Expand comments and add an assertion in nodeModifyTable.c.

Most comments concern RELKIND_VIEW.  One addresses the ExecUpdate()
"tupleid" parameter.  A later commit will rely on these facts, but they
hold already.  Back-patch to v12 (all supported versions), the plan for
that commit.

Reviewed (in an earlier version) by Robert Haas.

Discussion: https://postgr.es/m/20240512232923.aa.nmisch@google.com

src/backend/executor/nodeModifyTable.c

@@ -24,6 +24,14 @@
  *		values plus row-locating info for UPDATE and MERGE cases, or just the
  *		row-locating info for DELETE cases.
  *
+ *		The relation to modify can be an ordinary table, a view having an
+ *		INSTEAD OF trigger, or a foreign table.  Earlier processing already
+ *		pointed ModifyTable to the underlying relations of any automatically
+ *		updatable view not using an INSTEAD OF trigger, so code here can
+ *		assume it won't have one as a modification target.  This node does
+ *		process ri_WithCheckOptions, which may have expressions from those
+ *		automatically updatable views.
+ *
  *		MERGE runs a join between the source relation and the target
  *		table; if any WHEN NOT MATCHED clauses are present, then the
  *		join is an outer join.  In this case, any unmatched tuples will
@@ -1401,18 +1409,18 @@ ExecDeleteEpilogue(ModifyTableContext *context, ResultRelInfo *resultRelInfo,
  *		DELETE is like UPDATE, except that we delete the tuple and no
  *		index modifications are needed.
  *
- *		When deleting from a table, tupleid identifies the tuple to
+ *		When deleting from a table, tupleid identifies the tuple to delete and
- *		delete and oldtuple is NULL.  When deleting from a view,
+ *		oldtuple is NULL.  When deleting through a view INSTEAD OF trigger,
- *		oldtuple is passed to the INSTEAD OF triggers and identifies
+ *		oldtuple is passed to the triggers and identifies what to delete, and
- *		what to delete, and tupleid is invalid.  When deleting from a
+ *		tupleid is invalid.  When deleting from a foreign table, tupleid is
- *		foreign table, tupleid is invalid; the FDW has to figure out
+ *		invalid; the FDW has to figure out which row to delete using data from
- *		which row to delete using data from the planSlot.  oldtuple is
+ *		the planSlot.  oldtuple is passed to foreign table triggers; it is
- *		passed to foreign table triggers; it is NULL when the foreign
+ *		NULL when the foreign table has no relevant triggers.  We use
- *		table has no relevant triggers.  We use tupleDeleted to indicate
+ *		tupleDeleted to indicate whether the tuple is actually deleted,
- *		whether the tuple is actually deleted, callers can use it to
+ *		callers can use it to decide whether to continue the operation.  When
- *		decide whether to continue the operation.  When this DELETE is a
+ *		this DELETE is a part of an UPDATE of partition-key, then the slot
- *		part of an UPDATE of partition-key, then the slot returned by
+ *		returned by EvalPlanQual() is passed back using output parameter
- *		EvalPlanQual() is passed back using output parameter epqreturnslot.
+ *		epqreturnslot.
  *
  *		Returns RETURNING result if any, otherwise NULL.
  * ----------------------------------------------------------------
@@ -2244,21 +2252,22 @@ ExecCrossPartitionUpdateForeignKey(ModifyTableContext *context,
  *		is, we don't want to get stuck in an infinite loop
  *		which corrupts your database..
  *
- *		When updating a table, tupleid identifies the tuple to
+ *		When updating a table, tupleid identifies the tuple to update and
- *		update and oldtuple is NULL.  When updating a view, oldtuple
+ *		oldtuple is NULL.  When updating through a view INSTEAD OF trigger,
- *		is passed to the INSTEAD OF triggers and identifies what to
+ *		oldtuple is passed to the triggers and identifies what to update, and
- *		update, and tupleid is invalid.  When updating a foreign table,
+ *		tupleid is invalid.  When updating a foreign table, tupleid is
- *		tupleid is invalid; the FDW has to figure out which row to
+ *		invalid; the FDW has to figure out which row to update using data from
- *		update using data from the planSlot.  oldtuple is passed to
+ *		the planSlot.  oldtuple is passed to foreign table triggers; it is
- *		foreign table triggers; it is NULL when the foreign table has
+ *		NULL when the foreign table has no relevant triggers.
- *		no relevant triggers.
  *
  *		slot contains the new tuple value to be stored.
  *		planSlot is the output of the ModifyTable's subplan; we use it
  *		to access values from other input tables (for RETURNING),
  *		row-ID junk columns, etc.
  *
- *		Returns RETURNING result if any, otherwise NULL.
+ *		Returns RETURNING result if any, otherwise NULL.  On exit, if tupleid
+ *		had identified the tuple to update, it will identify the tuple
+ *		actually updated after EvalPlanQual.
  * ----------------------------------------------------------------
  */
 static TupleTableSlot *
@@ -3800,8 +3809,8 @@ ExecModifyTable(PlanState *pstate)
 			 * know enough here to set t_tableOid.  Quite separately from
 			 * this, the FDW may fetch its own junk attrs to identify the row.
 			 *
-			 * Other relevant relkinds, currently limited to views, always
+			 * Other relevant relkinds, currently limited to views having
-			 * have a wholerow attribute.
+			 * INSTEAD OF triggers, always have a wholerow attribute.
 			 */
 			else if (AttributeNumberIsValid(resultRelInfo->ri_RowIdAttNo))
 			{