How the position of LEFT JOIN influence the query and the optimizer? - mysql

Question

i see a strange behavior of Mysql when i launch two different query with one small difference, the position of one left join.
The slow query:
SELECT i.id_affiliate, i.id_franchising, i.Codice
FROM network_configuration_affiliate AS c
INNER JOIN franchising AS fr ON fr.id = c.id_franchising
INNER JOIN network_selected_car AS i ON c.id_affiliate = i.id_affiliate
INNER JOIN (
select T1.id_car, T1.id_network, T1.id_franchising, T1.id_agencie
from network_car_destinations as T1
where T1.id_network='12' and ( T1.id_franchising = 968 or T1.id_franchising = 974 )
) AS n ON n.id_franchising=i.id_franchising AND n.id_car=i.id_car AND c.id_network=n.id_network
INNER JOIN affiliate_tipologies AS t ON t.id_tipology_ag=i.idCategory AND t.id_franchising=i.id_franchising
INNER JOIN network_assoc_tipologies AS p ON p.id_network=c.id_network AND p.id_default=t.id_tipology_net
LEFT JOIN network_conf_users as ce on ce.id_affiliate = i.id_affiliate and ce.id_user = i.id_user
WHERE c.id_network='12' and c.code_affiliate='69842' AND c.configured = 'yes' AND i.Code_car not like '' and p.code != '0'
GROUP BY i.Code_car
The quick:
select T2.* from (
SELECT i.`id_affiliate`, i.id_franchising, i.Code_car, i.id_user
FROM network_configuration_affiliate AS c
INNER JOIN franchising AS fr ON fr.id = c.id_franchising
INNER JOIN network_selected_car AS i ON c.`id_affiliate` = i.`id_affiliate`
INNER JOIN (
select T1.id_car, T1.id_network, T1.id_franchising, T1.id_agencie
from network_car_destinations as T1
where T1.id_network='12' and ( T1.id_franchising = 968 or T1.id_franchising = 974 )
) AS n ON n.id_franchising=i.id_franchising AND n.id_car=i.id_car AND c.id_network=n.id_network
INNER JOIN affiliate_tipologies AS t ON t.id_tipology_ag=i.idCategory AND t.id_franchising=i.id_franchising
INNER JOIN network_assoc_tipologies AS p ON p.id_network=c.id_network AND p.id_default=t.id_tipology_net
WHERE c.id_network='12' and c.code_affiliate='69842' AND c.configured = 'yes' AND i.Code_car not like ''
GROUP BY i.Code_car
) as T2
LEFT JOIN network_conf_users as ce on ce.`id_affiliate` = T2.`id_affiliate` and ce.id_user = T2.id_user
WHERE c.id_network='12' and c.code_affiliate='69842' AND c.configured = 'yes' AND i.Code_car not like '' and p.code != '0'
GROUP BY i.Code_car
The EXPLAIN are almost the same, the result is the same for both query, but the first query is taking 20 secs to end, the second 0.02, how the position of the left join can influence so much the execution of the query?
EXPLAIN - Slow query:
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY c index_merge id_affiliate,id_network,configured,code_affiliate code_affiliate,id_network 203,5 NULL 1 100.00 Using intersect(code_affiliate,id_network); Using where; Using temporary; Using filesort
1 PRIMARY fr eq_ref PRIMARY PRIMARY 4 c.id_franchising 1 100.00 Using index
1 PRIMARY <derived2> ref <auto_key1> <auto_key1> 5 const 10 100.00 Using where
1 PRIMARY t ref id_franchising,id_tipology_ag,id_tipology_net id_franchising 5 n.id_franchising 13 100.00 Using where
1 PRIMARY p ref id_network,id_default id_default 5 t.id_tipology 26 100.00 Using where
1 PRIMARY i ref id_car,id_affiliate,id_franchising,idCategory id_car 5 n.id_car 3 100.00 Using where
1 PRIMARY ce ALL id_affiliate,id_user NULL NULL NULL 4 75.00 Using where; Using join buffer (Block Nested Loop)
2 DERIVED T1 ref id_franchising,id_network id_network 5 const 136952 100.00 Using where
SHOW WARNINGS:
Level Code Message
Note 1003 /* select#1 */
select `i`.`id_affiliate` AS `id_affiliate`,
`i`.`id_franchising` AS `id_franchising`,
`i`.`Code_car` AS `Code_car`
from `network_configuration_affiliate` `c`
join `franchising` `fr`
join `network_selected_car` `i`
join (/* select#2 */
select `T1`.`id_car` AS `id_car`,
`T1`.`id_network` AS `id_network`,
`T1`.`id_franchising` AS `id_franchising`,
`T1`.`id_agencie` AS `id_agencie`
from `network_car_destinations` `T1`
where ((`T1`.`id_network` = '12')
and ((`T1`.`id_franchising` = 968)
or (`T1`.`id_franchising` = 974)))
) `n`
join `affiliate_tipologies` `t`
join `network_assoc_tipologies` `p`
left join `network_conf_users` `ce` on(((`ce`.`id_user` = `i`.`id_user`)
and (`ce`.`id_affiliate` = `c`.`id_affiliate`))
)
where ((`fr`.`id` = `c`.`id_franchising`)
and (`i`.`id_affiliate` = `c`.`id_affiliate`)
and (`i`.`id_car` = `n`.`id_car`)
and (`t`.`id_franchising` = `n`.`id_franchising`)
and (`i`.`id_franchising` = `n`.`id_franchising`)
and (`i`.`idCategory` = `t`.`id_tipology_ag`)
and (`p`.`id_default` = `t`.`id_tipology_net`)
and (`n`.`id_network` = `c`.`id_network`)
and (`p`.`id_network` = `c`.`id_network`)
and (`c`.`configured` = 'yes')
and (`c`.`code_affiliate` = '69842')
and (`c`.`id_network` = '12')
and (not((`i`.`Code` like '')))
and (`p`.`code` <> '0')
)
group by `i`.`Code_car`
Fast query:
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 7280 100.00 NULL
1 PRIMARY ce ALL id_affiliate,id_user NULL NULL NULL 4 75.00 Using where; Using join buffer (Block Nested Loop)
2 DERIVED c index_merge id_affiliate,id_network,configured,code_affiliate code_affiliate,id_network 203,5 NULL 1 100.00 Using intersect(codice_affiliato,id_network); Using where; Using temporary; Using filesort
2 DERIVED fr eq_ref PRIMARY PRIMARY 4 c.id_franchising 1 100.00 Using index
2 DERIVED <derived3> ref <auto_key1> <auto_key1> 5 const 10 100.00 Using where
2 DERIVED i ref id_car,id_affiliate,id_franchising,idCategory id_car 5 n.id_car 2 100.00 Using where
2 DERIVED t ref id_franchising,id_tipology_ag,id_tipology_net id_franchising 5 n.id_franchising 14 100.00 Using where
2 DERIVED p ref id_network,id_default id_default 5 t.id_tipology_net 26 100.00 Using where
3 DERIVED T1 ref id_franchising,id_network id_network 5 const 133324 100.00 Using where
SHOW WARNINGS:
Level Code Message
Note 1003 /* select#1 */
select `T2`.id_affiliate AS id_affiliate,
`T2`.`id_franchising` AS `id_franchising`,
`T2`.Code_car AS Code_car,`T2`.id_user AS id_user
from (/* select#2 */
select `i`.id_affiliate AS id_affiliate,
`i`.`id_franchising` AS `id_franchising`,
`i`.Code_car AS Code_car,`i`.id_user AS id_user
from network_configuration_affiliate `c`
join `franchising` `fr`
join network_selected_car `i`
join (/* select#3 */
select `T1`.id_car AS id_car,
`T1`.`id_network` AS `id_network`,
`T1`.`id_franchising` AS `id_franchising`,
`T1`.`id_agencie` AS `id_agencie`
from network_car_destinations `T1`
where ((`T1`.`id_network` = '12')
and ((`T1`.`id_franchising` = 968)
or (`T1`.`id_franchising` = 974)))
) `n`
join affiliate_tipologies `t`
join network_assoc_tipologies `p`
where ((`fr`.`id` = `c`.`id_franchising`)
and (`i`.id_affiliate = `c`.id_affiliate)
and (`i`.id_car = `n`.id_car)
and (`i`.`id_franchising` = `n`.`id_franchising`)
and (`t`.`id_franchising` = `n`.`id_franchising`)
and (`t`.`id_tipology_ag` = `i`.`idCategory`)
and (`p`.`id_default` = `t`.`id_tipology_net`)
and (`n`.`id_network` = `c`.`id_network`)
and (`p`.`id_network` = `c`.`id_network`)
and (`c`.`configured` = 'yes')
and (`c`.`code_affiliate` = '69842')
and (`c`.`id_network` = '12')
and (not((`i`.Code_car like '')))
)
group by `i`.Code_car
) `T2`
left join network_conf_users `ce` on(((`ce`.id_affiliate = `T2`.id_affiliate)
and (`ce`.id_user = `T2`.id_user))
)
where 1

Answer 1

The left join finds those fields to the left and retrieves those from the right, while inner joining will lookup all fields. Keeping it simple by not needing to organize the extra columns the query completes much quicker.

Answer 2

Please show us the EXPLAINs.
The Optimizer will turn LEFT JOIN into JOIN if it decides that it makes no difference. Please do EXPLAIN EXTENDED SELECT ... followed immediately by SHOW WARNINGS; so we can see whether this happened.
The Optimizer will try a variety of orders for JOINing (in the absence of LEFT or RIGHT). So, if the LEFT was really redundant, I would expect the EXPLAINs to have the tables in the same order.
Normally, the Optimizer will start with a "derived" table, n in your examples. But it may be hidden behind the LEFT.
Do you have any "composite" indexes? These are likely to be beneficial:
T1: INDEX(id_network, id_franchising) -- in this order
c: INDEX(id_network, code_affiliate, configured)
Revise
With the recent changes to the query, I see this pattern:
Slow:
SELECT ...
FROM ...
JOIN ...
GROUP BY ...
Quick:
SELECT ...
FROM ( SELECT ...
FROM ...
GROUP BY ... )
JOIN ...
(LEFT is only partially important to the question; any JOIN can exhibit the problem.)
I call the slow one "inflate-deflate". By that I mean that first it did join(s), thereby inflating the number of rows. Then it did a GROUP BY, which deflated the results.
The quick one deflated before doing the final join, there by leading to less effort overall.
index merge intersect can almost always be improved by using a 'composite' index. In this case, the one I suggest for c. (This would speed up both versions of the query.)