Disallow scrolling of FOR UPDATE/FOR SHARE cursors, so as to avoid problems

in corner cases such as re-fetching a just-deleted row.  We may be able to
relax this someday, but let's find out how many people really care before
we invest a lot of work in it.  Per report from Heikki and subsequent
discussion.

While in the neighborhood, make the combination of INSENSITIVE and FOR UPDATE
throw an error, since they are semantically incompatible.  (Up to now we've
accepted but just ignored the INSENSITIVE option of DECLARE CURSOR.)

Author: tglsfdc

doc/src/sgml/ref/declare.sgml

@@ -1,5 +1,5 @@
 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/declare.sgml,v 1.41 2007/06/11 01:16:21 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/declare.sgml,v 1.42 2007/10/24 23:27:07 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -172,7 +172,7 @@ DECLARE <replaceable class="parameter">name</replaceable> [ BINARY ] [ INSENSITI
     transaction.  Thus, <command>DECLARE</> without <literal>WITH
     HOLD</literal> is useless outside a transaction block: the cursor would
     survive only to the completion of the statement.  Therefore
-    <productname>PostgreSQL</productname> reports an error if this
+    <productname>PostgreSQL</productname> reports an error if such a
     command is used outside a transaction block.
     Use
     <xref linkend="sql-begin" endterm="sql-begin-title">,
@@ -230,6 +230,11 @@ DECLARE <replaceable class="parameter">name</replaceable> [ BINARY ] [ INSENSITI
     will have no effect if the row was changed meanwhile.
    </para>
 
+   <para>
+    <literal>SCROLL</literal> may not be specified when the query
+    includes <literal>FOR UPDATE</> or <literal>FOR SHARE</>.
+   </para>
+
    <para>
     The SQL standard only makes provisions for cursors in embedded
     <acronym>SQL</acronym>.  The <productname>PostgreSQL</productname>
@@ -265,10 +270,11 @@ DECLARE liahona CURSOR FOR SELECT * FROM films;
   <title>Compatibility</title>
 
   <para>
-   The SQL standard specifies that by default, cursors are sensitive to
-   concurrent updates of the underlying data.  In
+   The SQL standard says that it is implementation-dependent whether cursors
+   are sensitive to concurrent updates of the underlying data by default.  In
    <productname>PostgreSQL</productname>, cursors are insensitive by default,
-   and can be made sensitive by specifying <literal>FOR UPDATE</>.
+   and can be made sensitive by specifying <literal>FOR UPDATE</>.  Other
+   products may work differently.
   </para>
 
   <para>

src/backend/commands/portalcmds.c

@@ -14,7 +14,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.65 2007/04/27 22:05:47 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.66 2007/10/24 23:27:08 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -102,12 +102,13 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
 	 *
 	 * If the user didn't specify a SCROLL type, allow or disallow scrolling
 	 * based on whether it would require any additional runtime overhead to do
-	 * so.
+	 * so.  Also, we disallow scrolling for FOR UPDATE cursors.
 	 */
 	portal->cursorOptions = cstmt->options;
 	if (!(portal->cursorOptions & (CURSOR_OPT_SCROLL | CURSOR_OPT_NO_SCROLL)))
 	{
-		if (ExecSupportsBackwardScan(stmt->planTree))
+		if (stmt->rowMarks == NIL &&
+			ExecSupportsBackwardScan(stmt->planTree))
 			portal->cursorOptions |= CURSOR_OPT_SCROLL;
 		else
 			portal->cursorOptions |= CURSOR_OPT_NO_SCROLL;

src/backend/executor/spi.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/spi.c,v 1.181 2007/09/20 17:56:31 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/spi.c,v 1.182 2007/10/24 23:27:08 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -975,12 +975,29 @@ SPI_cursor_open(const char *name, SPIPlanPtr plan,
 	{
 		if (list_length(stmt_list) == 1 &&
 			IsA((Node *) linitial(stmt_list), PlannedStmt) &&
+			((PlannedStmt *) linitial(stmt_list))->rowMarks == NIL &&
 			ExecSupportsBackwardScan(((PlannedStmt *) linitial(stmt_list))->planTree))
 			portal->cursorOptions |= CURSOR_OPT_SCROLL;
 		else
 			portal->cursorOptions |= CURSOR_OPT_NO_SCROLL;
 	}
 
+	/*
+	 * Disallow SCROLL with SELECT FOR UPDATE.  This is not redundant with
+	 * the check in transformDeclareCursorStmt because the cursor options
+	 * might not have come through there.
+	 */
+	if (portal->cursorOptions & CURSOR_OPT_SCROLL)
+	{
+		if (list_length(stmt_list) == 1 &&
+			IsA((Node *) linitial(stmt_list), PlannedStmt) &&
+			((PlannedStmt *) linitial(stmt_list))->rowMarks != NIL)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("DECLARE CURSOR SCROLL ... FOR UPDATE/SHARE is not supported"),
+					 errdetail("Scrollable cursors must be READ ONLY.")));
+	}
+
 	/*
 	 * If told to be read-only, we'd better check for read-only queries.
 	 * This can't be done earlier because we need to look at the finished,

src/backend/parser/analyze.c

@@ -17,7 +17,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- *	$PostgreSQL: pgsql/src/backend/parser/analyze.c,v 1.367 2007/06/23 22:12:51 tgl Exp $
+ *	$PostgreSQL: pgsql/src/backend/parser/analyze.c,v 1.368 2007/10/24 23:27:08 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1623,6 +1623,20 @@ transformDeclareCursorStmt(ParseState *pstate, DeclareCursorStmt *stmt)
 			  errmsg("DECLARE CURSOR WITH HOLD ... FOR UPDATE/SHARE is not supported"),
 				 errdetail("Holdable cursors must be READ ONLY.")));
 
+	/* FOR UPDATE and SCROLL are not compatible */
+	if (result->rowMarks != NIL && (stmt->options & CURSOR_OPT_SCROLL))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			  errmsg("DECLARE CURSOR SCROLL ... FOR UPDATE/SHARE is not supported"),
+				 errdetail("Scrollable cursors must be READ ONLY.")));
+
+	/* FOR UPDATE and INSENSITIVE are not compatible */
+	if (result->rowMarks != NIL && (stmt->options & CURSOR_OPT_INSENSITIVE))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			  errmsg("DECLARE CURSOR INSENSITIVE ... FOR UPDATE/SHARE is not supported"),
+				 errdetail("Insensitive cursors must be READ ONLY.")));
+
 	/* We won't need the raw querytree any more */
 	stmt->query = NULL;
 

src/include/nodes/parsenodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/parsenodes.h,v 1.353 2007/09/03 18:46:30 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/parsenodes.h,v 1.354 2007/10/24 23:27:08 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1479,7 +1479,7 @@ typedef struct CommentStmt
 #define CURSOR_OPT_BINARY		0x0001		/* BINARY */
 #define CURSOR_OPT_SCROLL		0x0002		/* SCROLL explicitly given */
 #define CURSOR_OPT_NO_SCROLL	0x0004		/* NO SCROLL explicitly given */
-#define CURSOR_OPT_INSENSITIVE	0x0008		/* INSENSITIVE (unimplemented) */
+#define CURSOR_OPT_INSENSITIVE	0x0008		/* INSENSITIVE */
 #define CURSOR_OPT_HOLD			0x0010		/* WITH HOLD */
 #define CURSOR_OPT_FAST_PLAN	0x0020		/* prefer fast-start plan */
 

src/test/regress/expected/portals.out

@@ -1073,40 +1073,31 @@ SELECT * FROM uctest;
  23 | three
 (2 rows)
 
--- sensitive cursor should show effects of updates or deletes
--- XXX current behavior is WRONG
-FETCH RELATIVE 0 FROM c1;
+DELETE FROM uctest WHERE CURRENT OF c1;
+SELECT * FROM uctest;
  f1 | f2  
 ----+-----
   8 | one
 (1 row)
 
-DELETE FROM uctest WHERE CURRENT OF c1;
-SELECT * FROM uctest;
- f1 |  f2   
-----+-------
- 23 | three
-(1 row)
-
 DELETE FROM uctest WHERE CURRENT OF c1; -- no-op
 SELECT * FROM uctest;
- f1 |  f2   
-----+-------
- 23 | three
+ f1 | f2  
+----+-----
+  8 | one
 (1 row)
 
 UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1; -- no-op
 SELECT * FROM uctest;
- f1 |  f2   
-----+-------
- 23 | three
+ f1 | f2  
+----+-----
+  8 | one
 (1 row)
 
+--- sensitive cursors can't currently scroll back, so this is an error:
 FETCH RELATIVE 0 FROM c1;
- f1 | f2 
-----+----
-(0 rows)
-
+ERROR:  cursor can only scan forward
+HINT:  Declare it with SCROLL option to enable backward scan.
 ROLLBACK;
 SELECT * FROM uctest;
  f1 |  f2   

src/test/regress/sql/portals.sql

@@ -376,15 +376,13 @@ UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1;
 SELECT * FROM uctest;
 UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1;
 SELECT * FROM uctest;
--- sensitive cursor should show effects of updates or deletes
--- XXX current behavior is WRONG
-FETCH RELATIVE 0 FROM c1;
 DELETE FROM uctest WHERE CURRENT OF c1;
 SELECT * FROM uctest;
 DELETE FROM uctest WHERE CURRENT OF c1; -- no-op
 SELECT * FROM uctest;
 UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1; -- no-op
 SELECT * FROM uctest;
+--- sensitive cursors can't currently scroll back, so this is an error:
 FETCH RELATIVE 0 FROM c1;
 ROLLBACK;
 SELECT * FROM uctest;