Quals not pushed down into lateral

Hi,

During citus development we noticed that restrictions aren't pushed down
into lateral subqueries, even if they semantically could. For example,
in this dumbed down example:

postgres[31776][1]=# CREATE TABLE t_2(id serial primary key);
postgres[31776][1]=# CREATE TABLE t_1(id serial primary key);

Comparing:

postgres[31776][1]=# EXPLAIN SELECT * FROM t_1 JOIN (SELECT * FROM t_2 GROUP BY id) s ON (t_1.id = s.id) WHERE t_1.id = 3;
┌─────────────────────────────────────────────────────────────────────────────────────┐
│ QUERY PLAN │
├─────────────────────────────────────────────────────────────────────────────────────┤
│ Nested Loop (cost=0.31..16.37 rows=1 width=8) │
│ -> Index Only Scan using t_1_pkey on t_1 (cost=0.15..8.17 rows=1 width=4) │
│ Index Cond: (id = 3) │
│ -> Group (cost=0.15..8.17 rows=1 width=4) │
│ Group Key: t_2.id │
│ -> Index Only Scan using t_2_pkey on t_2 (cost=0.15..8.17 rows=1 width=4) │
│ Index Cond: (id = 3) │
└─────────────────────────────────────────────────────────────────────────────────────┘
(7 rows)

with:

postgres[31776][1]=# EXPLAIN SELECT * FROM t_1, LATERAL (SELECT * FROM t_2 WHERE t_1.id = t_2.id GROUP BY id) s WHERE t_1.id = 3;
┌─────────────────────────────────────────────────────────────────────────────────────┐
│ QUERY PLAN │
├─────────────────────────────────────────────────────────────────────────────────────┤
│ Nested Loop (cost=0.31..16.37 rows=1 width=8) │
│ -> Index Only Scan using t_1_pkey on t_1 (cost=0.15..8.17 rows=1 width=4) │
│ Index Cond: (id = 3) │
│ -> Group (cost=0.15..8.17 rows=1 width=4) │
│ Group Key: t_2.id │
│ -> Index Only Scan using t_2_pkey on t_2 (cost=0.15..8.17 rows=1 width=4) │
│ Index Cond: (id = t_1.id) │
└─────────────────────────────────────────────────────────────────────────────────────┘

it's noticeable that the former has id = 3 pushed down into both
relations index scans, whereas the latter doesn't.

This seems like a worthwhile future optimization opportunity.

I've not looked into this in any detail, but the proximate source is
that set_subquery_pathlist() doesn't see any baserstrictinfos to push
down. Which makes sense, because t_1.id = t_2.id isn't "visible" (in
the sense of deconstruct_jointree dealing with it) to the outside.

It seems possible to look into rel->lateral_vars, check whether that's
member of some equivclass, and then push the relevant equivalences down
(after taking care that the Var from the outside is known as a Param on
the inside).

I'm not planning to work on this anytime soon, but I thought it'd be
useful to have a searchable reference point about the topic. If
somebody wants to work on it...

Greetings,

Andres Freund