Convert ExecComputeStoredGenerated to use tuple slots

This code was still using the old style of forming a heap tuple rather
than using tuple slots.  This would be less efficient if a non-heap
access method was used.  And using tuple slots is actually quite a bit
faster when using heap as well.

Also add some test cases for generated columns with null values and
with varlena values.  This lack of coverage was discovered while
working on this patch.

Discussion: https://www.postgresql.org/message-id/flat/20190331025744.ugbsyks7czfcoksd%40alap3.anarazel.de

src/backend/executor/nodeModifyTable.c

@@ -53,6 +53,7 @@
 #include "storage/bufmgr.h"
 #include "storage/lmgr.h"
 #include "utils/builtins.h"
+#include "utils/datum.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
 
@@ -254,9 +255,6 @@ ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot)
 	MemoryContext oldContext;
 	Datum	   *values;
 	bool	   *nulls;
-	bool	   *replaces;
-	HeapTuple	oldtuple, newtuple;
-	bool		should_free;
 
 	Assert(tupdesc->constr && tupdesc->constr->has_generated_stored);
 
@@ -294,11 +292,15 @@ ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot)
 
 	values = palloc(sizeof(*values) * natts);
 	nulls = palloc(sizeof(*nulls) * natts);
-	replaces = palloc0(sizeof(*replaces) * natts);
+
+	slot_getallattrs(slot);
+	memcpy(nulls, slot->tts_isnull, sizeof(*nulls) * natts);
 
 	for (int i = 0; i < natts; i++)
 	{
-		if (TupleDescAttr(tupdesc, i)->attgenerated == ATTRIBUTE_GENERATED_STORED)
+		Form_pg_attribute attr = TupleDescAttr(tupdesc, i);
+
+		if (attr->attgenerated == ATTRIBUTE_GENERATED_STORED)
 		{
 			ExprContext *econtext;
 			Datum		val;
@@ -311,20 +313,19 @@ ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot)
 
 			values[i] = val;
 			nulls[i] = isnull;
-			replaces[i] = true;
+		}
+		else
+		{
+			if (!nulls[i])
+				values[i] = datumCopy(slot->tts_values[i], attr->attbyval, attr->attlen);
 		}
 	}
 
-	oldtuple = ExecFetchSlotHeapTuple(slot, true, &should_free);
-	newtuple = heap_modify_tuple(oldtuple, tupdesc, values, nulls, replaces);
-	/*
-	 * The tuple will be freed by way of the memory context - the slot might
-	 * only be cleared after the context is reset, and we'd thus potentially
-	 * double free.
-	 */
-	ExecForceStoreHeapTuple(newtuple, slot, false);
-	if (should_free)
-		heap_freetuple(oldtuple);
+	ExecClearTuple(slot);
+	memcpy(slot->tts_values, values, sizeof(*values) * natts);
+	memcpy(slot->tts_isnull, nulls, sizeof(*nulls) * natts);
+	ExecStoreVirtualTuple(slot);
+	ExecMaterializeSlot(slot);
 
 	MemoryContextSwitchTo(oldContext);
 }

src/test/regress/expected/generated.out

@@ -226,15 +226,16 @@ NOTICE:  merging multiple inherited definitions of column "b"
 ERROR:  inherited column "b" has a generation conflict
 DROP TABLE gtesty;
 -- test stored update
-CREATE TABLE gtest3 (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 3) STORED);
-INSERT INTO gtest3 (a) VALUES (1), (2), (3);
+CREATE TABLE gtest3 (a int, b int GENERATED ALWAYS AS (a * 3) STORED);
+INSERT INTO gtest3 (a) VALUES (1), (2), (3), (NULL);
 SELECT * FROM gtest3 ORDER BY a;
  a | b 
 ---+---
  1 | 3
  2 | 6
  3 | 9
-(3 rows)
+   |  
+(4 rows)
 
 UPDATE gtest3 SET a = 22 WHERE a = 2;
 SELECT * FROM gtest3 ORDER BY a;
@@ -243,7 +244,29 @@ SELECT * FROM gtest3 ORDER BY a;
   1 |  3
   3 |  9
  22 | 66
-(3 rows)
+    |   
+(4 rows)
+
+CREATE TABLE gtest3a (a text, b text GENERATED ALWAYS AS (a || '+' || a) STORED);
+INSERT INTO gtest3a (a) VALUES ('a'), ('b'), ('c'), (NULL);
+SELECT * FROM gtest3a ORDER BY a;
+ a |  b  
+---+-----
+ a | a+a
+ b | b+b
+ c | c+c
+   | 
+(4 rows)
+
+UPDATE gtest3a SET a = 'bb' WHERE a = 'b';
+SELECT * FROM gtest3a ORDER BY a;
+ a  |   b   
+----+-------
+ a  | a+a
+ bb | bb+bb
+ c  | c+c
+    | 
+(4 rows)
 
 -- COPY
 TRUNCATE gtest1;

src/test/regress/sql/generated.sql

@@ -95,12 +95,18 @@ CREATE TABLE gtest1_2 () INHERITS (gtest1, gtesty);  -- error
 DROP TABLE gtesty;
 
 -- test stored update
-CREATE TABLE gtest3 (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 3) STORED);
-INSERT INTO gtest3 (a) VALUES (1), (2), (3);
+CREATE TABLE gtest3 (a int, b int GENERATED ALWAYS AS (a * 3) STORED);
+INSERT INTO gtest3 (a) VALUES (1), (2), (3), (NULL);
 SELECT * FROM gtest3 ORDER BY a;
 UPDATE gtest3 SET a = 22 WHERE a = 2;
 SELECT * FROM gtest3 ORDER BY a;
 
+CREATE TABLE gtest3a (a text, b text GENERATED ALWAYS AS (a || '+' || a) STORED);
+INSERT INTO gtest3a (a) VALUES ('a'), ('b'), ('c'), (NULL);
+SELECT * FROM gtest3a ORDER BY a;
+UPDATE gtest3a SET a = 'bb' WHERE a = 'b';
+SELECT * FROM gtest3a ORDER BY a;
+
 -- COPY
 TRUNCATE gtest1;
 INSERT INTO gtest1 (a) VALUES (1), (2);