Don't evaluate the FOR PORTION OF target under EXPLAIN

ExecInitModifyTable() evaluated ForPortionOfExpr.targetRange
unconditionally at executor start-up, with no EXEC_FLAG_EXPLAIN_ONLY
guard. This caused three problems:

- EXPLAIN (GENERIC_PLAN) failed with "no value found for parameter 1".

- Plain EXPLAIN with FOR PORTION OF (null) raised "FOR PORTION OF
  target must not be null".

- Plain EXPLAIN ran user functions appearing in the target a second
  time, beyond the one evaluation the planner already does for
  selectivity estimation.

Now we skip the evaluation and the accompanying null check in
explain-only mode.

Reported-by: Noah Misch <noah@leadboat.com>
Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/flat/CA%2BrenyXci9khYXJ8uSwzF30DVw%2Bzc4QwGb1RdsuOu_YtrFuy1g%40mail.gmail.com

src/backend/executor/nodeModifyTable.c

@@ -5619,9 +5619,6 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 		TupleDesc	tupDesc;
 		ForPortionOfExpr *forPortionOf;
 		Datum		targetRange;
-		bool		isNull;
-		ExprContext *econtext;
-		ExprState  *exprState;
 		ForPortionOfState *fpoState;
 
 		rootRelInfo = mtstate->resultRelInfo;
@@ -5632,22 +5629,31 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 		forPortionOf = (ForPortionOfExpr *) node->forPortionOf;
 
 		/* Eval the FOR PORTION OF target */
-		if (mtstate->ps.ps_ExprContext == NULL)
+		if (!(eflags & EXEC_FLAG_EXPLAIN_ONLY))
-			ExecAssignExprContext(estate, &mtstate->ps);
+		{
-		econtext = mtstate->ps.ps_ExprContext;
+			bool		isNull;
+			ExprContext *econtext;
+			ExprState  *exprState;
 
-		exprState = ExecPrepareExpr((Expr *) forPortionOf->targetRange, estate);
+			if (mtstate->ps.ps_ExprContext == NULL)
-		targetRange = ExecEvalExpr(exprState, econtext, &isNull);
+				ExecAssignExprContext(estate, &mtstate->ps);
+			econtext = mtstate->ps.ps_ExprContext;
 
-		/*
+			exprState = ExecPrepareExpr((Expr *) forPortionOf->targetRange, estate);
-		 * FOR PORTION OF ... TO ... FROM should never give us a NULL target,
+			targetRange = ExecEvalExpr(exprState, econtext, &isNull);
-		 * but FOR PORTION OF (...) could.
+
-		 */
+			/*
-		if (isNull)
+			 * FOR PORTION OF ... FROM ... TO should never give us a NULL
-			ereport(ERROR,
+			 * target, but FOR PORTION OF (...) could.
-					(errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
+			 */
-					 errmsg("FOR PORTION OF target must not be null"),
+			if (isNull)
-					 executor_errposition(estate, forPortionOf->targetLocation)));
+				ereport(ERROR,
+						(errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
+						 errmsg("FOR PORTION OF target must not be null"),
+						 executor_errposition(estate, forPortionOf->targetLocation)));
+		}
+		else
+			targetRange = (Datum) 0;
 
 		/* Create state for FOR PORTION OF operation */
 

src/test/regress/expected/for_portion_of.out

@@ -2895,4 +2895,93 @@ SELECT definition FROM pg_rules WHERE rulename = 'fpo_param_r';
 
 DROP TABLE fpo_param_src;
 DROP TABLE fpo_param;
+--
+-- EXPLAIN without ANALYZE must only plan the statement, so it must not
+-- evaluate the FOR PORTION OF target.
+--
+CREATE TABLE fpo_explain (
+  id int4range,
+  valid_at daterange,
+  name text
+);
+INSERT INTO fpo_explain (id, valid_at, name) VALUES
+  ('[1,2)', daterange('2000-01-01', '2010-01-01'), 'one');
+-- GENERIC_PLAN exists precisely so that a statement containing parameter
+-- placeholders can be planned without supplying any parameter values.  The
+-- hand-written equivalent qual is the control case.
+EXPLAIN (COSTS OFF, GENERIC_PLAN)
+  UPDATE fpo_explain SET name = 'q' WHERE valid_at && $1::daterange;
+            QUERY PLAN            
+----------------------------------
+ Update on fpo_explain
+   ->  Seq Scan on fpo_explain
+         Filter: (valid_at && $1)
+(3 rows)
+
+EXPLAIN (COSTS OFF, GENERIC_PLAN)
+  UPDATE fpo_explain FOR PORTION OF valid_at ($1::daterange) SET name = 'q';
+            QUERY PLAN            
+----------------------------------
+ Update on fpo_explain
+   ->  Seq Scan on fpo_explain
+         Filter: (valid_at && $1)
+(3 rows)
+
+EXPLAIN (COSTS OFF, GENERIC_PLAN)
+  DELETE FROM fpo_explain FOR PORTION OF valid_at ($1::daterange);
+            QUERY PLAN            
+----------------------------------
+ Delete on fpo_explain
+   ->  Seq Scan on fpo_explain
+         Filter: (valid_at && $1)
+(3 rows)
+
+-- A null target is a run-time error, so plain EXPLAIN should still report
+-- the plan.
+EXPLAIN (COSTS OFF)
+  UPDATE fpo_explain FOR PORTION OF valid_at (NULL::daterange) SET name = 'q';
+              QUERY PLAN               
+---------------------------------------
+ Update on fpo_explain
+   ->  Result
+         Replaces: Scan on fpo_explain
+         One-Time Filter: false
+(4 rows)
+
+EXPLAIN (COSTS OFF)
+  DELETE FROM fpo_explain FOR PORTION OF valid_at (NULL::daterange);
+              QUERY PLAN               
+---------------------------------------
+ Delete on fpo_explain
+   ->  Result
+         Replaces: Scan on fpo_explain
+         One-Time Filter: false
+(4 rows)
+
+-- Actually running it is still an error.
+UPDATE fpo_explain FOR PORTION OF valid_at (NULL::daterange) SET name = 'q';
+ERROR:  FOR PORTION OF target must not be null
+LINE 1: UPDATE fpo_explain FOR PORTION OF valid_at (NULL::daterange)...
+                                                    ^
+DELETE FROM fpo_explain FOR PORTION OF valid_at (NULL::daterange);
+ERROR:  FOR PORTION OF target must not be null
+LINE 1: DELETE FROM fpo_explain FOR PORTION OF valid_at (NULL::dater...
+                                                         ^
+-- EXPLAIN ANALYZE does execute, so the null check must still fire there.
+EXPLAIN (ANALYZE, COSTS OFF, TIMING OFF, SUMMARY OFF, BUFFERS OFF)
+  UPDATE fpo_explain FOR PORTION OF valid_at (NULL::daterange) SET name = 'q';
+ERROR:  FOR PORTION OF target must not be null
+LINE 2:   UPDATE fpo_explain FOR PORTION OF valid_at (NULL::daterang...
+                                                      ^
+UPDATE fpo_explain FOR PORTION OF valid_at FROM '2002-01-01' TO '2003-01-01'
+  SET name = 'q';
+SELECT * FROM fpo_explain ORDER BY valid_at;
+  id   |        valid_at         | name 
+-------+-------------------------+------
+ [1,2) | [2000-01-01,2002-01-01) | one
+ [1,2) | [2002-01-01,2003-01-01) | q
+ [1,2) | [2003-01-01,2010-01-01) | one
+(3 rows)
+
+DROP TABLE fpo_explain;
 RESET datestyle;

src/test/regress/sql/for_portion_of.sql

@@ -1920,4 +1920,48 @@ SELECT definition FROM pg_rules WHERE rulename = 'fpo_param_r';
 DROP TABLE fpo_param_src;
 DROP TABLE fpo_param;
 
+--
+-- EXPLAIN without ANALYZE must only plan the statement, so it must not
+-- evaluate the FOR PORTION OF target.
+--
+
+CREATE TABLE fpo_explain (
+  id int4range,
+  valid_at daterange,
+  name text
+);
+INSERT INTO fpo_explain (id, valid_at, name) VALUES
+  ('[1,2)', daterange('2000-01-01', '2010-01-01'), 'one');
+
+-- GENERIC_PLAN exists precisely so that a statement containing parameter
+-- placeholders can be planned without supplying any parameter values.  The
+-- hand-written equivalent qual is the control case.
+EXPLAIN (COSTS OFF, GENERIC_PLAN)
+  UPDATE fpo_explain SET name = 'q' WHERE valid_at && $1::daterange;
+EXPLAIN (COSTS OFF, GENERIC_PLAN)
+  UPDATE fpo_explain FOR PORTION OF valid_at ($1::daterange) SET name = 'q';
+EXPLAIN (COSTS OFF, GENERIC_PLAN)
+  DELETE FROM fpo_explain FOR PORTION OF valid_at ($1::daterange);
+
+-- A null target is a run-time error, so plain EXPLAIN should still report
+-- the plan.
+EXPLAIN (COSTS OFF)
+  UPDATE fpo_explain FOR PORTION OF valid_at (NULL::daterange) SET name = 'q';
+EXPLAIN (COSTS OFF)
+  DELETE FROM fpo_explain FOR PORTION OF valid_at (NULL::daterange);
+
+-- Actually running it is still an error.
+UPDATE fpo_explain FOR PORTION OF valid_at (NULL::daterange) SET name = 'q';
+DELETE FROM fpo_explain FOR PORTION OF valid_at (NULL::daterange);
+
+-- EXPLAIN ANALYZE does execute, so the null check must still fire there.
+EXPLAIN (ANALYZE, COSTS OFF, TIMING OFF, SUMMARY OFF, BUFFERS OFF)
+  UPDATE fpo_explain FOR PORTION OF valid_at (NULL::daterange) SET name = 'q';
+
+UPDATE fpo_explain FOR PORTION OF valid_at FROM '2002-01-01' TO '2003-01-01'
+  SET name = 'q';
+SELECT * FROM fpo_explain ORDER BY valid_at;
+
+DROP TABLE fpo_explain;
+
 RESET datestyle;