Author: Milo Hyson <milo(at)cyberlifelabs(dot)com>
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Clustering can correct situations where the query planner chooses the wrong method. We had a two-column table of about 25,000 rows with an index on the second column. Even after vacuuming and analyzing, the query planner estimated an index scan would take almost 8 times a long as a sequential scan. Forcing index usage proved otherwise. Clustering fixed the problem and even halved the query times.
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Manual page: http://wwwmaster.postgresql.org/docs/7.4/interactive/sql-cluster.html
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