Fix slow Patch counting query

Fix slow Patch counting query

Stephen Rothwell noticed (way back in September - sorry Stephen!) that
the following query is really slow on OzLabs:

SELECT COUNT(*) AS "__count" FROM "patchwork_patch"
    INNER JOIN "patchwork_submission" ON
        ("patchwork_patch"."submission_ptr_id" = "patchwork_submission"."id")
    WHERE ("patchwork_submission"."project_id" = 14 AND
           "patchwork_patch"."state_id" IN
	       (SELECT U0."id" AS Col1 FROM "patchwork_state" U0
                WHERE U0."action_required" = true
		ORDER BY U0."ordering" ASC));

I think this is really slow because we have to join the patch and
submission table to get the project id, which we need to filter the
patches.

Duplicate the project id in the patch table itself, which allows us to
avoid the JOIN.

The new query reads as:
SELECT COUNT(*) AS "__count" FROM "patchwork_patch"
    WHERE ("patchwork_patch"."patch_project_id" = 1 AND
           "patchwork_patch"."state_id" IN
	       (SELECT U0."id" AS Col1 FROM "patchwork_state" U0
	        WHERE U0."action_required" = true
		ORDER BY U0."ordering" ASC));

Very simple testing on a small, artifical Postgres instance (3
projects, 102711 patches), shows speed gains of ~1.5-5x for this
query. Looking at Postgres' cost estimates (EXPLAIN) of the first
query vs the second query, we see a ~1.75x improvement there too.

I suspect the gains will be bigger on OzLabs.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Daniel Axtens <dja@axtens.net>