The following bug has been logged online:

Bug reference: 1588
Logged by: Lionel Bouton
Email address: lionel-subscription (AT) bouton (DOT) name
PostgreSQL version: 8.0.1
Operating system: Linux 2.6, glibc 2.3.4
Description: pg_autovacuum sleep parameter overflow
Details:

pg_autovacuum doesn't sleep when the -s parameter is set above 2147
(PostgreSQL then is under constant stress), seems like a 32-bit signed
integer overflow (the time in µs then gets past 2^31-1) somewhere.

This worked with 7.4.6 (at least with -s 3600).

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