SELECT EXISTS vs. LIMIT 1

To my eye the second statement is problematic in that it will not return a row if the condition is not satisfied.

PostgreSQL seems to clever enough to treat both statements alike as you can clearly see in your execution plans.

My tests with a local table with ~150000 rows and ~100 selected out of that by the condition also show the same behaviour

The bottomline is: it doesn't really matter which one you use, but you should be aware that other DBMS might not behave in the same way.

When you use EXPLAIN you can see that the first statement will execute an additional subquery and the second does not.

That is why I would prefer using limit instead of exists

Example:

explain SELECT EXISTS (SELECT 1 FROM checkout WHERE id = 3);
explain SELECT 1 FROM checkout WHERE id = 3 limit 1;