Question

EXPLAIN SELECT * 
FROM (
`phppos_items`
)
WHERE (
name LIKE  'AB10LA2%'
OR item_number LIKE  'AB10LA2%'
OR category LIKE  'AB10LA2%'
)
AND deleted =0
ORDER BY  `name` ASC 
LIMIT 16

+----+-------------+--------------+-------+-----------------------------------+------+---------+------+------+-------------+
| id | select_type | table        | type  | possible_keys                     | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------------+-------+-----------------------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | phppos_items | index | item_number,name,category,deleted | name | 257     | NULL |   32 | Using where |
+----+-------------+--------------+-------+-----------------------------------+------+---------+------+------+-------------+

此查询需要9秒才能运行（该表有100万+行）。

我有一个关于item_number，name，category的索引，单独删除。如何加快查询速度？

Answer 1

最好我知道，MySQL不知道如何执行位图或索引扫描。但是你可以将它重写为三个查询的联合，以强制它做这样的事情，如果你在每个字段上有一个索引。如果是这样，这将非常快：

select *
from (
  select * from (
           select *
           from phppos_items
           where name like 'AB10LA2%' and deleted = 0
           order by `name` limit 16
           ) t
  union
  select * from (
           select *
           from phppos_items
           where item_number like 'AB10LA2%' and deleted = 0
           order by `name` limit 16
           ) t
  union
  select * from (
           select *
           from phppos_items
           where category like 'AB10LA2%' and deleted = 0
           order by `name` limit 16
           ) t
  ) as top rows
order by `name` limit 16

Answer 2

OR运算符可能是执行计划的毒药。您可以尝试使用等效OR替换UNION子句来重新表达您的查询：

SELECT * 
FROM (
  SELECT * FROM `phppos_items`
  WHERE name LIKE 'AB10LA2%'
  UNION
  SELECT * FROM `phppos_items`
  WHERE item_number LIKE 'AB10LA2%'
  UNION
  SELECT * FROM `phppos_items`
  WHERE category LIKE  'AB10LA2%'
)
WHERE deleted =0
ORDER BY  `name` ASC 
LIMIT 16

这将允许MySQL在将UNION运算符应用于每个子查询的结果之前并行运行多个子查询。我知道这对Oracle有很大帮助。也许MySQL可以做类似的事情？注意：我假设LIKE 'AB10LA2%'是一个非常有选择性的过滤器。否则，由于延迟排序和限制执行计划，这可能无法改善。有关更一般的方法，请参阅Denis's答案。

在任何情况下，我认为多列索引对您没有帮助，因为您的搜索表达式中有'%'个符号。这样，只能使用多列索引中的第一列，其余的仍然需要索引扫描或全表扫描。

我需要多列索引吗？

2 个答案: