# explain
# select i.item_id, u.user_id from items i
# left join users u on u.user_id = i.user_id
# where item_id = 169946840;
QUERY PLAN
----------------------------------------------------------------------------------------------
Nested Loop Left Join (cost=0.00..397.14 rows=1 width=16)
-> Index Scan using items_item_ux on items i (cost=0.00..358.84 rows=1 width=16)
Index Cond: (item_id = 169946840)
-> Index Only Scan using users_user_id_pkey on users u (cost=0.00..38.30 rows=1 width=8)
Index Cond: (user_id = i.user_id)
time: 55919.910 ms
# set enable_mergejoin to off;
# explain
select i.item_id, u.user_id from items i
left join users u on u.user_id = i.user_id
where item_id = 169946840;
QUERY PLAN
----------------------------------------------------------------------------------------------
Nested Loop Left Join (cost=0.00..397.14 rows=1 width=16)
-> Index Scan using items_item_ux on items i (cost=0.00..358.84 rows=1 width=16)
Index Cond: (item_id = 169946840)
-> Index Only Scan using users_user_id_pkey on users u (cost=0.00..38.30 rows=1 width=8)
Index Cond: (user_id = i.user_id)
time: 28.874 ms
--
Sergey Burladyan