有一本书Learning PostgreSQL有一段介绍，供参考：

PostgreSQL also provides tools to maintain the system objects, mainly clusterdb and
reindexdb. clusterdb is a wrapper around the CLUSTER statement, which is used to
physically reorder the table based on certain index information. This can increase databaseperformance read operations due to the locality of reference principles, mainly the spatial
locality. Clustering the table helps to retrieve data from adjacent storage blocks and thus
reduces the hard-disk-access cost.