In this example, reading 4 million rows has taken more than 100 milliseconds. From a performance point of view, it is a total disaster. To figure out what goes wrong, PostgreSQL offers the EXPLAIN command:
test=# \h EXPLAIN Command: EXPLAIN Description: show the execution plan of a statement Syntax: EXPLAIN [ ( option [, ...] ) ] statement EXPLAIN [ ANALYZE ] [ VERBOSE ] statement where option can be one of: ANALYZE [ boolean ] VERBOSE [ boolean ] COSTS [ boolean ] BUFFERS [ boolean ] TIMING [ boolean ] FORMAT { TEXT | XML | JSON | YAML }
When you have a feeling that a query is not performing well, EXPLAIN will help you to reveal the real performance problem.
Here is how it works:
test=# EXPLAIN SELECT * FROM t_test ...
