postgres_fdw: Fix assertion in estimate_path_cost_size().

Commit 08d2d58a2 added this assertion assuming that the retrieved_rows
estimate for a foreign relation is set to at least one row in
estimate_path_cost_size(), but the assumption isn't correct because if
the relation is a foreign table with tuples=0, the estimate would be set
to 0 in there when using local stats, and if the query's WHERE clause
has a NULL condition, the estimate could be set to 0 in there when using
remote estimates.  (Note: even in the latter case the assertion could be
reachable when costing foreign final paths.)  Repair.

Per bug #17713 from Robins Tharakan.  This patch was already applied to
v13 or later; apply it to v12 as well where the aforementioned commit
was added.  Thanks to Richard Guo for investigation.

Discussion: http://postgr.es/m/17713-92cce66de7e81c04%40postgresql.org
Discussion: http://postgr.es/m/CAEP4nAza%2B0fTCLkgkKYux3JDo3tUBTQORehP%2BaCxSNURpSFpHw%40mail.gmail.com

diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index 92cff268ca8cb4bccaefde9eea42e227fa46a12a..e3cc20ff4bfffa4b03d55c9488b3e3943343e414 100644 (file)
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -590,6 +590,24 @@ SELECT t1."C 1", t2.c1, t3.c1 FROM "S 1"."T 1" t1 full join ft1 t2 full join ft2
 
 RESET enable_hashjoin;
 RESET enable_nestloop;
+-- Test executing assertion in estimate_path_cost_size() that makes sure that
+-- retrieved_rows for foreign rel re-used to cost pre-sorted foreign paths is
+-- a sensible value even when the rel has tuples=0
+CREATE TABLE loct_empty (c1 int NOT NULL, c2 text);
+CREATE FOREIGN TABLE ft_empty (c1 int NOT NULL, c2 text)
+  SERVER loopback OPTIONS (table_name 'loct_empty');
+INSERT INTO loct_empty
+  SELECT id, 'AAA' || to_char(id, 'FM000') FROM generate_series(1, 100) id;
+DELETE FROM loct_empty;
+ANALYZE ft_empty;
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM ft_empty ORDER BY c1;
+                                  QUERY PLAN                                   
+-------------------------------------------------------------------------------
+ Foreign Scan on public.ft_empty
+   Output: c1, c2
+   Remote SQL: SELECT c1, c2 FROM public.loct_empty ORDER BY c1 ASC NULLS LAST
+(3 rows)
+
 -- ===================================================================
 -- WHERE with remotely-executable conditions
 -- ===================================================================

diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index a4d0377f31f91cbd20672d108b72fdb126cf2adb..7aa6d7c549c56f90bd1f306be9daadbc22889d7e 100644 (file)
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -2749,7 +2749,7 @@ estimate_path_cost_size(PlannerInfo *root,
                 */
                if (fpinfo->rel_startup_cost >= 0 && fpinfo->rel_total_cost >= 0)
                {
-                       Assert(fpinfo->retrieved_rows >= 1);
+                       Assert(fpinfo->retrieved_rows >= 0);
 
                        rows = fpinfo->rows;
                        retrieved_rows = fpinfo->retrieved_rows;

diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 3118ac5468b31a007f05c77558cb3140f3eb3356..6cc8b251ccc667a41d5865d3ac31cdc62d7e81d0 100644 (file)
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -285,6 +285,18 @@ SELECT t1."C 1", t2.c1, t3.c1 FROM "S 1"."T 1" t1 full join ft1 t2 full join ft2
 RESET enable_hashjoin;
 RESET enable_nestloop;
 
+-- Test executing assertion in estimate_path_cost_size() that makes sure that
+-- retrieved_rows for foreign rel re-used to cost pre-sorted foreign paths is
+-- a sensible value even when the rel has tuples=0
+CREATE TABLE loct_empty (c1 int NOT NULL, c2 text);
+CREATE FOREIGN TABLE ft_empty (c1 int NOT NULL, c2 text)
+  SERVER loopback OPTIONS (table_name 'loct_empty');
+INSERT INTO loct_empty
+  SELECT id, 'AAA' || to_char(id, 'FM000') FROM generate_series(1, 100) id;
+DELETE FROM loct_empty;
+ANALYZE ft_empty;
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM ft_empty ORDER BY c1;
+
 -- ===================================================================
 -- WHERE with remotely-executable conditions
 -- ===================================================================