Re: Questions about indexes with text_pattern_ops

"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Gregory Stark <stark(at)enterprisedb(dot)com> writes:
>> Hm, for a simple = or <> I think it doesn't matter which operator class you
>> use. For < or > it would produce different answers. Postgres isn't clever enough
>> to notice that this is equivalent though so I think you would have to do
>> something like (untested):
>
>> CREATE INDEX new_index ON a (b text_pattern_ops) WHERE b ~<>~ '';
>
>> That uses the same operator that the LIKE clause will use for the index range.
>
> I'm intending to get rid of ~=~ and ~<>~ for 8.4; there's no longer any
> reason why those slots in the pattern_ops classes can't be filled by the
> plain = and <> operators. (There *was* a reason when they were first
> invented --- but now that texteq will only return true for exact bitwise
> match, I think it's OK to assume these are equivalent.)

The only question is whether we'll keep that forever. I thought it was a good
idea at the time but I'm starting to wonder about the implications for
multi-key indexes.

> In the meantime, though, I think the only way that Kaare's query can use
> that index is if he writes
> WHERE b LIKE 'whatever' AND b <> '';
> (with whatever spelling of <> the index predicate has). There is not
> anything in the predicate proving machinery that knows enough about LIKE
> to be able to show that "b LIKE 'whatever'" implies "b <> ''".

I was thinking that the inequalities that the LIKE index scan introduces would
imply the inequality. I take it we generate those inequalities too late in the
planning process to use them for other planning?

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's 24x7 Postgres support!