Remove useless GROUP BY columns considering unique index

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