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commit    71901ab6daaad65c0168c05e016e4208efe5b71a
Author:   Robert Haas <rhaas@postgresql.org>
Date:     Mon Sep 09 20:25:29 2013 +0000

    Introduce InvalidCommandId.
    
    This allows a 32-bit field to represent an *optional* command ID
    without a separate flag bit.
    
    Andres Freund


 src/backend/access/transam/xact.c | 4 ++--
 src/include/c.h                   | 1 +
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 31e868d4bc7..0591f3fd562 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -766,12 +766,12 @@ CommandCounterIncrement(void)
 	if (currentCommandIdUsed)
 	{
 		currentCommandId += 1;
-		if (currentCommandId == FirstCommandId) /* check for overflow */
+		if (currentCommandId == InvalidCommandId)
 		{
 			currentCommandId -= 1;
 			ereport(ERROR,
 					(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
-					 errmsg("cannot have more than 2^32-1 commands in a transaction")));
+					 errmsg("cannot have more than 2^32-2 commands in a transaction")));
 		}
 		currentCommandIdUsed = false;
 
diff --git a/src/include/c.h b/src/include/c.h
index 596118342df..14bfdcd4dab 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -368,6 +368,7 @@ typedef uint32 MultiXactOffset;
 typedef uint32 CommandId;
 
 #define FirstCommandId	((CommandId) 0)
+#define InvalidCommandId	(~(CommandId)0)
 
 /*
  * Array indexing support


[parent: 9c68834bfc2a]