Check if string starts with character

In ruby I would use something like

"PostgreSQL".downcase.start_with?("p")
=> true

and the equivalent in a query would be:

select lower(left('PostgreSQL', 1)) = 'p';

 ?column?
----------
 t
(1 row)

and if you have the citext extension enabled you could do:

select left('PostgreSQL', 1)::citext = 'p';

 ?column?
----------
 t
(1 row)

Other things:

select *
from users
where left(display_name, 1)::citext = 'a'
;