Flatten join alias Vars before pulling up targetlist items from a subquery.

pullup_replace_vars()'s decisions about whether a pulled-up replacement
expression needs to be wrapped in a PlaceHolderVar depend on the assumption
that what looks like a Var behaves like a Var.  However, if the Var is a
join alias reference, later flattening of join aliases might replace the
Var with something that's not a Var at all, and should have been wrapped.

To fix, do a forcible pass of flatten_join_alias_vars() on the subquery
targetlist before we start to copy items out of it.  We'll re-run that
processing on the pulled-up expressions later, but that's harmless.

Per report from Ken Tanzer; the added regression test case is based on his
example.  This bug has been there since the PlaceHolderVar mechanism was
invented, but has escaped detection because the circumstances that trigger
it are fairly narrow.  You need a flattenable query underneath an outer
join, which contains another flattenable query inside a join of its own,
with a dangerous expression (a constant or something else non-strict)
in that one's targetlist.

Having seen this, I'm wondering if it wouldn't be prudent to do all
alias-variable flattening earlier, perhaps even in the rewriter.
But that would probably not be a back-patchable change.

diff --git a/src/backend/optimizer/prep/prepjointree.c b/src/backend/optimizer/prep/prepjointree.c
index e874064fcdd1c2c71249bd48eb40bd205981910d..3c492c0199be5262ca19f0c7243d2bb87b00f025 100644 (file)
--- a/src/backend/optimizer/prep/prepjointree.c
+++ b/src/backend/optimizer/prep/prepjointree.c
@@ -729,6 +729,18 @@ pull_up_simple_subquery(PlannerInfo *root, Node *jtnode, RangeTblEntry *rte,
                return jtnode;
        }
 
+       /*
+        * We must flatten any join alias Vars in the subquery's targetlist,
+        * because pulling up the subquery's subqueries might have changed their
+        * expansions into arbitrary expressions, which could affect
+        * pullup_replace_vars' decisions about whether PlaceHolderVar wrappers
+        * are needed for tlist entries.  (Likely it'd be better to do
+        * flatten_join_alias_vars on the whole query tree at some earlier stage,
+        * maybe even in the rewriter; but for now let's just fix this case here.)
+        */
+       subquery->targetList = (List *)
+               flatten_join_alias_vars(subroot, (Node *) subquery->targetList);
+
        /*
         * Adjust level-0 varnos in subquery so that we can append its rangetable
         * to upper query's.  We have to fix the subquery's append_rel_list as

diff --git a/src/backend/optimizer/util/var.c b/src/backend/optimizer/util/var.c
index 89450cbb426023aecb7eb20d7a75ea09ed44ecbe..360a358c4fdbccb23c05614d24f454c9ed44c8a3 100644 (file)
--- a/src/backend/optimizer/util/var.c
+++ b/src/backend/optimizer/util/var.c
@@ -750,16 +750,14 @@ flatten_join_alias_vars_mutator(Node *node,
                                /* Ignore dropped columns */
                                if (newvar == NULL)
                                        continue;
+                               newvar = copyObject(newvar);
 
                                /*
                                 * If we are expanding an alias carried down from an upper
                                 * query, must adjust its varlevelsup fields.
                                 */
                                if (context->sublevels_up != 0)
-                               {
-                                       newvar = copyObject(newvar);
                                        IncrementVarSublevelsUp(newvar, context->sublevels_up, 0);
-                               }
                                /* Recurse in case join input is itself a join */
                                /* (also takes care of setting inserted_sublink if needed) */
                                newvar = flatten_join_alias_vars_mutator(newvar, context);
@@ -779,16 +777,14 @@ flatten_join_alias_vars_mutator(Node *node,
                Assert(var->varattno > 0);
                newvar = (Node *) list_nth(rte->joinaliasvars, var->varattno - 1);
                Assert(newvar != NULL);
+               newvar = copyObject(newvar);
 
                /*
                 * If we are expanding an alias carried down from an upper query, must
                 * adjust its varlevelsup fields.
                 */
                if (context->sublevels_up != 0)
-               {
-                       newvar = copyObject(newvar);
                        IncrementVarSublevelsUp(newvar, context->sublevels_up, 0);
-               }
 
                /* Recurse in case join input is itself a join */
                newvar = flatten_join_alias_vars_mutator(newvar, context);

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 4463985cf28295c07e110b8770617705d261bc3b..be3df5e4735e32e32373fcbca8b478916d69b094 100644 (file)
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -2673,6 +2673,58 @@ select f1, unique2, case when unique2 is null then f1 else 0 end
   0 |       0 |    0
 (1 row)
 
+--
+-- check handling of join aliases when flattening multiple levels of subquery
+--
+explain (verbose, costs off)
+select foo1.join_key as foo1_id, foo3.join_key AS foo3_id, bug_field from
+  (values (0),(1)) foo1(join_key)
+left join
+  (select join_key, bug_field from
+    (select ss1.join_key, ss1.bug_field from
+      (select f1 as join_key, 666 as bug_field from int4_tbl i1) ss1
+    ) foo2
+   left join
+    (select unique2 as join_key from tenk1 i2) ss2
+   using (join_key)
+  ) foo3
+using (join_key);
+                                                                                                    QUERY PLAN                                                                                                     
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Nested Loop Left Join
+   Output: "*VALUES*".column1, i1.f1, (666)
+   Join Filter: ("*VALUES*".column1 = i1.f1)
+   ->  Values Scan on "*VALUES*"
+         Output: "*VALUES*".column1
+   ->  Materialize
+         Output: i1.f1, (666)
+         ->  Nested Loop Left Join
+               Output: i1.f1, 666
+               ->  Seq Scan on public.int4_tbl i1
+                     Output: i1.f1
+               ->  Index Scan using tenk1_unique2 on public.tenk1 i2
+                     Output: i2.unique1, i2.unique2, i2.two, i2.four, i2.ten, i2.twenty, i2.hundred, i2.thousand, i2.twothousand, i2.fivethous, i2.tenthous, i2.odd, i2.even, i2.stringu1, i2.stringu2, i2.string4
+                     Index Cond: (i1.f1 = i2.unique2)
+(14 rows)
+
+select foo1.join_key as foo1_id, foo3.join_key AS foo3_id, bug_field from
+  (values (0),(1)) foo1(join_key)
+left join
+  (select join_key, bug_field from
+    (select ss1.join_key, ss1.bug_field from
+      (select f1 as join_key, 666 as bug_field from int4_tbl i1) ss1
+    ) foo2
+   left join
+    (select unique2 as join_key from tenk1 i2) ss2
+   using (join_key)
+  ) foo3
+using (join_key);
+ foo1_id | foo3_id | bug_field 
+---------+---------+-----------
+       0 |       0 |       666
+       1 |         |          
+(2 rows)
+
 --
 -- test ability to push constants through outer join clauses
 --

diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index b8e69e10185c2782928e2975ee0d4a411ca7d2ec..9328c8c250301d02d5ec980520e951baa5f20ab1 100644 (file)
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -690,6 +690,37 @@ select f1, unique2, case when unique2 is null then f1 else 0 end
   from int4_tbl a left join tenk1 b on f1 = unique2
   where (case when unique2 is null then f1 else 0 end) = 0;
 
+--
+-- check handling of join aliases when flattening multiple levels of subquery
+--
+
+explain (verbose, costs off)
+select foo1.join_key as foo1_id, foo3.join_key AS foo3_id, bug_field from
+  (values (0),(1)) foo1(join_key)
+left join
+  (select join_key, bug_field from
+    (select ss1.join_key, ss1.bug_field from
+      (select f1 as join_key, 666 as bug_field from int4_tbl i1) ss1
+    ) foo2
+   left join
+    (select unique2 as join_key from tenk1 i2) ss2
+   using (join_key)
+  ) foo3
+using (join_key);
+
+select foo1.join_key as foo1_id, foo3.join_key AS foo3_id, bug_field from
+  (values (0),(1)) foo1(join_key)
+left join
+  (select join_key, bug_field from
+    (select ss1.join_key, ss1.bug_field from
+      (select f1 as join_key, 666 as bug_field from int4_tbl i1) ss1
+    ) foo2
+   left join
+    (select unique2 as join_key from tenk1 i2) ss2
+   using (join_key)
+  ) foo3
+using (join_key);
+
 --
 -- test ability to push constants through outer join clauses
 --