*** a/src/backend/commands/variable.c
--- b/src/backend/commands/variable.c
***************
*** 544,549 **** show_log_timezone(void)
--- 544,580 ----
  
  
  /*
+  * 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 value, bool doit, GucSource source)
+ {
+ 	if (FirstSnapshotSet)
+ 	{
+ 		ereport(GUC_complaint_elevel(source),
+ 				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+ 				 errmsg("read-only property must be set before any query")));
+ 		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
+ 		if (source != PGC_S_OVERRIDE)
+ 			return false;
+ 	}
+ 	else if (IsSubTransaction())
+ 	{
+ 		ereport(GUC_complaint_elevel(source),
+ 				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+ 				 errmsg("read-only propery may not be changed in a subtransaction")));
+ 		/* source == PGC_S_OVERRIDE means do it anyway, eg at xact abort */
+ 		if (source != PGC_S_OVERRIDE)
+ 			return false;
+ 	}
+ 
+ 	return true;
+ }
+ 
+ /*
   * SET TRANSACTION ISOLATION LEVEL
   */
  
*** a/src/backend/utils/misc/guc.c
--- b/src/backend/utils/misc/guc.c
***************
*** 169,175 **** 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);
--- 169,174 ----
***************
*** 7837,7870 **** 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)
  {
--- 7836,7841 ----
*** 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;
--- 43,58 ----
  -- 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:  read-only property must be set before any query
+ 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;
--- 39,50 ----
  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;
+ 
  SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;
  
  DROP TABLE writetest; -- fail