Hello,
>
> The distribution of the data is that all but 120,000 rows have null
> values in the _revision_expired column.
>
A shot in the dark - will a partial index on the above
column help?
http://www.postgresql.org/docs/current/interactive/indexes-partial.html
http://en.wikipedia.org/wiki/Partial_index
One link with discussion about it...
http://www.devheads.net/database/postgresql/general/when-can-postgresql-use-partial-not-null-index-seems-depend-size-clause-even-enable-seqscan.htm
Regards,
Jayadevan
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