lazy vacuum sleeps with exclusive lock on table

Hi,

I noticed that lazy vacuum acquires an exclusive lock at the end, to be
able to truncate the table. This is not a surprise. If it cannot
acquire the lock, it simply skips truncating the table and goes on with
life.

However, what's problematic is that if a non-zero cost delay has been
set, it will happily take naps while determining what to truncate :-(
This seems a bad idea. It also may explain why some people is seeing
autovacuum blocking other processes. It also readily explains why this
is so when there are no non-granted locks for autovacuum.

Comments? I think we should remove the sleep in the truncate phase.

--
Alvaro Herrera http://www.CommandPrompt.com/
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