问题
I have the fowlloing query
select * from mapping_channel_fqdn_virtual_host where id in (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
explaining the above query gives me the following result:
explain select * from mapping_channel_fqdn_virtual_host where id in (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
+----+-------------+-----------------------------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------------------------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | mapping_channel_fqdn_virtual_host | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 10 | 100.00 | Using where |
+----+-------------+-----------------------------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0,00 sec)
id is the primary key of the table. It looks like this is a range type of query and it uses the Primary Key as index for this one.
When I try to explain the following query I get different results:
explain select * from mapping_channel_fqdn_virtual_host where id in (select max(id) from mapping_channel_fqdn_virtual_host group by channel_id, fqdn_virtual_host_id);
+----+-------------+-----------------------------------+------------+-------+------------------+------------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------------------------------+------------+-------+------------------+------------------+---------+------+------+----------+-------------+
| 1 | PRIMARY | mapping_channel_fqdn_virtual_host | NULL | ALL | NULL | NULL | NULL | NULL | 4849 | 100.00 | Using where |
| 2 | SUBQUERY | mapping_channel_fqdn_virtual_host | NULL | index | idx_channel_fqdn | idx_channel_fqdn | 8 | NULL | 4849 | 100.00 | Using index |
+----+-------------+-----------------------------------+------------+-------+------------------+------------------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0,00 sec)
idx_channel_fqdn is a composite index key for the column pair used in the groupby clause. But when using subquery the Primary query stops using the index like it did before. Can you explain why this is happening?
Tried the JOIN query Anouar suggested:
explain select * from mapping_channel_fqdn_virtual_host as x join (select max(id) as ids from mapping_channel_fqdn_virtual_host group by channel_id, fqdn_virtual_host_id) as y on x.id=y.ids;
+----+-------------+-----------------------------------+------------+--------+------------------+------------------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------------------------------+------------+--------+------------------+------------------+---------+-------+------+----------+-------------+
| 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 4849 | 100.00 | Using where |
| 1 | PRIMARY | x | NULL | eq_ref | PRIMARY | PRIMARY | 4 | y.ids | 1 | 100.00 | NULL |
| 2 | DERIVED | mapping_channel_fqdn_virtual_host | NULL | index | idx_channel_fqdn | idx_channel_fqdn | 8 | NULL | 4849 | 100.00 | Using index |
+----+-------------+-----------------------------------+------------+--------+------------------+------------------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0,00 sec)
Judging by the index and eq_ref types it looks like it is better to use the JOIN than the subquery? Can you explain a bit more the outcome of the join explain expressiong?
回答1:
You can see the answers for this question You will fine ideas.
I am quoting from some answers
Sometimes MySQL does not use an index, even if one is available. One circumstance under which this occurs is when the optimizer estimates that using the index would require MySQL to access a very large percentage of the rows in the table. (In this case, a table scan is likely to be much faster because it requires fewer seeks.)
Briefly, Try forcing the index:
SELECT *
FROM mapping_channel_fqdn_virtual_host FORCE INDEX (name of the index you want to use)
WHERE (mapping_channel_fqdn_virtual_host.id IN (1,2,3,4,5,6,7,8,9,10));
Or use JOIN instead and see the explain
SELECT * FROM mapping_channel_fqdn_virtual_host mcf
JOIN (select max(id) as ids from mapping_channel_fqdn_virtual_host group by channel_id, fqdn_virtual_host_id)) AS mcfv
ON mcf.id = mcfv.ids;
来源：https://stackoverflow.com/questions/47097508/using-in-with-a-subquery-doesnt-use-index