Re: Query performance issue

Jonathan Gray wrote:
> Chris,
>
> Creating indexes on the customerclass table does speed up the queries but
> still does not create the plan we are looking for (using the double index
> with a backward index scan on the orders table).

Stupid question - why is that particular plan your "goal" plan?

> The plans we now get, with times on par or slightly better than with the
> plpgsql hack, are:
>
> EXPLAIN ANALYZE
> SELECT o.orderid,o.orderstamp FROM indextest.orders o
> INNER JOIN indextest.customerclass cc ON (cc.classid = 2)
> WHERE o.customerid = cc.customerid ORDER BY o.orderstamp DESC LIMIT 5;

Didn't notice this before...

Shouldn't this be:

INNER JOIN indextest.customerclass cc ON (o.customerid = cc.customerid)
WHERE cc.classid = 2

ie join on the common field not the classid one which doesn't appear in
the 2nd table?

> As I said, this is a hypothetical test case we have arrived at that
> describes our situation as best as we can given a simple case. We're
> interested in potential issues with the approach, why postgres would not
> attempt something like it, and how we might go about implementing it
> ourselves at a lower level than we currently have (in SPI, libpq, etc).
>
> If it could be generalized then we could use it in cases where we aren't
> pulling from just one table (the orders table) but rather trying to merge,
> in sorted order, results from different conditions on different tables.
> Right now we use something like the plpgsql or plpythonu functions in the
> example and they outperform our regular SQL queries by a fairly significant
> margin.

I'm sure if you posted the queries you are running with relevant info
you'd get some help ;)

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