EXPLAIN SQL - Language Statements EXPLAIN Shows statement execution details 1999-07-20 EXPLAIN [ VERBOSE ] query 1998-09-01 VERBOSE Flag to show detailed query plan. query Any query. 1998-04-15 NOTICE: QUERY PLAN: plan Explicit query plan from the Postgres backend. EXPLAIN Flag sent after query plan is shown. 1998-04-15 This command outputs details about the supplied query. The default output is the computed query cost. The cost value is only meaningful to the optimizer in comparing various query plans. VERBOSE displays the full query plan and cost to your screen, and pretty-prints the plan to the postmaster log file. 1998-04-15 There is only sparse documentation on the optimizer's use of cost information in Postgres. General information on cost estimation for query optimization can be found in database textbooks. Refer to the Programmer's Guide in the chapters on indexes and the genetic query optimizer for more information. To show a query plan for a simple query on a table with a single int4 column and 128 rows: EXPLAIN SELECT * FROM foo; NOTICE: QUERY PLAN: Seq Scan on foo (cost=5.22 rows=128 width=4) EXPLAIN For the same table with an index to support an equijoin condition on the query, EXPLAIN will show a different plan: EXPLAIN SELECT * FROM foo WHERE i = 4; NOTICE: QUERY PLAN: Index Scan using fi on foo (cost=2.05 rows=1 width=4) EXPLAIN And finally, for the same table with an index to support an equijoin condition on the query, EXPLAIN will show the following for a query using an aggregate function: EXPLAIN SELECT sum(i) FROM foo WHERE i = 4; NOTICE: QUERY PLAN: Aggregate (cost=2.05 rows=1 width=4) -> Index Scan using fi on foo (cost=2.05 rows=1 width=4) 1998-09-01 There is no EXPLAIN statement defined in SQL92.