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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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
