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- to all supported branches.

Reported by Nishant Sharma.   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 852b5b4707eef27393faba88869be1dd56180ef5..f6d3b8ec08ecfbee673cfba68a517bed1af234b7 100644 (file)
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -2316,6 +2316,32 @@ 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 2fe8abc7af47e7323ec82bd81d2c3e2d2ad6fd79..436feee396b1e60fec2076a907a0372f76db05ff 100644 (file)
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -640,6 +640,9 @@ 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 f047ad9ba4688601bc24a73d58af6463b362c659..4b58936fa4cd2151b2204a5f3a553a794a8f9a91 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;
 
@@ -321,13 +323,25 @@ 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);
@@ -335,7 +349,8 @@ 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 d6d26a2b515cf89f6b55f589d3550efa2d764e8e..c1fbbb6bfeefb429473731cdc646126f428c1766 100644 (file)
--- a/src/backend/optimizer/util/restrictinfo.c
+++ b/src/backend/optimizer/util/restrictinfo.c
@@ -549,6 +549,36 @@ 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 &&
+           !rinfo_is_constant_true(rinfo))
+           return true;
+   }
+   return false;
+}
+
 /*
  * join_clause_is_movable_to
  *     Test whether a join clause is a safe candidate for parameterization

diff --git a/src/include/optimizer/restrictinfo.h b/src/include/optimizer/restrictinfo.h
index e140e619ace72a844938fcdcdeaeceb903431362..14cdce750cb7fd5f085303f83d25cfe17981e895 100644 (file)
--- a/src/include/optimizer/restrictinfo.h
+++ b/src/include/optimizer/restrictinfo.h
@@ -43,6 +43,8 @@ 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,