Disallow replacing joins with scans in problematic cases.

Commit e7cb7ee14, which introduced the infrastructure for FDWs and
custom scan providers to replace joins with scans, failed to add support
handling of pseudoconstant quals assigned to replaced joins in
createplan.c, leading to an incorrect plan without a gating Result node
when postgres_fdw replaced a join with such a qual.

To fix, we could add the support by 1) modifying the ForeignPath and
CustomPath structs to store the list of RestrictInfo nodes to apply to
the join, as in JoinPaths, if they represent foreign and custom scans
replacing a join with a scan, and by 2) modifying create_scan_plan() in
createplan.c to use that list in that case, instead of the
baserestrictinfo list, to get pseudoconstant quals assigned to the join;
but #1 would cause an ABI break.  So fix by modifying the infrastructure
to just disallow replacing joins with such quals.

Back-patch to all supported branches.

Reported by Nishant Sharma.  Patch by me, reviewed by Nishant Sharma and
Richard Guo.

Discussion: https://postgr.es/m/CADrsxdbcN1vejBaf8a%2BQhrZY5PXL-04mCd4GDu6qm6FigDZd6Q%40mail.gmail.com

diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index 17c95da25e448df42e5a62c0031d14f1f0fc88d7..08f1b356f298d961a07f93ecc82db940d538a26e 100644 (file)
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -2293,6 +2293,32 @@ SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM
    1
 (10 rows)
 
+-- join with pseudoconstant quals, not pushed down.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1 AND CURRENT_USER = SESSION_USER) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
+                                  QUERY PLAN                                   
+-------------------------------------------------------------------------------
+ Limit
+   Output: t1.c1, t2.c1, t1.c3
+   ->  Sort
+         Output: t1.c1, t2.c1, t1.c3
+         Sort Key: t1.c3, t1.c1
+         ->  Result
+               Output: t1.c1, t2.c1, t1.c3
+               One-Time Filter: (CURRENT_USER = SESSION_USER)
+               ->  Hash Join
+                     Output: t1.c1, t1.c3, t2.c1
+                     Hash Cond: (t2.c1 = t1.c1)
+                     ->  Foreign Scan on public.ft2 t2
+                           Output: t2.c1
+                           Remote SQL: SELECT "C 1" FROM "S 1"."T 1"
+                     ->  Hash
+                           Output: t1.c1, t1.c3
+                           ->  Foreign Scan on public.ft1 t1
+                                 Output: t1.c1, t1.c3
+                                 Remote SQL: SELECT "C 1", c3 FROM "S 1"."T 1"
+(19 rows)
+
 -- non-Var items in targetlist of the nullable rel of a join preventing
 -- push-down in some cases
 -- unable to push {ft1, ft2}

diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index ad3f7880fe49b3f63d51cfd6ea3a9d66ebb57c98..0839f8ea3d470851756fc209ee8b0b48f000f971 100644 (file)
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -629,6 +629,9 @@ SELECT t1c1, avg(t1c1 + t2c1) FROM (SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM ft1 t2, ft2 t3 WHERE t2.c1 = t3.c1 AND t2.c2 = t1.c2) q ORDER BY t1."C 1" OFFSET 10 LIMIT 10;
 SELECT t1."C 1" FROM "S 1"."T 1" t1, LATERAL (SELECT DISTINCT t2.c1, t3.c1 FROM ft1 t2, ft2 t3 WHERE t2.c1 = t3.c1 AND t2.c2 = t1.c2) q ORDER BY t1."C 1" OFFSET 10 LIMIT 10;
+-- join with pseudoconstant quals, not pushed down.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1 AND CURRENT_USER = SESSION_USER) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
 
 -- non-Var items in targetlist of the nullable rel of a join preventing
 -- push-down in some cases

diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index cda9d4c1a8f639c74de947ea043aa37866a2c1af..c09f0e1df3ea400f617217d3e1dfa6a0e4f8258c 100644 (file)
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -24,6 +24,7 @@
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
 #include "optimizer/planmain.h"
+#include "optimizer/restrictinfo.h"
 #include "utils/typcache.h"
 
 /* Hook for plugins to get control in add_paths_to_joinrel() */
@@ -130,6 +131,7 @@ add_paths_to_joinrel(PlannerInfo *root,
 {
    JoinPathExtraData extra;
    bool        mergejoin_allowed = true;
+   bool        consider_join_pushdown = false;
    ListCell   *lc;
    Relids      joinrelids;
 
@@ -318,13 +320,25 @@ add_paths_to_joinrel(PlannerInfo *root,
        hash_inner_and_outer(root, joinrel, outerrel, innerrel,
                             jointype, &extra);
 
+   /*
+    * createplan.c does not currently support handling of pseudoconstant
+    * clauses assigned to joins pushed down by extensions; check if the
+    * restrictlist has such clauses, and if so, disallow pushing down joins.
+    */
+   if ((joinrel->fdwroutine &&
+        joinrel->fdwroutine->GetForeignJoinPaths) ||
+       set_join_pathlist_hook)
+       consider_join_pushdown = !has_pseudoconstant_clauses(root,
+                                                            restrictlist);
+
    /*
     * 5. If inner and outer relations are foreign tables (or joins) belonging
     * to the same server and assigned to the same user to check access
     * permissions as, give the FDW a chance to push down joins.
     */
    if (joinrel->fdwroutine &&
-       joinrel->fdwroutine->GetForeignJoinPaths)
+       joinrel->fdwroutine->GetForeignJoinPaths &&
+       consider_join_pushdown)
        joinrel->fdwroutine->GetForeignJoinPaths(root, joinrel,
                                                 outerrel, innerrel,
                                                 jointype, &extra);
@@ -332,7 +346,8 @@ add_paths_to_joinrel(PlannerInfo *root,
    /*
     * 6. Finally, give extensions a chance to manipulate the path list.
     */
-   if (set_join_pathlist_hook)
+   if (set_join_pathlist_hook &&
+       consider_join_pushdown)
        set_join_pathlist_hook(root, joinrel, outerrel, innerrel,
                               jointype, &extra);
 }

diff --git a/src/backend/optimizer/util/restrictinfo.c b/src/backend/optimizer/util/restrictinfo.c
index ef8df3d098e5a655c654d8541e371d024fbc3895..e7764f4309102fff60370fad7a0af8c4752cc7c3 100644 (file)
--- a/src/backend/optimizer/util/restrictinfo.c
+++ b/src/backend/optimizer/util/restrictinfo.c
@@ -497,6 +497,35 @@ extract_actual_join_clauses(List *restrictinfo_list,
    }
 }
 
+/*
+ * has_pseudoconstant_clauses
+ *
+ * Returns true if 'restrictinfo_list' includes pseudoconstant clauses.
+ *
+ * This is used when we determine whether to allow extensions to consider
+ * pushing down joins in add_paths_to_joinrel().
+ */
+bool
+has_pseudoconstant_clauses(PlannerInfo *root,
+                          List *restrictinfo_list)
+{
+   ListCell   *l;
+
+   /* No need to look if we know there are no pseudoconstants */
+   if (!root->hasPseudoConstantQuals)
+       return false;
+
+   /* See if there are pseudoconstants in the RestrictInfo list */
+   foreach(l, restrictinfo_list)
+   {
+       RestrictInfo *rinfo = lfirst_node(RestrictInfo, l);
+
+       if (rinfo->pseudoconstant)
+           return true;
+   }
+   return false;
+}
+
 
 /*
  * join_clause_is_movable_to

diff --git a/src/include/optimizer/restrictinfo.h b/src/include/optimizer/restrictinfo.h
index 6d30bd5e9d2cc13a052227e57d0e5ca0764829ff..fd7a0ec69df96a7824df92d0e320af5a1bd1da0a 100644 (file)
--- a/src/include/optimizer/restrictinfo.h
+++ b/src/include/optimizer/restrictinfo.h
@@ -41,6 +41,8 @@ extern void extract_actual_join_clauses(List *restrictinfo_list,
                                        Relids joinrelids,
                                        List **joinquals,
                                        List **otherquals);
+extern bool has_pseudoconstant_clauses(PlannerInfo *root,
+                                      List *restrictinfo_list);
 extern bool join_clause_is_movable_to(RestrictInfo *rinfo, RelOptInfo *baserel);
 extern bool join_clause_is_movable_into(RestrictInfo *rinfo,
                                        Relids currentrelids,