Hi,
While investigating a CTE-related query, I've noticed that we don't
really remove all unreachable CTEs. For example, for this query
with a as (select 1), b as (select * from a), c as (select * from b) select 2;
where none of the CTEs if (directly or indirectly) referenced from the
query, we get a plan like this:
QUERY PLAN
---------------------------------------------------------Result (cost=0.03..0.04 rows=1 width=4) CTE a -> Result
(cost=0.00..0.01rows=1 width=4) CTE b -> CTE Scan on a (cost=0.00..0.02 rows=1 width=4)
(5 rows)
So we only remove the top-level CTE, but we fail to remove the other
CTEs because we don't tweak the refcount in SS_process_ctes().
Of course, it's harmless as none of those CTEs gets actually executed,
but is this intentional, or do we want/need to fix it? I don't see
anything about this in the docs, but it seems a bit awkward and
confusing to remove only some of the CTEs - I think we should either
remove all or none of them.
I don't think that should be particularly difficult - ISTM we need to
make SS_process_ctes a bit smarter, essentially by adding a loop to
remove the CTEs recursively (and decrease the refcount).
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services