Stopping expensive queries before they start

Today we're releasing a code for a small PostgreSQL module called plan_filter that lets you stop queries from even starting if they meet certain criteria. Currently the module implements one such criterion: the estimated cost of the query.

After you have built and installed it, you add a couple of settings to the postgresql.conf file, like this:

shared_preload_libraries = 'plan_filter'
plan_filter.statement_cost_limit = 100000.0

Then if the planner estimates the cost as higher than the statement_cost_limit it will raise an error rather than allowing the query to run.

This module follows an idea from a discussion on the postgresql-hackers mailing list some time ago. It was developed by PostgreSQL Experts Inc for our client Twitch.TV, who have generously allowed us to make it publicly available.