Martijn van Oosterhout wrote:
On Tue, Aug 08, 2006 at 01:55:35PM +0200, Florian G. Pflug wrote:
Hi
Since the discussion about how to force a specific plan has
come up, I though I'd post an idea I had for this a while ago.
It's not reall well though out yet, but anyway.
<snip>
Image a query like "select ... from t1 join t2 on t1.t2_id = t2.id and
<expr>". Lets say that "<expr>" is true for only 1% of the rows in t2 -
but those are exactly the rows that have matching rows in t1.
Postgres would probably guess that this join will produce about 1/100
of the rows that t1 has - but I _know_ that it will produce 100 (!)
times more rows.
ISTM theat the easiest way would be to introduce a sort of predicate
like so:
SELECT * FROM foo, bar WHERE pg_selectivity(foo.a = bar.a, 0.1);
Ideally, though it needs to be defined upon the table(s) in question,
possibly with a WHERE clause as with indexes:
CREATE STATISTIC <...defn here...>
ON invoices (cli_id), clients (id)
WHERE invoices.paid = false
WITH PRIORITY 100;
(I'm thinking the priority so you can delete any rules with a low
priority while keeping ones you think are vital)
--
Richard Huxton
Archonet Ltd
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