Re: What type of index do I need for this JOIN?

On 08/14/2013 06:05 AM, Robert James wrote:
> I'm doing a JOIN which is very slow:
>
> JOIN t ON t.f1 LIKE (q.f1 || '%')
>
> t1 has an INDEX on (f1, f2) which I thought would help for this. But
> Postgres seems to still use a (very slow) Nested Loop. What type of
> index would be appropriate for this?

You'll need a text_pattern_ops index.

http://www.postgresql.org/docs/current/static/indexes-opclass.html

http://www.postgresql.org/docs/current/static/indexes-types.html

"The optimizer can also use a B-tree index for queries involving the
pattern matching operators LIKE and ~ if the pattern is a constant and
is anchored to the beginning of the string — for example, col LIKE
'foo%' or col ~ '^foo', but not col LIKE '%bar'. However, if your
database does not use the C locale you will need to create the index
with a special operator class to support indexing of pattern-matching
queries; see Section 11.9 below."

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services