PostGIS Cookbook - Second Edition

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Using the CLUSTER statement on the caschools table did not result in a significant performance boost. The lesson here is that, despite the fact that the data is physically reordered based on the index information in order to optimize searching, there is no guarantee that query performance will improve on a clustered table. Clustering should be reserved for tables with many large records only after adding the appropriate indices to and optimizing queries for the tables in question.