The following bug has been logged online:

Bug reference: 1891
Logged by: Elmival de Sousa
Email address: elmival (AT) yahoo (DOT) com.br
PostgreSQL version: 8.0.3
Operating system: Linux Suse Sles 9
Description: Out Of Memory.
Details:

how to accomplish the correction from mistake. I already tried the sysctl
(sysctl vm.overcommit_memory=value), but even so it continues with mistake.

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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