The following bug has been logged online:

Bug reference: 2596
Logged by: Anderson Nonato Santos
Email address: andersonnsantos (AT) hotmail (DOT) com
PostgreSQL version: 8.1.0
Operating system: Windows 2003
Description: Invalid memory alloc request size
Details:

I have this error after a Select in table.
What is this error?

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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