age(xid) function bug

I see we recommend using the age(xid) function to check for XID
wraparound:

http://www.postgresql.org/docs/8.3/static/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND

However, the function only does a subtraction with no adjustment for
overflow, and considering it returns a signed int, it can't even display
the full range of valid values:

test=> \df age
List of functions
Schema | Name | Result data type | Argument data types
------------+------+------------------+----------------------------------------------------------
...
pg_catalog | age | integer | xid

For example:

test=> select txid_current();
txid_current
--------------
397
(1 row)

test=> select age('10000'::xid);
age
-------
-9602
(1 row)

Seems this is a bug. I think the fix is to do the arithmetic in int8
and return an int8.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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