While not "SELECT APPROXIMATELY COUNT (*)," both MySQL and PostgreSQL both offer various metadata tables with approximate (albeit completely unfiltered) total row counts.

Many DB systems have some sort of HLL function to provide a similar approximation (although I think you’re overestimating the costs that MVCC impose on large datasets).

Note that HLL is used for counting `unique` things. You don't need HLL for counting the rows of the table.

Doesn't SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED do that? Or maybe a NOLOCK hint equivalent in MySQL?