mysql - 查询庞大的 MySQL DB

Question

我有一个带有两列的 MySQL 数据库。“钥匙”和“二手”。Key 是一个字符串，Used 是一个整数。有没有一种非常快速的方法来搜索特定的键，然后在一个包含 6000000 行数据的巨大 MySQL 数据库中返回使用。

Answer 1

key您可以通过在字段上创建索引来加快速度：

CREATE INDEX mytable_key_idx ON mytable (`key`);

实际上，您可以通过在两个(key, used)字段上创建覆盖索引来加快阅读速度：

CREATE INDEX mytable_key_used_idx ON mytable (`key`, `used`);

在这种情况下，当读取时，MySQL 可以used从索引本身中检索值，而无需读取表（仅索引扫描）。但是，如果您有很多写入活动，覆盖索引可能会运行得更慢，因为现在它必须同时更新索引和实际表。

Answer 2

The normative SQL for that would be:

SELECT t.key, t.used FROM mytable t WHERE t.key = 'particularvalue' ; 

The output from

EXPLAIN 
SELECT t.key, t.used FROM mytable t WHERE t.key = 'particularvalue' ; 

Would give details about the access plan, what indexes are being considered, etc.

The output from a

SHOW CREATE TABLE mytable ; 

would give information about the table, the engine being used and the available indexes, as well as the datatypes.

Slow performance on a query like this is usually indicative of a suboptimal access plan, either because suitable indexes are not available, or not being used. Sometimes, a characterset mismatch between the column datatype and the literal datatype in the predicate can make an index "unusable" by a particular query.