select myword,tsearch from mytable;
myword tsearch
dark 'dark':1
darkness 'dark':1
darked 'dark':1
darks 'dark':1
DARK 'dark':1
goat 'goat':1
tin can 'tin':1
dark sun 'dark':1 'sun':2
DarkAges 'darkag':1
darkage 'darkag':1
my dog sunshine 'dog':2 'sunshin':3

(11 rows)

select myword,tsearch from mytable where tsearch @@ to_tsquery('dark');
myword tsearch
dark 'dark':1
darkness 'dark':1
darked 'dark':1
darks 'dark':1
DARK 'dark':1
dark sun 'dark':1 'sun':2

(6 rows)

select myword,tsearch from mytable where tsearch @@ to_tsquery('darks');
myword tsearch
dark 'dark':1
darkness 'dark':1
darked 'dark':1
darks 'dark':1
DARK 'dark':1
dark sun 'dark':1 'sun':2

(6 rows)

select myword,tsearch from mytable where tsearch @@ to_tsquery('darkness');
myword tsearch
dark 'dark':1
darkness 'dark':1
darked 'dark':1
darks 'dark':1
DARK 'dark':1
dark sun 'dark':1 'sun':2

(6 rows)