How can you use match specifications to filter ETS data efficiently?

A match specification is a low-level, compiled query format — a list of {Pattern, Guards, Result} tuples — that ets:select/2 executes directly inside the table engine, so filtering happens without copying every row out to the calling process first.

%% Find {Key, Value} pairs where Value > 100, returning just the Key
ets:select(Tab, [{{'$1', '$2'}, [{'>', '$2', 100}], ['$1']}]).

Writing match specs by hand is error-prone, so most code generates them with the ets:fun2ms/1 parse-transform, which lets you write an ordinary-looking fun and have it converted into the equivalent match spec at compile time:

ets:fun2ms(fun({K, V}) when V > 100 -> K end).

The performance benefit over pulling all rows and filtering in Erlang code is significant on large tables, since the match spec engine avoids constructing and copying rows that don't pass the guard at all.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.