Support ON CONFLICT DO SELECT on hypertables for PostgreSQL 19

PostgreSQL 19 added INSERT ... ON CONFLICT DO SELECT, which returns the
conflicting row instead of updating it and can optionally lock it.

Upstream changes:
Add support for INSERT ... ON CONFLICT DO SELECT.
postgres/postgres@88327092ff0

Author: svenklemm

.unreleased/pr_10279

@@ -0,0 +1 @@
+Implements: #10279 Support ON CONFLICT DO SELECT with hypertables

src/chunk_insert_state.c

@@ -230,14 +230,17 @@ setup_on_conflict_state(ResultRelInfo *ht_rri, ModifyTableState *mtstate, ChunkI
 	Relation hyper_rel = ht_rri->ri_RelationDesc;
 	ModifyTable *mt = castNode(ModifyTable, mtstate->ps.plan);
 
+	/* DO UPDATE has a SET clause; DO SELECT only fetches the conflicting row. */
+	bool do_update = mt->onConflictAction == ONCONFLICT_UPDATE;
+
 	OnConflictActionState *onconfl = makeNode(OnConflictActionState);
 	memcpy(onconfl, ht_rri->ri_onConflict, sizeof(OnConflictActionState));
 	chunk_rri->ri_onConflict = onconfl;
 
 	chunk_rri->ri_RootToChildMap = map;
 	chunk_rri->ri_RootToChildMapValid = true;
 
-	Assert(mt->onConflictSet);
+	Assert(!do_update || mt->onConflictSet);
 	Assert(ht_rri->ri_onConflict != NULL);
 
 	/*
@@ -271,18 +274,6 @@ setup_on_conflict_state(ResultRelInfo *ht_rri, ModifyTableState *mtstate, ChunkI
 	}
 	else
 	{
-		List *onconflset;
-		List *onconflcols;
-
-		/*
-		 * Translate expressions in onConflictSet to account for
-		 * different attribute numbers.  For that, map partition
-		 * varattnos twice: first to catch the EXCLUDED
-		 * pseudo-relation (INNER_VAR), and second to handle the main
-		 * target relation (firstVarno).
-		 */
-		onconflset = copyObject(mt->onConflictSet);
-
 		Assert(map->outdesc == RelationGetDescr(chunk_rel));
 
 		if (!chunk_map)
@@ -291,33 +282,52 @@ setup_on_conflict_state(ResultRelInfo *ht_rri, ModifyTableState *mtstate, ChunkI
 				convert_tuples_by_name(RelationGetDescr(chunk_rel), RelationGetDescr(hyper_rel));
 		}
 
-		onconflset = translate_clause(onconflset, chunk_map, ht_rri->ri_RangeTableIndex, chunk_rel);
-
 		chunk_rri->ri_ChildToRootMap = chunk_map;
 		chunk_rri->ri_ChildToRootMapValid = true;
 
-		/* Finally, adjust the target colnos to match the chunk. */
-		if (chunk_map)
+		if (do_update)
 		{
-			onconflcols = adjust_chunk_colnos(mt->onConflictCols, chunk_rri);
+			List *onconflset;
+			List *onconflcols;
+
+			/*
+			 * Translate expressions in onConflictSet to account for
+			 * different attribute numbers.  For that, map partition
+			 * varattnos twice: first to catch the EXCLUDED
+			 * pseudo-relation (INNER_VAR), and second to handle the main
+			 * target relation (firstVarno).
+			 */
+			onconflset = copyObject(mt->onConflictSet);
+			onconflset =
+				translate_clause(onconflset, chunk_map, ht_rri->ri_RangeTableIndex, chunk_rel);
+
+			/*
+			 * Finally, adjust the target colnos to match the chunk. When the
+			 * descriptors match (e.g. a direct chunk insert) there is no
+			 * child-to-root map and the parent colnos apply unchanged.
+			 */
+			if (chunk_map)
+			{
+				onconflcols = adjust_chunk_colnos(mt->onConflictCols, chunk_rri);
+			}
+			else
+			{
+				onconflcols = mt->onConflictCols;
+			}
+
+			/* create the tuple slot for the UPDATE SET projection */
+			onconfl->oc_ProjSlot = table_slot_create(chunk_rel, NULL);
+			state->conflproj_slot = onconfl->oc_ProjSlot;
+
+			/* build UPDATE SET projection state */
+			onconfl->oc_ProjInfo = ExecBuildUpdateProjection(onconflset,
+															 true,
+															 onconflcols,
+															 RelationGetDescr(chunk_rel),
+															 mtstate->ps.ps_ExprContext,
+															 onconfl->oc_ProjSlot,
+															 &mtstate->ps);
 		}
-		else
-		{
-			onconflcols = mt->onConflictCols;
-		}
-
-		/* create the tuple slot for the UPDATE SET projection */
-		onconfl->oc_ProjSlot = table_slot_create(chunk_rel, NULL);
-		state->conflproj_slot = onconfl->oc_ProjSlot;
-
-		/* build UPDATE SET projection state */
-		onconfl->oc_ProjInfo = ExecBuildUpdateProjection(onconflset,
-														 true,
-														 onconflcols,
-														 RelationGetDescr(chunk_rel),
-														 mtstate->ps.ps_ExprContext,
-														 onconfl->oc_ProjSlot,
-														 &mtstate->ps);
 
 		Node *onconflict_where = mt->onConflictWhere;
 
@@ -406,7 +416,11 @@ adjust_projections(ResultRelInfo *ht_rri, ModifyTableState *mtstate, ChunkInsert
 	{
 		set_arbiter_indexes(cis, ht_rri->ri_onConflictArbiterIndexes);
 
-		if (onConflictAction == ONCONFLICT_UPDATE)
+		if (onConflictAction == ONCONFLICT_UPDATE
+#if PG19_GE
+			|| onConflictAction == ONCONFLICT_SELECT
+#endif
+		)
 		{
 			setup_on_conflict_state(ht_rri, mtstate, cis, chunk_map);
 		}