Postgres constraint for unique datetime range

You can keep your separate timestamp columns and still use an exclusion constraint on an expression:

CREATE TABLE tbl ( tbl_id serial PRIMARY KEY , starts_at timestamp , ends_at timestamp , EXCLUDE USING gist (tsrange(starts_at, ends_at) WITH &&) -- no overlap ); 

Constructing a tsrange value without explicit bounds as tsrange(starts_at, ends_at) assumes default bounds: inclusive lower and exclusive upper - '[)', which is typically best.

db<>fiddle here
_{Old sqlfiddle}

Related:

• Preventing adjacent/overlapping entries with EXCLUDE in PostgreSQL

Add constraint to existing table

ALTER TABLE tbl ADD CONSTRAINT tbl_no_overlapping_time_ranges EXCLUDE USING gist (tsrange(starts_at, ends_at) WITH &&) 

Syntax details are the same as for CREATE TABLE.