should INSERT SELECT use a BulkInsertState?

Seems to me it should, at least conditionally.  At least if there's a function
scan or a relation or ..

I mentioned a bit about our use-case here:
https://www.postgresql.org/message-id/20200219173742.GA30939%40telsasoft.com
=> I'd prefer our loaders to write their own data rather than dirtying large
fractions of buffer cache and leaving it around for other backends to clean up.

commit 7f9e061363e58f30eee0cccc8a0e46f637bf137b
Author: Justin Pryzby <pryzbyj@telsasoft.com>
Date:   Fri May 8 02:17:32 2020 -0500

    Make INSERT SELECT use a BulkInsertState

diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index 20a4c474cc..6da4325225 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -578,7 +578,7 @@ ExecInsert(ModifyTableState *mtstate,
             table_tuple_insert_speculative(resultRelationDesc, slot,
                                            estate->es_output_cid,
                                            0,
-                                           NULL,
+                                           mtstate->bistate,
                                            specToken);
 
             /* insert index entries for tuple */
@@ -617,7 +617,7 @@ ExecInsert(ModifyTableState *mtstate,
             /* insert the tuple normally */
             table_tuple_insert(resultRelationDesc, slot,
                                estate->es_output_cid,
-                               0, NULL);
+                               0, mtstate->bistate);
 
             /* insert index entries for tuple */
             if (resultRelInfo->ri_NumIndices > 0)
@@ -2332,6 +2332,14 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 
     mtstate->mt_arowmarks = (List **) palloc0(sizeof(List *) * nplans);
     mtstate->mt_nplans = nplans;
+    mtstate->bistate = NULL;
+    if (operation == CMD_INSERT)
+    {
+        Plan *p = linitial(node->plans);
+        Assert(nplans == 1);
+        if (!IsA(p, Result) && !IsA(p, ValuesScan))
+            mtstate->bistate = GetBulkInsertState();
+    }
 
     /* set up epqstate with dummy subplan data for the moment */
     EvalPlanQualInit(&mtstate->mt_epqstate, estate, NULL, NIL, node->epqParam);
@@ -2809,6 +2817,9 @@ ExecEndModifyTable(ModifyTableState *node)
      */
     for (i = 0; i < node->mt_nplans; i++)
         ExecEndNode(node->mt_plans[i]);
+
+    if (node->bistate)
+        FreeBulkInsertState(node->bistate);
 }
 
 void
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 4fee043bb2..daf365f181 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -14,6 +14,7 @@
 #ifndef EXECNODES_H
 #define EXECNODES_H
 
+#include "access/heapam.h"
 #include "access/tupconvert.h"
 #include "executor/instrument.h"
 #include "fmgr.h"
@@ -1177,6 +1178,7 @@ typedef struct ModifyTableState
     List      **mt_arowmarks;    /* per-subplan ExecAuxRowMark lists */
     EPQState    mt_epqstate;    /* for evaluating EvalPlanQual rechecks */
     bool        fireBSTriggers; /* do we need to fire stmt triggers? */
+    BulkInsertState    bistate;    /* State for bulk insert like INSERT SELECT */
 
     /*
      * Slot for storing tuples in the root partitioned table's rowtype during