it would be nice to make compression methods pluggable.
Agreed.
My thinking is that this should be combined with work to make use of the compressed data, which is why Alvaro, Tomas, David have been working on Col Store API for about 18 months and work on that continues with more submissions for 9.6 due.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services