Richard Huxton wrote:
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.
>
What version of PostgreSQL is it?
8.1.4
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.
No more than 10,000.
Can you share the EXPLAIN ANALYSE output? You might want to try
increasing work_mem for this one query to speed any sorting.
Real table and colum names are obfuscated because of NDA, sorry.
explain analyze select distinct a, b from tbl
EXPLAIN ANALYZE output is:
Unique (cost=500327.32..525646.88 rows=1848 width=6) (actual
time=52719.868..56126.356 rows=5390 loops=1)
-> Sort (cost=500327.32..508767.17 rows=3375941 width=6) (actual
time=52719.865..54919.989 rows=3378864 loops=1)
Sort Key: a, b
-> Seq Scan on tbl (cost=0.00..101216.41 rows=3375941
width=6) (actual time=16.643..20652.610 rows=3378864 loops=1)
Total runtime: 57307.394 ms
How often is the table updated? Clustering might buy you some
improvements (but not a huge amount I suspect).
It is updated once per 3-5 seconds.
And one more thing. I don't know if it helps, but column 'a' can have
value from a limited set: 0, 1 or 2. Column 'b' is also an integer
(foreign key, actually).
--
Igor Lobanov
Internal Development Engineer
SWsoft, Inc.
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