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(at)telsasoft(dot)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