The following bug has been logged online:

Bug reference: 1777
Logged by: nicolas
Email address: nrecabarren (AT) adexus (DOT) cl
PostgreSQL version: 7.2
Operating system: Linux Debian
Description: Cache lookup filed
Details:

ERROR: fmgr_info: function 16556: cache lookup failed

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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