Re: Index Skip Scan

On Fri, Jun 21, 2019 at 1:20 AM Jesper Pedersen
<jesper(dot)pedersen(at)redhat(dot)com> wrote:
> Attached is v20, since the last patch should have been v19.

I took this for a quick spin today. The DISTINCT ON support is nice
and I think it will be very useful. I've signed up to review it and
will have more to say later. But today I had a couple of thoughts
after looking into how src/backend/optimizer/plan/planagg.c works and
wondering how to do some more skipping tricks with the existing
machinery.

1. SELECT COUNT(DISTINCT i) FROM t could benefit from this. (Or
AVG(DISTINCT ...) or any other aggregate). Right now you get a seq
scan, with the sort/unique logic inside the Aggregate node. If you
write SELECT COUNT(*) FROM (SELECT DISTINCT i FROM t) ss then you get
a skip scan that is much faster in good cases. I suppose you could
have a process_distinct_aggregates() in planagg.c that recognises
queries of the right form and generates extra paths a bit like
build_minmax_path() does. I think it's probably better to consider
that in the grouping planner proper instead. I'm not sure.

2. SELECT i, MIN(j) FROM t GROUP BY i could benefit from this if
you're allowed to go forwards. Same for SELECT i, MAX(j) FROM t GROUP
BY i if you're allowed to go backwards. Those queries are equivalent
to SELECT DISTINCT ON (i) i, j FROM t ORDER BY i [DESC], j [DESC]
(though as Floris noted, the backwards version gives the wrong answers
with v20). That does seem like a much more specific thing applicable
only to MIN and MAX, and I think preprocess_minmax_aggregates() could
be taught to handle that sort of query, building an index only scan
path with skip scan in build_minmax_path().

--
Thomas Munro
https://enterprisedb.com