Make Vars be outer-join-aware.

Traditionally we used the same Var struct to represent the value
of a table column everywhere in parse and plan trees.  This choice
predates our support for SQL outer joins, and it's really a pretty
bad idea with outer joins, because the Var's value can depend on
where it is in the tree: it might go to NULL above an outer join.
So expression nodes that are equal() per equalfuncs.c might not
represent the same value, which is a huge correctness hazard for
the planner.

To improve this, decorate Var nodes with a bitmapset showing
which outer joins (identified by RTE indexes) may have nulled
them at the point in the parse tree where the Var appears.
This allows us to trust that equal() Vars represent the same value.
A certain amount of klugery is still needed to cope with cases
where we re-order two outer joins, but it's possible to make it
work without sacrificing that core principle.  PlaceHolderVars
receive similar decoration for the same reason.

In the planner, we include these outer join bitmapsets into the relids
that an expression is considered to depend on, and in consequence also
add outer-join relids to the relids of join RelOptInfos.  This allows
us to correctly perceive whether an expression can be calculated above
or below a particular outer join.

This change affects FDWs that want to plan foreign joins.  They *must*
follow suit when labeling foreign joins in order to match with the
core planner, but for many purposes (if postgres_fdw is any guide)
they'd prefer to consider only base relations within the join.
To support both requirements, redefine ForeignScan.fs_relids as
base+OJ relids, and add a new field fs_base_relids that's set up by
the core planner.

Large though it is, this commit just does the minimum necessary to
install the new mechanisms and get check-world passing again.
Follow-up patches will perform some cleanup.  (The README additions
and comments mention some stuff that will appear in the follow-up.)

Patch by me; thanks to Richard Guo for review.

Discussion: https://postgr.es/m/830269.1656693747@sss.pgh.pa.us

1 files changed, 8 insertions, 0 deletions

diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 89335d95e7b..fbbbe647a43 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1090,6 +1090,14 @@ typedef struct RangeTblEntry
 	 * alias Vars are generated only for merged columns).  We keep these
 	 * entries only because they're needed in expandRTE() and similar code.
 	 *
+	 * Vars appearing within joinaliasvars are marked with varnullingrels sets
+	 * that describe the nulling effects of this join and lower ones.  This is
+	 * essential for FULL JOIN cases, because the COALESCE expression only
+	 * describes the semantics correctly if its inputs have been nulled by the
+	 * join.  For other cases, it allows expandRTE() to generate a valid
+	 * representation of the join's output without consulting additional
+	 * parser state.
+	 *
 	 * Within a Query loaded from a stored rule, it is possible for non-merged
 	 * joinaliasvars items to be null pointers, which are placeholders for
 	 * (necessarily unreferenced) columns dropped since the rule was made.