Add a comment noting that FDWs don't have to implement EXCEPT or LIMIT TO.

tglsfdc · tglsfdc · commit 5f36096b77fe · 2015-12-31T17:59:10.000-05:00
postgresImportForeignSchema pays attention to IMPORT's EXCEPT and LIMIT TO
options, but only as an efficiency hack, not for correctness' sake.  The
FDW documentation does explain that, but someone using postgres_fdw.c
as a coding guide might not remember it, so let's add a comment here.
Per question from Regina Obe.
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
@@ -2892,7 +2892,11 @@ postgresImportForeignSchema(ImportForeignSchemaStmt *stmt, Oid serverOid)
 
 		/*
 		 * Fetch all table data from this schema, possibly restricted by
-		 * EXCEPT or LIMIT TO.
+		 * EXCEPT or LIMIT TO.  (We don't actually need to pay any attention
+		 * to EXCEPT/LIMIT TO here, because the core code will filter the
+		 * statements we return according to those lists anyway.  But it
+		 * should save a few cycles to not process excluded tables in the
+		 * first place.)
 		 *
 		 * Note: because we run the connection with search_path restricted to
 		 * pg_catalog, the format_type() and pg_get_expr() outputs will always
