cached plans and enable_partition_pruning

[Amit Langote]

It seems that because enable_partition_pruning's value is only checked
during planning, turning it off *after* a plan is created and cached does
not work as expected.

create table p (a int) partition by list (a);
create table p1 partition of p for values in (1);
create table p1 partition of p for values in (2);

-- force a generic plan so that run-time pruning is used in the plan
reset enable_partition_pruning;
set plan_cache_mode to force_generic_plan;
prepare p as select * from p where a = $1;

explain (costs off, analyze) execute p (1);
                           QUERY PLAN
────────────────────────────────────────────────────────────────
 Append (actual time=0.079..0.106 rows=1 loops=1)
   Subplans Removed: 1
   ->  Seq Scan on p2 (actual time=0.058..0.068 rows=1 loops=1)
         Filter: (a = $1)
 Planning Time: 17.573 ms
 Execution Time: 0.396 ms
(6 rows)

set enable_partition_pruning to off;

explain (costs off, analyze) execute p (1);
                           QUERY PLAN
────────────────────────────────────────────────────────────────
 Append (actual time=0.108..0.135 rows=1 loops=1)
   Subplans Removed: 1
   ->  Seq Scan on p2 (actual time=0.017..0.028 rows=1 loops=1)
         Filter: (a = $1)
 Planning Time: 0.042 ms
 Execution Time: 0.399 ms
(6 rows)

Pruning still occurs, whereas one would expect it not to, because the plan
(the Append node) contains run-time pruning information, which was
initialized because enable_partition_pruning was turned on when the plan
was created.

Should we check its value during execution too, as done in the attached?

Thanks,
Amit

diff --git a/src/backend/executor/nodeAppend.c 
b/src/backend/executor/nodeAppend.c
index 1fc55e90d7..d67abed330 100644
--- a/src/backend/executor/nodeAppend.c
+++ b/src/backend/executor/nodeAppend.c
@@ -61,6 +61,7 @@
 #include "executor/execPartition.h"
 #include "executor/nodeAppend.h"
 #include "miscadmin.h"
+#include "optimizer/cost.h"
 
 /* Shared state for parallel-aware Append. */
 struct ParallelAppendState
@@ -128,8 +129,12 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
        /* Let choose_next_subplan_* function handle setting the first subplan 
*/
        appendstate->as_whichplan = INVALID_SUBPLAN_INDEX;
 
-       /* If run-time partition pruning is enabled, then set that up now */
-       if (node->part_prune_info != NULL)
+       /*
+        * If run-time partition pruning is enabled, then set that up now.
+        * However, enable_partition_pruning may have been turned off since when
+        * the plan was created, so recheck its value.
+        */
+       if (node->part_prune_info != NULL && enable_partition_pruning)
        {
                PartitionPruneState *prunestate;
 
diff --git a/src/backend/executor/nodeMergeAppend.c 
b/src/backend/executor/nodeMergeAppend.c
index c7eb18e546..5252930b17 100644
--- a/src/backend/executor/nodeMergeAppend.c
+++ b/src/backend/executor/nodeMergeAppend.c
@@ -43,6 +43,7 @@
 #include "executor/nodeMergeAppend.h"
 #include "lib/binaryheap.h"
 #include "miscadmin.h"
+#include "optimizer/cost.h"
 
 /*
  * We have one slot for each item in the heap array.  We use SlotNumber
@@ -89,8 +90,12 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int 
eflags)
        mergestate->ps.ExecProcNode = ExecMergeAppend;
        mergestate->ms_noopscan = false;
 
-       /* If run-time partition pruning is enabled, then set that up now */
-       if (node->part_prune_info != NULL)
+       /*
+        * If run-time partition pruning is enabled, then set that up now.
+        * However, enable_partition_pruning may have been turned off since when
+        * the plan was created, so recheck its value.
+        */
+       if (node->part_prune_info != NULL && enable_partition_pruning)
        {
                PartitionPruneState *prunestate;