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)