Set a default autovacuum vacuum_cost_delay value of 20ms, to avoid excessive

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 53d2951be7687089885865f31949eda87439a80b
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2007-07-24T01:53:56Z
Releases: 8.3.0
Set a default autovacuum vacuum_cost_delay value of 20ms, to avoid excessive
I/O utilization, per discussion.

While at it, lower the autovacuum vacuum and analyze threshold values to 50
tuples.  It is a bit higher (i.e. more conservative) than what I originally
proposed but much better than the old values for small tables.

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