..17.02 rows=5 width=33)
> Index Cond: ((word)::text = 'abc'::text)
> Filter: ((word)::text <> 'the'::text)
> (3 rows)
>
> You may have unrealistic expectations about the planner's ability to
> prove that the index predicate condition is implied by the query
> WHERE clause. This will not use the index:
>
> regression=# explain select * from token where word = 'abc';
> QUERY PLAN
> -------------------------------------------------------
> Seq Scan on token (cost=0.00..22.50 rows=5 width=33)
> Filter: ((word)::text = 'abc'::text)
> (2 rows)
>
> You know and I know that "word = 'abc'" implies "not (word = 'the')",
> but the planner cannot make that deduction. The pred_test() routine
> doesn't really have any intelligence about conditions involving NOT.
>
> regards, tom lane
>