PostgreSQL, Second Edition

Table Statistics

You've seen all the operators that PostgreSQL can use to execute a query. Remember that the goal of the optimizer is to find the plan with the least overall expense. Each operator uses a different algorithm for estimating its cost of execution. The cost estimators need some basic statistical information to make educated estimates.

Table statistics are stored in two places in a PostgreSQL database: pg_class and pg_statistic.

The pg_class system table contains one row for each table defined in your database (it also contains information about views, indexes, and sequences). For any given table, the pg_class.relpages column contains an estimate of the number of 8KB pages required to hold the table. The pg_class.reltuples column ...