(By re-cluster, I of course mean pg_repack.)
Purely OLTP tables (that are only accessed randomly) can of course live with 0% correlation, but lots of tables are mixed-use, and so benefit from physical ordering on a carefully chosen field..
SELECT abs(correlation)::numeric(3,2) as correlation
FROM pg_stats
WHERE schemaname = 'foo' AND tablename = 'bar'
AND attname = 'blarge';
correlation
-------------
0.84
(1 row)
Obviously 84% is no need to worry, but what about 60% or 40%? Currently, I use 60%, but would like to do better.
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