Re: Index Skip Scan

On Fri, 14 Jun 2019 at 04:32, Jesper Pedersen
<jesper(dot)pedersen(at)redhat(dot)com> wrote:
> Here is a rebased version.

Hi Jesper,

I read over this thread a few weeks ago while travelling back from
PGCon. (I wish I'd read it on the outward trip instead since it would
have been good to talk about it in person.)

First off. I think this is a pretty great feature. It certainly seems
worthwhile working on it.

I've looked over the patch just to get a feel for how the planner part
works and I have a few ideas to share.

The code in create_distinct_paths() I think should work a different
way. I think it would be much better to add a new field to Path and
allow a path to know what keys it is distinct for. This sort of goes
back to an idea I thought about when developing unique joins
(9c7f5229ad) about an easier way to detect fields that a relation is
unique for. I've been calling these "UniqueKeys" in a few emails [1].
The idea was to tag these onto RelOptInfo to mention which columns or
exprs a relation is unique by so that we didn't continuously need to
look at unique indexes in all the places that call
relation_has_unique_index_for(). The idea there was that unique joins
would know when a join was unable to duplicate rows. If the outer side
of a join didn't duplicate the inner side, then the join RelOptInfo
could keep the UniqueKeys from the inner rel, and vice-versa. If both
didn't duplicate then the join rel would obtain the UniqueKeys from
both sides of the join. The idea here is that this would be a better
way to detect unique joins, and also when it came to the grouping
planner we'd also know if the distinct or group by should be a no-op.
DISTINCT could be skipped, and GROUP BY could do a group aggregate
without any sort.

I think these UniqueKeys ties into this work, perhaps not adding
UniqueKeys to RelOptInfo, but just to Path so that we create paths
that have UniqueKeys during create_index_paths() based on some
uniquekeys that are stored in PlannerInfo, similar to how we create
index paths in build_index_paths() by checking if the index
has_useful_pathkeys(). Doing it this way would open up more
opportunities to use skip scans. For example, semi-joins and
anti-joins could make use of them if the uniquekeys covered the entire
join condition. With this idea, the code you've added in
create_distinct_paths() can just search for the cheapest path that has
the correct uniquekeys, or if none exist then just do the normal
sort->unique or hash agg. I'm not entirely certain how we'd instruct
a semi/anti joined relation to build such paths, but that seems like a
problem that could be dealt with when someone does the work to allow
skip scans to be used for those.

Also, I'm not entirely sure that these UniqueKeys should make use of
PathKey since there's no real need to know about pk_opfamily,
pk_strategy, pk_nulls_first as those all just describe how the keys
are ordered. We just need to know if they're distinct or not. All
that's left after removing those fields is pk_eclass, so could
UniqueKeys just be a list of EquivalenceClass? or perhaps even a
Bitmapset with indexes into PlannerInfo->ec_classes (that might be
premature for not since we've not yet got
https://commitfest.postgresql.org/23/1984/ or
https://commitfest.postgresql.org/23/2019/ ) However, if we did use
PathKey, that does allow us to quickly check if the UniqueKeys are
contained within the PathKeys, since pathkeys are canonical which
allows us just to compare their memory address to know if two are
equal, however, if you're storing eclasses we could probably get the
same just by comparing the address of the eclass to the pathkey's
pk_eclass.

Otherwise, I think how you're making use of paths in
create_distinct_paths() and create_skipscan_unique_path() kind of
contradicts how they're meant to be used.

I also agree with James that this should not be limited to Index Only
Scans. From testing the patch, the following seems pretty strange to
me:

# create table abc (a int, b int, c int);
CREATE TABLE
# insert into abc select a,b,1 from generate_Series(1,1000) a,
generate_Series(1,1000) b;
INSERT 0 1000000
# create index on abc(a,b);
CREATE INDEX
# explain analyze select distinct on (a) a,b from abc order by a,b; --
this is fast.
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Index Only Scan using abc_a_b_idx on abc (cost=0.42..85.00 rows=200
width=8) (actual time=0.260..20.518 rows=1000 loops=1)
Scan mode: Skip scan
Heap Fetches: 1000
Planning Time: 5.616 ms
Execution Time: 21.791 ms
(5 rows)

# explain analyze select distinct on (a) a,b,c from abc order by a,b;
-- Add one more column and it's slow.
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------
Unique (cost=0.42..50104.43 rows=200 width=12) (actual
time=1.201..555.280 rows=1000 loops=1)
-> Index Scan using abc_a_b_idx on abc (cost=0.42..47604.43
rows=1000000 width=12) (actual time=1.197..447.683 rows=1000000
loops=1)
Planning Time: 0.102 ms
Execution Time: 555.407 ms
(4 rows)

[1] https://www.postgresql.org/search/?m=1&q=uniquekeys&l=1&d=-1&s=r

--
David Rowley http://www.2ndQuadrant.com/
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