Make postgres_fdw's "Relations" output agree with the rest of EXPLAIN.

The relation aliases shown in the "Relations" line for a foreign scan
didn't always agree with those used in the rest of EXPLAIN's output.
The regression test result changes appearing here provide examples.

It's really impossible for postgres_fdw to duplicate EXPLAIN's alias
assignment logic during postgresGetForeignRelSize(), because of the
de-duplication that EXPLAIN does on a global basis --- and anyway,
trying to duplicate that would be unmaintainable.  Instead, just put
numeric rangetable indexes into the string, and convert those to
table names/aliases in postgresExplainForeignScan, which does have
access to the results of ruleutils.c's alias assignment logic.
Aside from being more reliable, this shifts some work from planning
to EXPLAIN, which is a good tradeoff for performance.  (I also
changed from using StringInfo to using psprintf, which makes the
code slightly simpler and reduces its memory consumption.)

A kluge required by this solution is that we have to reverse-engineer
the rtoffset applied by setrefs.c.  If that logic ever fails
(presumably because the member tables of a join got offset by
different amounts), we'll need some more cooperation with setrefs.c
to keep things straight.  But for now, there's no need for that.

Arguably this is a back-patchable bug fix, but since this is a mostly
cosmetic issue and there have been no field complaints, I'll refrain
for now.

Discussion: https://postgr.es/m/12424.1575168015@sss.pgh.pa.us

1 files changed, 7 insertions, 4 deletions

diff --git a/contrib/postgres_fdw/postgres_fdw.h b/contrib/postgres_fdw/postgres_fdw.h
index bdefe0c40e2..ea052872c3e 100644
--- a/contrib/postgres_fdw/postgres_fdw.h
+++ b/contrib/postgres_fdw/postgres_fdw.h
@@ -87,11 +87,14 @@ typedef struct PgFdwRelationInfo
 	int			fetch_size;		/* fetch size for this remote table */
 
 	/*
-	 * Name of the relation while EXPLAINing ForeignScan. It is used for join
-	 * relations but is set for all relations. For join relation, the name
-	 * indicates which foreign tables are being joined and the join type used.
+	 * Name of the relation, for use while EXPLAINing ForeignScan.  It is used
+	 * for join and upper relations but is set for all relations.  For a base
+	 * relation, this is really just the RT index as a string; we convert that
+	 * while producing EXPLAIN output.  For join and upper relations, the name
+	 * indicates which base foreign tables are included and the join type or
+	 * aggregation type used.
 	 */
-	StringInfo	relation_name;
+	char	   *relation_name;
 
 	/* Join information */
 	RelOptInfo *outerrel;