Hi, This idea first came from remove_useless_groupby_columns does not need to record constraint dependencie[0] which points out that unique index whose columns all have NOT NULL constraints could also take the work with primary key when removing useless GROUP BY columns. I study it and implement the idea.
Ex:
create temp table t2 (a int, b int, c int not null, primary key (a, b),
unique(c));
explain (costs off) select * from t2 group by a,b,c;
QUERY PLAN
----------------------
HashAggregate
Group Key: c
-> Seq Scan on t2
The plan drop column a, b as I did a little more.
For the query, as t2 has primary key (a, b), before this patch, we could drop
column c because {a, b} are PK.
And we have an unique index(c) with NOT NULL constraint now, we could drop
column {a, b}, just keep {c}.
While we have multiple choices, group by a, b (c is removed by PK) and group
by c (a, b are removed by unique not null index)
And I implement it to choose the one with less columns so that we can drop as
more columns as possible.
I think it’s good for planner to save some cost like Sort less columns.
There may be better one for some reason like: try to keep PK for planner?
I’m not sure about that and it seems not worth much complex.
The NOT NULL constraint may also be computed from primary keys, ex:
create temp table t2 (a int, b int, c int not null, primary key (a, b),
unique(a));
Primary key(a, b) ensure a is NOT NULL and we have a unique index(a), but it
will introduce more complex to check if a unique index could be used.
I also doubt it worths doing that..
So my patch make it easy: check unique index’s columns, it’s a valid candidate
if all of that have NOT NULL constraint.
And we choose a best one who has the least column numbers in
get_min_unique_not_null_attnos(), as the reason: less columns mean that more
group by columns could be removed.
create temp table t3 (a int, b int, c int not null, d int not null, primary key
(a, b), unique(c, d));
-- Test primary key beats unique not null index.
explain (costs off) select * from t3 group by a,b,c,d;
QUERY PLAN
----------------------
HashAggregate
Group Key: a, b
-> Seq Scan on t3
(3 rows)
create temp table t4 (a int, b int not null, c int not null, d int not null,
primary key (a, b), unique(b, c), unique(d));
-- Test unique not null index with less columns wins.
explain (costs off) select * from t4 group by a,b,c,d;
QUERY PLAN
----------------------
HashAggregate
Group Key: d
-> Seq Scan on t4
(3 rows)
The unique Indices could have overlaps with primary keys and indices themselves.
create temp table t5 (a int not null, b int not null, c int not null, d int not
null, unique (a, b), unique(b, c), unique(a, c, d));
-- Test unique not null indices have overlap.
explain (costs off) select * from t5 group by a,b,c,d;
QUERY PLAN
----------------------
HashAggregate
Group Key: a, b
-> Seq Scan on t5
(3 rows)
[0]https://www.postgresql.org/message-id/flat/CAApHDvrdYa%3DVhOoMe4ZZjZ-G4ALnD-xuAeUNCRTL%2BPYMVN8OnQ%40mail.gmail.com
Zhang Mingli
www.hashdata.xyz
v1-0001-Remove-useless-GROUP-BY-columns-considering-unique-i.patch
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