postgres_fdw: Add ORDER BY to some remote SQL queries.

If the join problem's entire ORDER BY clause can be pushed to the
remote server, consider a path that adds this ORDER BY clause.  If
use_remote_estimate is on, we cost this path using an additional
remote EXPLAIN.  If not, we just estimate that the path costs 20%
more, which is intended to be large enough that we won't request a
remote sort when it's not helpful, but small enough that we'll have
the remote side do the sort when in doubt.  In some cases, the remote
sort might actually be free, because the remote query plan might
happen to produce output that is ordered the way we need, but without
remote estimates we have no way of knowing that.

It might also be useful to request sorted output from the remote side
if it enables an efficient merge join, but this patch doesn't attempt
to handle that case.

Ashutosh Bapat with revisions by me.  Also reviewed by Fabrízio de Royes
Mello and Jeevan Chalke.

1 files changed, 53 insertions, 3 deletions

diff --git a/contrib/postgres_fdw/deparse.c b/contrib/postgres_fdw/deparse.c
index 697de60dfe5..3cb728fa693 100644
--- a/contrib/postgres_fdw/deparse.c
+++ b/contrib/postgres_fdw/deparse.c
@@ -193,9 +193,12 @@ is_foreign_expr(PlannerInfo *root,
 	if (!foreign_expr_walker((Node *) expr, &glob_cxt, &loc_cxt))
 		return false;
 
-	/* Expressions examined here should be boolean, ie noncollatable */
-	Assert(loc_cxt.collation == InvalidOid);
-	Assert(loc_cxt.state == FDW_COLLATE_NONE);
+	/*
+	 * If the expression has a valid collation that does not arise from a
+	 * foreign var, the expression can not be sent over.
+	 */
+	if (loc_cxt.state == FDW_COLLATE_UNSAFE)
+		return false;
 
 	/*
 	 * An expression which includes any mutable functions can't be sent over
@@ -1877,3 +1880,50 @@ printRemotePlaceholder(Oid paramtype, int32 paramtypmod,
 
 	appendStringInfo(buf, "((SELECT null::%s)::%s)", ptypename, ptypename);
 }
+
+/*
+ * Deparse ORDER BY clause according to the given pathkeys for given base
+ * relation. From given pathkeys expressions belonging entirely to the given
+ * base relation are obtained and deparsed.
+ */
+void
+appendOrderByClause(StringInfo buf, PlannerInfo *root, RelOptInfo *baserel,
+					List *pathkeys)
+{
+	ListCell			*lcell;
+	deparse_expr_cxt	context;
+	int					nestlevel;
+	char				*delim = " ";
+
+	/* Set up context struct for recursion */
+	context.root = root;
+	context.foreignrel = baserel;
+	context.buf = buf;
+	context.params_list = NULL;
+
+	/* Make sure any constants in the exprs are printed portably */
+	nestlevel = set_transmission_modes();
+
+	appendStringInfo(buf, " ORDER BY");
+	foreach(lcell, pathkeys)
+	{
+		PathKey				*pathkey = lfirst(lcell);
+		Expr				*em_expr;
+
+		em_expr = find_em_expr_for_rel(pathkey->pk_eclass, baserel);
+		Assert(em_expr != NULL);
+
+		appendStringInfoString(buf, delim);
+		deparseExpr(em_expr, &context);
+		if (pathkey->pk_strategy == BTLessStrategyNumber)
+			appendStringInfoString(buf, " ASC");
+		else
+			appendStringInfoString(buf, " DESC");
+
+		if (pathkey->pk_nulls_first)
+			appendStringInfoString(buf, " NULLS FIRST");
+
+		delim = ", ";
+	}
+	reset_transmission_modes(nestlevel);
+}