I see SELECT EXISTS used a lot like:

    if db.query("""SELECT EXISTS (SELECT 1 FROM checkoutWHERE checkout_id = %s)""" % checkout_id).getresult()[0][0] == 't':

vs. what i prefer:

    if db.query("""SELECT 1 FROM checkoutWHERE checkout_id = %sLIMIT 1""" % checkout_id).getresult():

Which one is preferred and why?

P.S. i am using Python and PosgreSQL.

cert=> explain SELECT EXISTS (SELECT 1 FROM checkout WHERE checkout_id = 3);QUERY PLAN                                      
--------------------------------------------------------------------------------------Result  (cost=4.03..4.03 rows=1 width=0)InitPlan->  Index Scan using checkout_pkey on checkout  (cost=0.00..4.03 rows=1 width=0)Index Cond: (checkout_id = 3)
(4 rows)cert=> explain SELECT 1 FROM checkout WHERE checkout_id = 3 limit 1;QUERY PLAN                                     
------------------------------------------------------------------------------------Limit  (cost=0.00..4.03 rows=1 width=0)->  Index Scan using checkout_pkey on checkout  (cost=0.00..4.03 rows=1 width=0)Index Cond: (checkout_id = 3)
(3 rows)

My point is, why getting a row from the result and check if it's first column is true, if i can just check if there are any rows at all, meaning the same?

To my eye the second statement is problematic in that it will not return a row if the condition is not satisfied.