Tuning PostgreSQL queries
PostgreSQL provides the means to figure out why a certain query is slow. PostgreSQL behind the scene analyzes the tables, collects statistics from them, and builds histograms using auto vacuuming. Auto vacuuming, in general, is used to recover disk space, update table statistics, and perform other maintenance tasks, such as preventing transaction ID wraparound. Table statistics allow PostgreSQL to pick up an execution plan at the least cost. The least cost is calculated by taking into account the IO and, naturally, CPU cost.
Also, PostgreSQL enables users to see the generated execution plan by providing the EXPLAIN
command.
For beginners, it is extremely useful to write the same query in different ways and compare the results. For example, in some cases, the NOT IN
construct can be converted to LEFT JOIN
or NOT EXIST
. Also, the IN
construct can be rewritten using INNER JOIN
as well as EXISTS
. Writing the query in several ways teaches the developer when to use or avoid...