Re: Allowing parallel-safe initplans

I wrote:
> Eventually I noticed that all the failing cases were instances of
> optimizing MIN()/MAX() aggregates into indexscans, and then I figured
> out what the problem is: we substitute Params for the optimized-away
> Aggref nodes in setrefs.c, *after* SS_finalize_plan has been run.
> That means we fail to account for those Params in extParam/allParam
> sets.  We've gotten away with that up to now because such Params
> could only appear where Aggrefs can appear, which is only in top-level
> (above scans and joins) nodes, which generally don't have any of the
> sorts of rescan optimizations that extParam/allParam bits control.
> But this patch results in needing to have a correct extParam set for
> the node just below Gather, and we don't.  I am not sure whether there
> are any reachable bugs without this patch; but there might be, or some
> future optimization might introduce one.

> It seems like the cleanest fix for this is to replace such optimized
> Aggrefs in a separate tree scan before running SS_finalize_plan.
> That's fairly annoying from a planner-runtime standpoint, although
> we could skip the extra pass in the typical case where no minmax aggs
> have been optimized.
> I also thought about swapping the order of operations so that we
> run SS_finalize_plan after setrefs.c.  That falls down because of
> set_param_references itself, which requires those bits to be
> calculated already.  But maybe we could integrate that computation
> into SS_finalize_plan instead?  There's certainly nothing very
> pretty about the way it's done now.

I tried both of those and concluded they'd be too messy for a patch
that we might find ourselves having to back-patch.  So 0001 attached
fixes it by teaching SS_finalize_plan to treat optimized MIN()/MAX()
aggregates as if they were already Params.  It's slightly annoying
to have knowledge of that optimization metastasizing into another
place, but the alternatives are even less palatable.

Having done that, if you adjust 0002 to inject Gathers even when
debug_parallel_query = regress, the only diffs in the core regression
tests are that some initPlans disappear from EXPLAIN output.  The
outputs of the actual queries are still correct, demonstrating that
e89a71fb4 does indeed make it work as long as the param bitmapsets
are correct.

I'm still resistant to the idea of kluging EXPLAIN to the extent
of hiding the EXPLAIN output changes.  It wouldn't be that hard
to do really, but I worry that such a kluge might hide real problems
in future.  So what I did in 0002 was to allow initPlans for an
injected Gather only if debug_parallel_query = on, so that there
will be a place for EXPLAIN to show them.  Other than the changes
in that area, 0002 is the same as the previous patch.

            regards, tom lane

From c0ae4618308f89de57b4a7d744cf2f734555d4a2 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 13 Jul 2023 16:50:13 -0400
Subject: [PATCH v2 1/2] Account for optimized MinMax aggregates during
 SS_finalize_plan.

We are capable of optimizing MIN() and MAX() aggregates on indexed
columns into subqueries that exploit the index, rather than the normal
thing of scanning the whole table.  When we do this, we replace the
Aggref node(s) with Params referencing subquery outputs.  Such Params
really ought to be included in the per-plan-node extParam/allParam
sets computed by SS_finalize_plan.  However, we've never done so
up to now because of an ancient implementation choice to perform
that substitution during set_plan_references, which runs after
SS_finalize_plan, so that SS_finalize_plan never sees these Params.

This seems like clearly a bug, yet there have been no field reports
of problems that could trace to it.  This may be because the types
of Plan nodes that could contain Aggrefs do not have any of the
rescan optimizations that are controlled by extParam/allParam.
Nonetheless it seems certain to bite us someday, so let's fix it
in a self-contained patch that can be back-patched if we find a
case in which there's a live bug pre-v17.

The cleanest fix would be to perform a separate tree walk to do
these substitutions before SS_finalize_plan runs.  That seems
unattractive, first because a whole-tree mutation pass is expensive,
and second because we lack infrastructure for visiting expression
subtrees in a Plan tree, so that we'd need a new function knowing
as much as SS_finalize_plan knows about that.  I also considered
swapping the order of SS_finalize_plan and set_plan_references,
but that fell foul of various assumptions that seem tricky to fix.
So the approach adopted here is to teach SS_finalize_plan itself
to check for such Aggrefs.  I refactored things a bit in setrefs.c
to avoid having three copies of the code that does that.

Given the lack of any currently-known bug, no test case here.

Discussion: https://postgr.es/m/2391880.1689025003@sss.pgh.pa.us
---
 src/backend/optimizer/plan/setrefs.c   | 68 ++++++++++++++++----------
 src/backend/optimizer/plan/subselect.c | 25 ++++++++--
 src/include/optimizer/planmain.h       |  2 +
 3 files changed, 66 insertions(+), 29 deletions(-)

diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index c63758cb2b..16e5537f7f 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -2181,22 +2181,14 @@ fix_scan_expr_mutator(Node *node, fix_scan_expr_context *context)
     if (IsA(node, Aggref))
     {
         Aggref       *aggref = (Aggref *) node;
+        Param       *aggparam;

         /* See if the Aggref should be replaced by a Param */
-        if (context->root->minmax_aggs != NIL &&
-            list_length(aggref->args) == 1)
+        aggparam = find_minmax_agg_replacement_param(context->root, aggref);
+        if (aggparam != NULL)
         {
-            TargetEntry *curTarget = (TargetEntry *) linitial(aggref->args);
-            ListCell   *lc;
-
-            foreach(lc, context->root->minmax_aggs)
-            {
-                MinMaxAggInfo *mminfo = (MinMaxAggInfo *) lfirst(lc);
-
-                if (mminfo->aggfnoid == aggref->aggfnoid &&
-                    equal(mminfo->target, curTarget->expr))
-                    return (Node *) copyObject(mminfo->param);
-            }
+            /* Make a copy of the Param for paranoia's sake */
+            return (Node *) copyObject(aggparam);
         }
         /* If no match, just fall through to process it normally */
     }
@@ -3225,22 +3217,14 @@ fix_upper_expr_mutator(Node *node, fix_upper_expr_context *context)
     if (IsA(node, Aggref))
     {
         Aggref       *aggref = (Aggref *) node;
+        Param       *aggparam;

         /* See if the Aggref should be replaced by a Param */
-        if (context->root->minmax_aggs != NIL &&
-            list_length(aggref->args) == 1)
+        aggparam = find_minmax_agg_replacement_param(context->root, aggref);
+        if (aggparam != NULL)
         {
-            TargetEntry *curTarget = (TargetEntry *) linitial(aggref->args);
-            ListCell   *lc;
-
-            foreach(lc, context->root->minmax_aggs)
-            {
-                MinMaxAggInfo *mminfo = (MinMaxAggInfo *) lfirst(lc);
-
-                if (mminfo->aggfnoid == aggref->aggfnoid &&
-                    equal(mminfo->target, curTarget->expr))
-                    return (Node *) copyObject(mminfo->param);
-            }
+            /* Make a copy of the Param for paranoia's sake */
+            return (Node *) copyObject(aggparam);
         }
         /* If no match, just fall through to process it normally */
     }
@@ -3395,6 +3379,38 @@ set_windowagg_runcondition_references(PlannerInfo *root,
     return newlist;
 }

+/*
+ * find_minmax_agg_replacement_param
+ *        If the given Aggref is one that we are optimizing into a subquery
+ *        (cf. planagg.c), then return the Param that should replace it.
+ *        Else return NULL.
+ *
+ * This is exported so that SS_finalize_plan can use it before setrefs.c runs.
+ * Note that it will not find anything until we have built a Plan from a
+ * MinMaxAggPath, as root->minmax_aggs will never be filled otherwise.
+ */
+Param *
+find_minmax_agg_replacement_param(PlannerInfo *root, Aggref *aggref)
+{
+    if (root->minmax_aggs != NIL &&
+        list_length(aggref->args) == 1)
+    {
+        TargetEntry *curTarget = (TargetEntry *) linitial(aggref->args);
+        ListCell   *lc;
+
+        foreach(lc, root->minmax_aggs)
+        {
+            MinMaxAggInfo *mminfo = (MinMaxAggInfo *) lfirst(lc);
+
+            if (mminfo->aggfnoid == aggref->aggfnoid &&
+                equal(mminfo->target, curTarget->expr))
+                return mminfo->param;
+        }
+    }
+    return NULL;
+}
+
+
 /*****************************************************************************
  *                    QUERY DEPENDENCY MANAGEMENT
  *****************************************************************************/
diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index da2d8abe01..250ba8edcb 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -2835,8 +2835,8 @@ finalize_plan(PlannerInfo *root, Plan *plan,
 }

 /*
- * finalize_primnode: add IDs of all PARAM_EXEC params appearing in the given
- * expression tree to the result set.
+ * finalize_primnode: add IDs of all PARAM_EXEC params that appear (or will
+ * appear) in the given expression tree to the result set.
  */
 static bool
 finalize_primnode(Node *node, finalize_primnode_context *context)
@@ -2853,7 +2853,26 @@ finalize_primnode(Node *node, finalize_primnode_context *context)
         }
         return false;            /* no more to do here */
     }
-    if (IsA(node, SubPlan))
+    else if (IsA(node, Aggref))
+    {
+        /*
+         * Check to see if the aggregate will be replaced by a Param
+         * referencing a subquery output during setrefs.c.  If so, we must
+         * account for that Param here.  (For various reasons, it's not
+         * convenient to perform that substitution earlier than setrefs.c, nor
+         * to perform this processing after setrefs.c.  Thus we need a wart
+         * here.)
+         */
+        Aggref       *aggref = (Aggref *) node;
+        Param       *aggparam;
+
+        aggparam = find_minmax_agg_replacement_param(context->root, aggref);
+        if (aggparam != NULL)
+            context->paramids = bms_add_member(context->paramids,
+                                               aggparam->paramid);
+        /* Fall through to examine the agg's arguments */
+    }
+    else if (IsA(node, SubPlan))
     {
         SubPlan    *subplan = (SubPlan *) node;
         Plan       *plan = planner_subplan_get_plan(context->root, subplan);
diff --git a/src/include/optimizer/planmain.h b/src/include/optimizer/planmain.h
index 5fc900737d..31c188176b 100644
--- a/src/include/optimizer/planmain.h
+++ b/src/include/optimizer/planmain.h
@@ -110,6 +110,8 @@ extern bool innerrel_is_unique(PlannerInfo *root,
  */
 extern Plan *set_plan_references(PlannerInfo *root, Plan *plan);
 extern bool trivial_subqueryscan(SubqueryScan *plan);
+extern Param *find_minmax_agg_replacement_param(PlannerInfo *root,
+                                                Aggref *aggref);
 extern void record_plan_function_dependency(PlannerInfo *root, Oid funcid);
 extern void record_plan_type_dependency(PlannerInfo *root, Oid typid);
 extern bool extract_query_dependencies_walker(Node *node, PlannerInfo *context);
--
2.39.3

From fa74d0a664e56aee98b9737779d3768deb2cfb7f Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 13 Jul 2023 17:30:14 -0400
Subject: [PATCH v2 2/2] Allow plan nodes with initPlans to be considered
 parallel-safe.

If the plan itself is parallel-safe, and the initPlans are too,
there's no reason anymore to prevent the plan from being marked
parallel-safe.  That restriction (dating to commit ab77a5a45) was
really a special case of the fact that we couldn't transmit subplans
to parallel workers at all.  We fixed that in commit 5e6d8d2bb and
follow-ons, but this case never got addressed.

We still forbid attaching initPlans to a Gather node that's
inserted pursuant to debug_parallel_query = regress.  That's because,
when we hide the Gather from EXPLAIN output, we'd hide the initPlans
too, causing cosmetic regression diffs.  It seems inadvisable to
kluge EXPLAIN to the extent required to make the output look the
same, so just don't do it in that case.

Along the way, this also takes care of some sloppiness about updating
path costs to match when we move initplans from one place to another
during createplan.c and setrefs.c.  Since all the planning decisions
are already made by that point, this is just cosmetic; but it seems
good to keep EXPLAIN output consistent with where the initplans are.

The diff in query_planner() might be worth remarking on.  I found that
one because after fixing things to allow parallel-safe initplans, one
partition_prune test case changed plans (as shown in the patch) ---
but only when debug_parallel_query was active.  The reason proved to
be that we only bothered to mark Result nodes as potentially
parallel-safe when debug_parallel_query is on.  This neglects the fact
that parallel-safety may be of interest for a sub-query even though
the Result itself doesn't parallelize.

Discussion: https://postgr.es/m/1129530.1681317832@sss.pgh.pa.us
---
 src/backend/optimizer/plan/createplan.c       | 15 +++-
 src/backend/optimizer/plan/planmain.c         | 15 ++--
 src/backend/optimizer/plan/planner.c          | 36 ++++++--
 src/backend/optimizer/plan/setrefs.c          | 32 ++++---
 src/backend/optimizer/plan/subselect.c        | 84 ++++++++++++++-----
 src/backend/optimizer/util/pathnode.c         | 18 +++-
 src/include/optimizer/subselect.h             |  3 +
 src/test/regress/expected/partition_prune.out | 37 ++++----
 8 files changed, 169 insertions(+), 71 deletions(-)

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index ec73789bc2..af48109058 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -6481,6 +6481,8 @@ materialize_finished_plan(Plan *subplan)
 {
     Plan       *matplan;
     Path        matpath;        /* dummy for result of cost_material */
+    Cost        initplan_cost;
+    bool        unsafe_initplans;

     matplan = (Plan *) make_material(subplan);

@@ -6488,20 +6490,25 @@ materialize_finished_plan(Plan *subplan)
      * XXX horrid kluge: if there are any initPlans attached to the subplan,
      * move them up to the Material node, which is now effectively the top
      * plan node in its query level.  This prevents failure in
-     * SS_finalize_plan(), which see for comments.  We don't bother adjusting
-     * the subplan's cost estimate for this.
+     * SS_finalize_plan(), which see for comments.
      */
     matplan->initPlan = subplan->initPlan;
     subplan->initPlan = NIL;

+    /* Move the initplans' cost delta, as well */
+    SS_compute_initplan_cost(matplan->initPlan,
+                             &initplan_cost, &unsafe_initplans);
+    subplan->startup_cost -= initplan_cost;
+    subplan->total_cost -= initplan_cost;
+
     /* Set cost data */
     cost_material(&matpath,
                   subplan->startup_cost,
                   subplan->total_cost,
                   subplan->plan_rows,
                   subplan->plan_width);
-    matplan->startup_cost = matpath.startup_cost;
-    matplan->total_cost = matpath.total_cost;
+    matplan->startup_cost = matpath.startup_cost + initplan_cost;
+    matplan->total_cost = matpath.total_cost + initplan_cost;
     matplan->plan_rows = subplan->plan_rows;
     matplan->plan_width = subplan->plan_width;
     matplan->parallel_aware = false;
diff --git a/src/backend/optimizer/plan/planmain.c b/src/backend/optimizer/plan/planmain.c
index 7afd434c60..fcc0eacd25 100644
--- a/src/backend/optimizer/plan/planmain.c
+++ b/src/backend/optimizer/plan/planmain.c
@@ -112,14 +112,17 @@ query_planner(PlannerInfo *root,
                  * quals are parallel-restricted.  (We need not check
                  * final_rel->reltarget because it's empty at this point.
                  * Anything parallel-restricted in the query tlist will be
-                 * dealt with later.)  This is normally pretty silly, because
-                 * a Result-only plan would never be interesting to
-                 * parallelize.  However, if debug_parallel_query is on, then
-                 * we want to execute the Result in a parallel worker if
-                 * possible, so we must do this.
+                 * dealt with later.)  We should always do this in a subquery,
+                 * since it might be useful to use the subquery in parallel
+                 * paths in the parent level.  At top level this is normally
+                 * not worth the cycles, because a Result-only plan would
+                 * never be interesting to parallelize.  However, if
+                 * debug_parallel_query is on, then we want to execute the
+                 * Result in a parallel worker if possible, so we must check.
                  */
                 if (root->glob->parallelModeOK &&
-                    debug_parallel_query != DEBUG_PARALLEL_OFF)
+                    (root->query_level > 1 ||
+                     debug_parallel_query != DEBUG_PARALLEL_OFF))
                     final_rel->consider_parallel =
                         is_parallel_safe(root, parse->jointree->quals);

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 0e12fdeb60..44efb1f4eb 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -432,16 +432,23 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
     /*
      * Optionally add a Gather node for testing purposes, provided this is
      * actually a safe thing to do.
-     */
-    if (debug_parallel_query != DEBUG_PARALLEL_OFF && top_plan->parallel_safe)
+     *
+     * We can add Gather even when top_plan has parallel-safe initPlans, but
+     * then we have to move the initPlans to the Gather node because of
+     * SS_finalize_plan's limitations.  That would cause cosmetic breakage of
+     * regression tests when debug_parallel_query = regress, because initPlans
+     * that would normally appear on the top_plan move to the Gather, causing
+     * them to disappear from EXPLAIN output.  That doesn't seem worth kluging
+     * EXPLAIN to hide, so skip it when debug_parallel_query = regress.
+     */
+    if (debug_parallel_query != DEBUG_PARALLEL_OFF &&
+        top_plan->parallel_safe &&
+        (top_plan->initPlan == NIL ||
+         debug_parallel_query != DEBUG_PARALLEL_REGRESS))
     {
         Gather       *gather = makeNode(Gather);
-
-        /*
-         * Top plan must not have any initPlans, else it shouldn't have been
-         * marked parallel-safe.
-         */
-        Assert(top_plan->initPlan == NIL);
+        Cost        initplan_cost;
+        bool        unsafe_initplans;

         gather->plan.targetlist = top_plan->targetlist;
         gather->plan.qual = NIL;
@@ -451,6 +458,10 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
         gather->single_copy = true;
         gather->invisible = (debug_parallel_query == DEBUG_PARALLEL_REGRESS);

+        /* Transfer any initPlans to the new top node */
+        gather->plan.initPlan = top_plan->initPlan;
+        top_plan->initPlan = NIL;
+
         /*
          * Since this Gather has no parallel-aware descendants to signal to,
          * we don't need a rescan Param.
@@ -470,6 +481,15 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
         gather->plan.parallel_aware = false;
         gather->plan.parallel_safe = false;

+        /*
+         * Delete the initplans' cost from top_plan.  We needn't add it to the
+         * Gather node, since the above coding already included it there.
+         */
+        SS_compute_initplan_cost(gather->plan.initPlan,
+                                 &initplan_cost, &unsafe_initplans);
+        top_plan->startup_cost -= initplan_cost;
+        top_plan->total_cost -= initplan_cost;
+
         /* use parallel mode for parallel plans. */
         root->glob->parallelModeNeeded = true;

diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index 16e5537f7f..97fa561e4e 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -23,6 +23,7 @@
 #include "optimizer/pathnode.h"
 #include "optimizer/planmain.h"
 #include "optimizer/planner.h"
+#include "optimizer/subselect.h"
 #include "optimizer/tlist.h"
 #include "parser/parse_relation.h"
 #include "tcop/utility.h"
@@ -1519,19 +1520,30 @@ clean_up_removed_plan_level(Plan *parent, Plan *child)
 {
     /*
      * We have to be sure we don't lose any initplans, so move any that were
-     * attached to the parent plan to the child.  If we do move any, the child
-     * is no longer parallel-safe.
+     * attached to the parent plan to the child.  If any are parallel-unsafe,
+     * the child is no longer parallel-safe.  As a cosmetic matter, also add
+     * the initplans' run costs to the child's costs.
      */
     if (parent->initPlan)
-        child->parallel_safe = false;
+    {
+        Cost        initplan_cost;
+        bool        unsafe_initplans;

-    /*
-     * Attach plans this way so that parent's initplans are processed before
-     * any pre-existing initplans of the child.  Probably doesn't matter, but
-     * let's preserve the ordering just in case.
-     */
-    child->initPlan = list_concat(parent->initPlan,
-                                  child->initPlan);
+        SS_compute_initplan_cost(parent->initPlan,
+                                 &initplan_cost, &unsafe_initplans);
+        child->startup_cost += initplan_cost;
+        child->total_cost += initplan_cost;
+        if (unsafe_initplans)
+            child->parallel_safe = false;
+
+        /*
+         * Attach plans this way so that parent's initplans are processed
+         * before any pre-existing initplans of the child.  Probably doesn't
+         * matter, but let's preserve the ordering just in case.
+         */
+        child->initPlan = list_concat(parent->initPlan,
+                                      child->initPlan);
+    }

     /*
      * We also have to transfer the parent's column labeling info into the
diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index 250ba8edcb..7a9fe88fec 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -1016,8 +1016,7 @@ SS_process_ctes(PlannerInfo *root)

         /*
          * CTE scans are not considered for parallelism (cf
-         * set_rel_consider_parallel), and even if they were, initPlans aren't
-         * parallel-safe.
+         * set_rel_consider_parallel).
          */
         splan->parallel_safe = false;
         splan->setParam = NIL;
@@ -2120,8 +2119,8 @@ SS_identify_outer_params(PlannerInfo *root)
  * If any initPlans have been created in the current query level, they will
  * get attached to the Plan tree created from whichever Path we select from
  * the given rel.  Increment all that rel's Paths' costs to account for them,
- * and make sure the paths get marked as parallel-unsafe, since we can't
- * currently transmit initPlans to parallel workers.
+ * and if any of the initPlans are parallel-unsafe, mark all the rel's Paths
+ * parallel-unsafe as well.
  *
  * This is separate from SS_attach_initplans because we might conditionally
  * create more initPlans during create_plan(), depending on which Path we
@@ -2132,6 +2131,7 @@ void
 SS_charge_for_initplans(PlannerInfo *root, RelOptInfo *final_rel)
 {
     Cost        initplan_cost;
+    bool        unsafe_initplans;
     ListCell   *lc;

     /* Nothing to do if no initPlans */
@@ -2140,17 +2140,10 @@ SS_charge_for_initplans(PlannerInfo *root, RelOptInfo *final_rel)

     /*
      * Compute the cost increment just once, since it will be the same for all
-     * Paths.  We assume each initPlan gets run once during top plan startup.
-     * This is a conservative overestimate, since in fact an initPlan might be
-     * executed later than plan startup, or even not at all.
+     * Paths.  Also check for parallel-unsafe initPlans.
      */
-    initplan_cost = 0;
-    foreach(lc, root->init_plans)
-    {
-        SubPlan    *initsubplan = (SubPlan *) lfirst(lc);
-
-        initplan_cost += initsubplan->startup_cost + initsubplan->per_call_cost;
-    }
+    SS_compute_initplan_cost(root->init_plans,
+                             &initplan_cost, &unsafe_initplans);

     /*
      * Now adjust the costs and parallel_safe flags.
@@ -2161,19 +2154,71 @@ SS_charge_for_initplans(PlannerInfo *root, RelOptInfo *final_rel)

         path->startup_cost += initplan_cost;
         path->total_cost += initplan_cost;
-        path->parallel_safe = false;
+        if (unsafe_initplans)
+            path->parallel_safe = false;
     }

     /*
-     * Forget about any partial paths and clear consider_parallel, too;
-     * they're not usable if we attached an initPlan.
+     * Adjust partial paths' costs too, or forget them entirely if we must
+     * consider the rel parallel-unsafe.
      */
-    final_rel->partial_pathlist = NIL;
-    final_rel->consider_parallel = false;
+    if (unsafe_initplans)
+    {
+        final_rel->partial_pathlist = NIL;
+        final_rel->consider_parallel = false;
+    }
+    else
+    {
+        foreach(lc, final_rel->partial_pathlist)
+        {
+            Path       *path = (Path *) lfirst(lc);
+
+            path->startup_cost += initplan_cost;
+            path->total_cost += initplan_cost;
+        }
+    }

     /* We needn't do set_cheapest() here, caller will do it */
 }

+/*
+ * SS_compute_initplan_cost - count up the cost delta for some initplans
+ *
+ * The total cost returned in *initplan_cost_p should be added to both the
+ * startup and total costs of the plan node the initplans get attached to.
+ * We also report whether any of the initplans are not parallel-safe.
+ *
+ * The primary user of this is SS_charge_for_initplans, but it's also
+ * used in adjusting costs when we move initplans to another plan node.
+ */
+void
+SS_compute_initplan_cost(List *init_plans,
+                         Cost *initplan_cost_p,
+                         bool *unsafe_initplans_p)
+{
+    Cost        initplan_cost;
+    bool        unsafe_initplans;
+    ListCell   *lc;
+
+    /*
+     * We assume each initPlan gets run once during top plan startup.  This is
+     * a conservative overestimate, since in fact an initPlan might be
+     * executed later than plan startup, or even not at all.
+     */
+    initplan_cost = 0;
+    unsafe_initplans = false;
+    foreach(lc, init_plans)
+    {
+        SubPlan    *initsubplan = lfirst_node(SubPlan, lc);
+
+        initplan_cost += initsubplan->startup_cost + initsubplan->per_call_cost;
+        if (!initsubplan->parallel_safe)
+            unsafe_initplans = true;
+    }
+    *initplan_cost_p = initplan_cost;
+    *unsafe_initplans_p = unsafe_initplans;
+}
+
 /*
  * SS_attach_initplans - attach initplans to topmost plan node
  *
@@ -2990,6 +3035,7 @@ SS_make_initplan_from_plan(PlannerInfo *root,
                                node->plan_id, prm->paramid);
     get_first_col_type(plan, &node->firstColType, &node->firstColTypmod,
                        &node->firstColCollation);
+    node->parallel_safe = plan->parallel_safe;
     node->setParam = list_make1_int(prm->paramid);

     root->init_plans = lappend(root->init_plans, node);
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 5f5596841c..f123fcb41e 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -3348,8 +3348,7 @@ create_minmaxagg_path(PlannerInfo *root,
     /* For now, assume we are above any joins, so no parameterization */
     pathnode->path.param_info = NULL;
     pathnode->path.parallel_aware = false;
-    /* A MinMaxAggPath implies use of initplans, so cannot be parallel-safe */
-    pathnode->path.parallel_safe = false;
+    pathnode->path.parallel_safe = true;    /* might change below */
     pathnode->path.parallel_workers = 0;
     /* Result is one unordered row */
     pathnode->path.rows = 1;
@@ -3358,13 +3357,15 @@ create_minmaxagg_path(PlannerInfo *root,
     pathnode->mmaggregates = mmaggregates;
     pathnode->quals = quals;

-    /* Calculate cost of all the initplans ... */
+    /* Calculate cost of all the initplans, and check parallel safety */
     initplan_cost = 0;
     foreach(lc, mmaggregates)
     {
         MinMaxAggInfo *mminfo = (MinMaxAggInfo *) lfirst(lc);

         initplan_cost += mminfo->pathcost;
+        if (!mminfo->path->parallel_safe)
+            pathnode->path.parallel_safe = false;
     }

     /* add tlist eval cost for each output row, plus cpu_tuple_cost */
@@ -3385,6 +3386,17 @@ create_minmaxagg_path(PlannerInfo *root,
         pathnode->path.total_cost += qual_cost.startup + qual_cost.per_tuple;
     }

+    /*
+     * If the initplans were all parallel-safe, also check safety of the
+     * target and quals.  (The Result node itself isn't parallelizable, but if
+     * we are in a subquery then it can be useful for the outer query to know
+     * that this one is parallel-safe.)
+     */
+    if (pathnode->path.parallel_safe)
+        pathnode->path.parallel_safe =
+            is_parallel_safe(root, (Node *) target->exprs) &&
+            is_parallel_safe(root, (Node *) quals);
+
     return pathnode;
 }

diff --git a/src/include/optimizer/subselect.h b/src/include/optimizer/subselect.h
index c03ffc56bf..44bc0bda7e 100644
--- a/src/include/optimizer/subselect.h
+++ b/src/include/optimizer/subselect.h
@@ -28,6 +28,9 @@ extern Node *SS_replace_correlation_vars(PlannerInfo *root, Node *expr);
 extern Node *SS_process_sublinks(PlannerInfo *root, Node *expr, bool isQual);
 extern void SS_identify_outer_params(PlannerInfo *root);
 extern void SS_charge_for_initplans(PlannerInfo *root, RelOptInfo *final_rel);
+extern void SS_compute_initplan_cost(List *init_plans,
+                                     Cost *initplan_cost_p,
+                                     bool *unsafe_initplans_p);
 extern void SS_attach_initplans(PlannerInfo *root, Plan *plan);
 extern void SS_finalize_plan(PlannerInfo *root, Plan *plan);
 extern Param *SS_make_initplan_output_param(PlannerInfo *root,
diff --git a/src/test/regress/expected/partition_prune.out b/src/test/regress/expected/partition_prune.out
index 2abf759385..1eb347503a 100644
--- a/src/test/regress/expected/partition_prune.out
+++ b/src/test/regress/expected/partition_prune.out
@@ -3903,30 +3903,25 @@ select explain_parallel_append(
 select * from listp where a = (select 2);');
                               explain_parallel_append
 -----------------------------------------------------------------------------------
- Append (actual rows=N loops=N)
-   ->  Gather (actual rows=N loops=N)
-         Workers Planned: 2
-         Params Evaluated: $0
-         Workers Launched: N
-         InitPlan 1 (returns $0)
-           ->  Result (actual rows=N loops=N)
-         ->  Parallel Append (actual rows=N loops=N)
-               ->  Seq Scan on listp_12_1 listp_1 (actual rows=N loops=N)
-                     Filter: (a = $0)
-               ->  Parallel Seq Scan on listp_12_2 listp_2 (never executed)
-                     Filter: (a = $0)
-   ->  Gather (actual rows=N loops=N)
-         Workers Planned: 2
-         Params Evaluated: $1
-         Workers Launched: N
-         InitPlan 2 (returns $1)
-           ->  Result (actual rows=N loops=N)
+ Gather (actual rows=N loops=N)
+   Workers Planned: 2
+   Workers Launched: N
+   ->  Parallel Append (actual rows=N loops=N)
          ->  Parallel Append (actual rows=N loops=N)
-               ->  Seq Scan on listp_12_1 listp_4 (never executed)
+               InitPlan 2 (returns $1)
+                 ->  Result (actual rows=N loops=N)
+               ->  Seq Scan on listp_12_1 listp_1 (never executed)
                      Filter: (a = $1)
-               ->  Parallel Seq Scan on listp_12_2 listp_5 (actual rows=N loops=N)
+               ->  Parallel Seq Scan on listp_12_2 listp_2 (actual rows=N loops=N)
                      Filter: (a = $1)
-(23 rows)
+         ->  Parallel Append (actual rows=N loops=N)
+               InitPlan 1 (returns $0)
+                 ->  Result (actual rows=N loops=N)
+               ->  Seq Scan on listp_12_1 listp_4 (actual rows=N loops=N)
+                     Filter: (a = $0)
+               ->  Parallel Seq Scan on listp_12_2 listp_5 (never executed)
+                     Filter: (a = $0)
+(18 rows)

 drop table listp;
 reset parallel_tuple_cost;
--
2.39.3