Index to get row count of read-only (immutable) PostgreSQL table?

Question

I have a script that runs several times a day, which records the row counts of several PostgreSQL tables.

Some of the tables though are read-only and never change. (No r

Answer 1

Unfortunately, in postgresql SELECT COUNT(*) is often slower than mysql to which it often get's compared to.

You can use the following query as an alternative to SELECT COUNT(*).

 SELECT reltuples FROM pg_class WHERE relname = 'mytable';

This is not always 100% upto date but for immutable tables it will be accurate every time. And instant. For very large tables the percentage error will be very small and thus well worth the massive saving in time.

If it does matter and the table does not contain nulls, you can use

 SELECT COUNT(primary_key_column) FROM table

and this will be significantly faster than SELECT COUNT(*)