The following bug has been logged online:

Bug reference: 2012
Logged by: Michael Enke
Email address: michael.enke (AT) wincor-nixdorf (DOT) com
PostgreSQL version: 8.0.3
Operating system: Linux SuSE 10
Description: SPI_fnumber sigsegv when compiled with 7.4.8
Details:

A 'C' function, which uses the SPI_fnumber,
which was compiled on 7.4.8 crashes the 8.0.3 server
with a SIGSEGV.
The same source code, compiled on 8.0.3
makes no trouble.

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"Michael Enke" <michael.enke (AT) wincor-nixdorf (DOT) com> writes:
This is not a bug, sorry. We have never promised compatibility of
dynamically loaded functions across PG versions.

regards, tom lane

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