My queries are slow or don make use of the indexes. Why?
Indexes are not automatically used by every query. Indexes are only used if the table is larger than a minimum size, and the query selects only a small percentage of the rows in the table. This is because the random disk access caused by an index scan can be slower than a straight read through the table, or sequential scan. To determine if an index should be used, PostgreSQL must have statistics about the table. These statistics are collected using VACUUM ANALYZE, or simply ANALYZE. Using statistics, the optimizer knows how many rows are in the table, and can better determine if indexes should be used. Statistics are also valuable in determining optimal join order and join methods. Statistics collection should be performed periodically as the contents of the table change. Indexes are normally not used for ORDER BY or to perform joins. A sequential scan followed by an explicit sort is usually faster than an index scan of a large table. However, LIMIT combined with ORDER BY often will us
