The following bug has been logged online:

Bug reference: 2508
Logged by: Soner Demiray
Email address: sdemiray (AT) gurmen (DOT) com.tr
PostgreSQL version: 8.1.4
Operating system: Gentoo Linux 2006.0 AMD64
Description: TransactionIdGetStatus returns wrong state
Details:

TransactionIdGetStatus function returns always
TRANSACTION_STATUS_IN_PROGRESS for any transaction which is just committed
or aborted.

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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match