From: | Richard Huxton <dev(at)archonet(dot)com> |
---|---|
To: | Igor Lobanov <ilobanov(at)swsoft(dot)com> |
Cc: | pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Querying distinct values from a large table |
Date: | 2007-01-30 09:12:51 |
Message-ID: | 45BF0C13.3070003@archonet.com |
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Thread: | |
Lists: | pgsql-performance |
Igor Lobanov wrote:
> Greetings!
>
> I have rather large table with about 5 millions of rows and a dozen of
> columns. Let's suppose that columns are named 'a', 'b', 'c' etc. I need
> to query distinct pairs of ('a';'b') from this table.
> Creating compound index on this table using following statement:
>
> CREATE INDEX tbl_a_b_idx ON tbl( a, b );
>
> gives no effect, postgres simply ignores it, at least according to the
> EXPLAIN output.
What version of PostgreSQL is it?
How many distinct values are you getting back from your 5 million rows?
If there are too many, an index isn't going to help.
Can you share the EXPLAIN ANALYSE output? You might want to try
increasing work_mem for this one query to speed any sorting.
How often is the table updated? Clustering might buy you some
improvements (but not a huge amount I suspect).
Richard Huxton
Archonet Ltd
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