How to optimize search restricted by boolean column?

I want to optimize a query¹ of the form

SELECT yid, xid FROM x WHERE is_principal;

Here, is_principal is a non-null boolean column of x.

If I run EXPLAIN QUERY PLAN on this query, the output I get is

0|0|0|SCAN TABLE x

I'd like to turn this SCAN into a SEARCH, so created the following index:

CREATE INDEX x_is_principal_idx ON x (is_principal);

...but it made no difference: the output of EXPLAIN QUERY PLAN remains unchanged.

Is there some other way to optimize this query?

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^{1 FWIW, this query is in fact a sub-query of a much larger query that I'm trying to optimize.}