Re: [SQL] Nested Views take forever

"Zot O'Connor" <zot(at)zotconsulting(dot)com> writes:
> consup=> EXPLAIN SELECT COUNT(*) from depth3;
> NOTICE: QUERY PLAN:
> Aggregate (cost=11.69 rows=233 width=4)
> -> Seq Scan on subcat (cost=11.69 rows=233 width=4)
> SubPlan
> -> Seq Scan on subcat (cost=11.69 rows=233 width=4)
> SubPlan
> -> Seq Scan on subcat (cost=11.69 rows=6 width=4)

The problem here is not views per se, it's that WHERE x in (sub-select)
is not a very efficient construct --- it basically always generates a
nested-loop plan. What you've got above is O(N^3) for an N-tuple table.

Try something like this instead:

CREATE VIEW depth2 AS SELECT ... FROM subcat, depth1 WHERE
subcat.scatscat = depth1.scatval;

CREATE VIEW depth3 AS SELECT ... FROM subcat, depth2 WHERE
subcat.scatscat = depth2.scatval;

Given indexes on scatscat and scatval, I'd expect this to produce a
merge-join plan, which should be reasonably quick --- better than
O(N^2) or O(N^3) anyway.

There's been some talk of reimplementing WHERE ... IN ... so that it
does something intelligent without help, but there are a lot of
higher-priority problems on the TODO list...

regards, tom lane