*** a/src/backend/commands/variable.c
--- b/src/backend/commands/variable.c
***************
*** 544,572 **** show_log_timezone(void)
  
  
  /*
   * SET TRANSACTION ISOLATION LEVEL
   */
  
  const char *
  assign_XactIsoLevel(const char *value, bool doit, GucSource source)
  {
! 	if (FirstSnapshotSet)
  	{
! 		ereport(GUC_complaint_elevel(source),
! 				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
! 				 errmsg("SET TRANSACTION ISOLATION LEVEL must be called before any query")));
! 		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
! 		if (source != PGC_S_OVERRIDE)
  			return NULL;
! 	}
! 	else if (IsSubTransaction())
! 	{
! 		ereport(GUC_complaint_elevel(source),
! 				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
! 				 errmsg("SET TRANSACTION ISOLATION LEVEL must not be called in a subtransaction")));
! 		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
! 		if (source != PGC_S_OVERRIDE)
  			return NULL;
  	}
  
  	if (strcmp(value, "serializable") == 0)
--- 544,617 ----
  
  
  /*
+  * SET TRANSACTION READ ONLY and SET TRANSACTION READ WRITE
+  *
+  * These should be transaction properties which can be set in exactly the
+  * same points in time that transaction isolation may be set.
+  */
+ bool
+ assign_transaction_read_only(bool newval, bool doit, GucSource source)
+ {
+ 	/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
+ 	if (source != PGC_S_OVERRIDE)
+ 	{
+ 		/* Can't go to r/w mode inside a r/o transaction */
+ 		if (newval == false && XactReadOnly && IsSubTransaction())
+ 		{
+ 			ereport(GUC_complaint_elevel(source),
+ 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ 					 errmsg("cannot set transaction read-write mode inside a read-only transaction")));
+ 			return false;
+ 		}
+ 		/* Top level transaction can't change this after first snapshot. */
+ 		if (FirstSnapshotSet && !IsSubTransaction())
+ 		{
+ 			ereport(GUC_complaint_elevel(source),
+ 					(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+ 					 errmsg(newval
+ 							? "transaction read-only mode must be set before any query"
+ 							: "transaction read-write mode must be set before any query")));
+ 			return false;
+ 		}
+ 		/* Can't go to r/w mode while recovery is still active */
+ 		if (newval == false && XactReadOnly && RecoveryInProgress())
+ 		{
+ 			ereport(GUC_complaint_elevel(source),
+ 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ 					 errmsg("cannot set transaction read-write mode during recovery")));
+ 			return false;
+ 		}
+ 	}
+ 
+ 	return true;
+ }
+ 
+ /*
   * SET TRANSACTION ISOLATION LEVEL
   */
+ extern char *XactIsoLevel_string;		/* in guc.c */
  
  const char *
  assign_XactIsoLevel(const char *value, bool doit, GucSource source)
  {
! 	/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
! 	if (source != PGC_S_OVERRIDE)
  	{
! 		if (FirstSnapshotSet)
! 		{
! 			ereport(GUC_complaint_elevel(source),
! 					(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
! 					 errmsg("SET TRANSACTION ISOLATION LEVEL must be called before any query")));
  			return NULL;
! 		}
! 		/* We ignore a subtransaction setting it to the existing value. */
! 		if (IsSubTransaction() && strcmp(value, XactIsoLevel_string) != 0)
! 		{
! 			ereport(GUC_complaint_elevel(source),
! 					(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
! 					 errmsg("SET TRANSACTION ISOLATION LEVEL must not be called in a subtransaction")));
  			return NULL;
+ 		}
  	}
  
  	if (strcmp(value, "serializable") == 0)
*** a/src/backend/utils/misc/guc.c
--- b/src/backend/utils/misc/guc.c
***************
*** 168,174 **** static bool assign_bonjour(bool newval, bool doit, GucSource source);
  static bool assign_ssl(bool newval, bool doit, GucSource source);
  static bool assign_stage_log_stats(bool newval, bool doit, GucSource source);
  static bool assign_log_stats(bool newval, bool doit, GucSource source);
- static bool assign_transaction_read_only(bool newval, bool doit, GucSource source);
  static const char *assign_canonical_path(const char *newval, bool doit, GucSource source);
  static const char *assign_timezone_abbreviations(const char *newval, bool doit, GucSource source);
  static const char *show_archive_command(void);
--- 168,173 ----
***************
*** 425,431 **** static int	server_version_num;
  static char *timezone_string;
  static char *log_timezone_string;
  static char *timezone_abbreviations_string;
- static char *XactIsoLevel_string;
  static char *custom_variable_classes;
  static int	max_function_args;
  static int	max_index_keys;
--- 424,429 ----
***************
*** 440,445 **** static int	effective_io_concurrency;
--- 438,444 ----
  /* should be static, but commands/variable.c needs to get at these */
  char	   *role_string;
  char	   *session_authorization_string;
+ char	   *XactIsoLevel_string;
  
  
  /*
***************
*** 7843,7876 **** assign_log_stats(bool newval, bool doit, GucSource source)
  	return true;
  }
  
- static bool
- assign_transaction_read_only(bool newval, bool doit, GucSource source)
- {
- 	/* Can't go to r/w mode inside a r/o transaction */
- 	if (newval == false && XactReadOnly && IsSubTransaction())
- 	{
- 		ereport(GUC_complaint_elevel(source),
- 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
- 				 errmsg("cannot set transaction read-write mode inside a read-only transaction")));
- 		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
- 		if (source != PGC_S_OVERRIDE)
- 			return false;
- 	}
- 
- 	/* Can't go to r/w mode while recovery is still active */
- 	if (newval == false && XactReadOnly && RecoveryInProgress())
- 	{
- 		ereport(GUC_complaint_elevel(source),
- 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
- 		  errmsg("cannot set transaction read-write mode during recovery")));
- 		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
- 		if (source != PGC_S_OVERRIDE)
- 			return false;
- 	}
- 
- 	return true;
- }
- 
  static const char *
  assign_canonical_path(const char *newval, bool doit, GucSource source)
  {
--- 7842,7847 ----
*** a/src/include/commands/variable.h
--- b/src/include/commands/variable.h
***************
*** 21,26 **** extern const char *show_timezone(void);
--- 21,28 ----
  extern const char *assign_log_timezone(const char *value,
  					bool doit, GucSource source);
  extern const char *show_log_timezone(void);
+ extern bool assign_transaction_read_only(bool value,
+ 					bool doit, GucSource source);
  extern const char *assign_XactIsoLevel(const char *value,
  					bool doit, GucSource source);
  extern const char *show_XactIsoLevel(void);
*** a/src/test/regress/expected/transactions.out
--- b/src/test/regress/expected/transactions.out
***************
*** 43,48 **** SELECT * FROM aggtest;
--- 43,123 ----
  -- Read-only tests
  CREATE TABLE writetest (a int);
  CREATE TEMPORARY TABLE temptest (a int);
+ BEGIN;
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+  a 
+ ---
+ (0 rows)
+ 
+ SET TRANSACTION READ WRITE; --fail
+ ERROR:  transaction read-write mode must be set before any query
+ COMMIT;
+ BEGIN;
+ SET TRANSACTION READ ONLY; -- ok
+ SET TRANSACTION READ WRITE; -- ok
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+  a 
+ ---
+ (0 rows)
+ 
+ SAVEPOINT x;
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+  a 
+ ---
+ (0 rows)
+ 
+ SET TRANSACTION READ ONLY; -- ok
+ SET TRANSACTION READ WRITE; --fail
+ ERROR:  cannot set transaction read-write mode inside a read-only transaction
+ COMMIT;
+ BEGIN;
+ SET TRANSACTION READ WRITE; -- ok
+ SAVEPOINT x;
+ SET TRANSACTION READ WRITE; -- ok
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+  a 
+ ---
+ (0 rows)
+ 
+ SET TRANSACTION READ ONLY; -- ok
+ SET TRANSACTION READ WRITE; --fail
+ ERROR:  cannot set transaction read-write mode inside a read-only transaction
+ COMMIT;
+ BEGIN;
+ SET TRANSACTION READ WRITE; -- ok
+ SAVEPOINT x;
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+  a 
+ ---
+ (0 rows)
+ 
+ ROLLBACK TO SAVEPOINT x;
+ SHOW transaction_read_only;  -- off
+  transaction_read_only 
+ -----------------------
+  off
+ (1 row)
+ 
+ SAVEPOINT y;
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+  a 
+ ---
+ (0 rows)
+ 
+ RELEASE SAVEPOINT y;
+ SHOW transaction_read_only;  -- off
+  transaction_read_only 
+ -----------------------
+  off
+ (1 row)
+ 
+ COMMIT;
  SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;
  DROP TABLE writetest; -- fail
  ERROR:  cannot execute DROP TABLE in a read-only transaction
*** a/src/test/regress/sql/transactions.sql
--- b/src/test/regress/sql/transactions.sql
***************
*** 39,44 **** SELECT * FROM aggtest;
--- 39,86 ----
  CREATE TABLE writetest (a int);
  CREATE TEMPORARY TABLE temptest (a int);
  
+ BEGIN;
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+ SET TRANSACTION READ WRITE; --fail
+ COMMIT;
+ 
+ BEGIN;
+ SET TRANSACTION READ ONLY; -- ok
+ SET TRANSACTION READ WRITE; -- ok
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+ SAVEPOINT x;
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+ SET TRANSACTION READ ONLY; -- ok
+ SET TRANSACTION READ WRITE; --fail
+ COMMIT;
+ 
+ BEGIN;
+ SET TRANSACTION READ WRITE; -- ok
+ SAVEPOINT x;
+ SET TRANSACTION READ WRITE; -- ok
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+ SET TRANSACTION READ ONLY; -- ok
+ SET TRANSACTION READ WRITE; --fail
+ COMMIT;
+ 
+ BEGIN;
+ SET TRANSACTION READ WRITE; -- ok
+ SAVEPOINT x;
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+ ROLLBACK TO SAVEPOINT x;
+ SHOW transaction_read_only;  -- off
+ SAVEPOINT y;
+ SET TRANSACTION READ ONLY; -- ok
+ SELECT * FROM writetest; -- ok
+ RELEASE SAVEPOINT y;
+ SHOW transaction_read_only;  -- off
+ COMMIT;
+ 
  SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;
  
  DROP TABLE writetest; -- fail