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

@@ -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;
 

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