How do I speed up counting rows in a PostgreSQL table?

Question

We need to count the number of rows in a PostgreSQL table. In our case, no conditions need to be met, and it would be perfectly acceptable to get a row estimate if that significantly improved query speed.

Basically, we want select count(id) from <table> to run as fast as possible, even if that implies not getting exact results.

Answer 1

For a very quick estimate:

SELECT reltuples FROM pg_class WHERE relname = 'my_table';

There are several caveats, though. For one, relname is not necessarily unique in pg_class. There can be multiple tables with the same relname in multiple schemas of the database. To be unambiguous:

SELECT reltuples::bigint FROM pg_class WHERE oid = 'my_schema.my_table'::regclass;

If you do not schema-qualify the table name, a cast to regclass observes the current search_path to pick the best match. And if the table does not exist (or cannot be seen) in any of the schemas in the search_path you get an error message. See Object Identifier Types in the manual.

The cast to bigint formats the real number nicely, especially for big counts.

Also, reltuples can be more or less out of date. There are ways to make up for this to some extent. See this later answer with new and improved options:

• Fast way to discover the row count of a table in PostgreSQL

And a query on pg_stat_user_tables is many times slower (though still much faster than full count), as that's a view on a couple of tables.