postgreSQL - pg_class question

Question

PostgreSQL stores statistics about tables in the system table called pg_class. The query planner accesses this table for every query. These statistics may only be updated using the analyze command. If the analyze command is not run often, the statistics in this table may not be accurate and the query planner may make poor decisions which can degrade system performance. Another strategy is for the query planner to generate these statistics for each query (including selects, inserts, updates, and deletes). This approach would allow the query planner to have the most up-to-date statistics possible.

Why postgres always rely on pg_class instead?

Answer 1

pg_class doesn't contain all the statistics needed by the planner, it only contains information about the structure of the table. Statistics generated by analyze command contain information about values existing in each column so when executing a command like:

SELECT * FORM tab WHERE cname = "pg";

the planner knows how much rows are in the table and how many rows have the value "pg" in the column cname. These information does not exist in pg_class.

Another nice feature of PostgreSQL is autovacuum, in 99,9999% of cases it should be enabled so the database actualizes statistics as soon as some (can be defined in config file) number of rows change. That minimizes the chance of wrong execution plan because of wrong table statistics.