Peter Geoghegan wrote:

> Now, that might not seem like that much of a difference, but if you
> consider how duplicates are handled in the B-Tree code, and how unique
> index enforcement works, I think it could be. It could lead to heavy
> buffer lock contention, because we sometimes do a lot of work with an
> exclusive buffer lock held.

Not to mention work done with a "buffer cleanup lock" held -- which is
compounded by the fact that acquiring such a lock is prone to starvation
if there are many scanners of that index.  I've seen a case where a very
hot table is scanned so heavily that vacuum is starved for days waiting
to acquire cleanup on a single page (vacuum was only able to finish
because the app using the table was restarted).  I'm sure that a uniform
distribution of keys, with a uniform distribution of values scanned,
would give a completely different behavior than a highly skewed
distribution where a single key receives a large fraction of the scans.

Álvaro Herrera      
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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