> Here is the updated version, which includes the restructuring you proposed.
> Other than the above issue and the alias issue we discussed, I addressed all
> your comments except one on testing; I tried to add test cases where the
> remote query is deparsed as nested subqueries, but I couldn't because IIUC,
> reduce_outer_joins reduced full joins to inner joins or left joins.

No always. It will do so in some cases as explained in the prologue of

 * More generally, an outer join can be reduced in strength if there is a
 * strict qual above it in the qual tree that constrains a Var from the
 * nullable side of the join to be non-null.  (For FULL joins this applies
 * to each side separately.)

>  So, I
> added two test cases: (1) the joining relations are both base relations
> (actually, we already have that) and (2) one of the joining relations is a
> base relation and the other is a join relation.  I rebased the patch to
> HEAD, so I added a test case with aggregate pushdown also.

This patch again has a lot of unrelated changes, esp. in
deparseSelectStmtForRel(). What was earlier part of deparseSelectSql()
and deparseFromExpr() is now flattened in deparseSelectStmtForRel(),
which seems unnecessary. Or there are changes like

-deparseSelectStmtForRel(StringInfo buf, PlannerInfo *root, RelOptInfo *rel,
-                       List *tlist, List *remote_conds, List *pathkeys,
+deparseSelectStmtForRel(StringInfo buf, PlannerInfo *root,
+                       RelOptInfo *foreignrel, List *tlist,
+                       List *remote_conds, List *pathkeys,

which arise because rel has been renamed as foreignrel. The patch will
work even without such renaming.

I think such refactoring, should be done in a separate patch.

Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company

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