Remove p_is_insert from struct ParseState.

The only place that used p_is_insert was transformAssignedExpr(),
which used it to distinguish INSERT from UPDATE when handling
indirection on assignment target columns -- see commit c1ca3a19df.
However, this information is already available to
transformAssignedExpr() via its exprKind parameter, which is always
either EXPR_KIND_INSERT_TARGET or EXPR_KIND_UPDATE_TARGET.

As noted in the commit message for c1ca3a19df, this use of
p_is_insert isn't particularly pretty, so have transformAssignedExpr()
use the exprKind parameter instead. This then allows p_is_insert to be
removed entirely, which simplifies state management in a few other
places across the parser.

Author: Viktor Holmberg <v@viktorh.net>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Discussion: https://postgr.es/m/badc3b4c-da73-4000-b8d3-638a6f53a769@Spark

src/backend/parser/analyze.c

@@ -657,7 +657,6 @@ transformInsertStmt(ParseState *pstate, InsertStmt *stmt)
 	Assert(pstate->p_ctenamespace == NIL);
 
 	qry->commandType = CMD_INSERT;
-	pstate->p_is_insert = true;
 
 	/* process the WITH clause independently of all else */
 	if (stmt->withClause)
@@ -1222,13 +1221,6 @@ transformOnConflictClause(ParseState *pstate,
 	/* Process DO UPDATE */
 	if (onConflictClause->action == ONCONFLICT_UPDATE)
 	{
-		/*
-		 * Expressions in the UPDATE targetlist need to be handled like UPDATE
-		 * not INSERT.  We don't need to save/restore this because all INSERT
-		 * expressions have been parsed already.
-		 */
-		pstate->p_is_insert = false;
-
 		/*
 		 * Add the EXCLUDED pseudo relation to the query namespace, making it
 		 * available in the UPDATE subexpressions.
@@ -2495,7 +2487,6 @@ transformUpdateStmt(ParseState *pstate, UpdateStmt *stmt)
 	Node	   *qual;
 
 	qry->commandType = CMD_UPDATE;
-	pstate->p_is_insert = false;
 
 	/* process the WITH clause independently of all else */
 	if (stmt->withClause)

src/backend/parser/parse_merge.c

@@ -307,8 +307,6 @@ transformMergeStmt(ParseState *pstate, MergeStmt *stmt)
 					List	   *icolumns;
 					List	   *attrnos;
 
-					pstate->p_is_insert = true;
-
 					icolumns = checkInsertTargets(pstate,
 												  mergeWhenClause->targetList,
 												  &attrnos);
@@ -381,12 +379,9 @@ transformMergeStmt(ParseState *pstate, MergeStmt *stmt)
 				}
 				break;
 			case CMD_UPDATE:
-				{
-					pstate->p_is_insert = false;
-					action->targetList =
-						transformUpdateTargetList(pstate,
-												  mergeWhenClause->targetList);
-				}
+				action->targetList =
+					transformUpdateTargetList(pstate,
+											  mergeWhenClause->targetList);
 				break;
 			case CMD_DELETE:
 				break;

src/backend/parser/parse_target.c

@@ -438,6 +438,7 @@ markTargetListOrigin(ParseState *pstate, TargetEntry *tle,
  * pstate		parse state
  * expr			expression to be modified
  * exprKind		indicates which type of statement we're dealing with
+ *				(EXPR_KIND_INSERT_TARGET or EXPR_KIND_UPDATE_TARGET)
  * colname		target column name (ie, name of attribute to be assigned to)
  * attrno		target attribute number
  * indirection	subscripts/field names for target column, if any
@@ -471,7 +472,8 @@ transformAssignedExpr(ParseState *pstate,
 	 * set p_expr_kind here because we can parse subscripts without going
 	 * through transformExpr().
 	 */
-	Assert(exprKind != EXPR_KIND_NONE);
+	Assert(exprKind == EXPR_KIND_INSERT_TARGET ||
+		   exprKind == EXPR_KIND_UPDATE_TARGET);
 	sv_expr_kind = pstate->p_expr_kind;
 	pstate->p_expr_kind = exprKind;
 
@@ -530,7 +532,7 @@ transformAssignedExpr(ParseState *pstate,
 	{
 		Node	   *colVar;
 
-		if (pstate->p_is_insert)
+		if (exprKind == EXPR_KIND_INSERT_TARGET)
 		{
 			/*
 			 * The command is INSERT INTO table (col.something) ... so there

src/include/parser/parse_node.h

@@ -153,10 +153,6 @@ typedef Node *(*CoerceParamHook) (ParseState *pstate, Param *param,
  *
  * p_grouping_nsitem: the ParseNamespaceItem that represents the grouping step.
  *
- * p_is_insert: true to process assignment expressions like INSERT, false
- * to process them like UPDATE.  (Note this can change intra-statement, for
- * cases like INSERT ON CONFLICT UPDATE.)
- *
  * p_windowdefs: list of WindowDefs representing WINDOW and OVER clauses.
  * We collect these while transforming expressions and then transform them
  * afterwards (so that any resjunk tlist items needed for the sort/group
@@ -209,7 +205,6 @@ struct ParseState
 	Relation	p_target_relation;	/* INSERT/UPDATE/DELETE/MERGE target rel */
 	ParseNamespaceItem *p_target_nsitem;	/* target rel's NSItem, or NULL */
 	ParseNamespaceItem *p_grouping_nsitem;	/* NSItem for grouping, or NULL */
-	bool		p_is_insert;	/* process assignment like INSERT not UPDATE */
 	List	   *p_windowdefs;	/* raw representations of window clauses */
 	ParseExprKind p_expr_kind;	/* what kind of expression we're parsing */
 	int			p_next_resno;	/* next targetlist resno to assign */