在MySQL中避免全表扫描

Question

如何避免在mysql上进行全表扫描？

Answer 1

通常，通过确保在WHERE ， JOIN和ORDER BY子句中出现的字段上有可用的索引。

Answer 2

Index your data.

Write queries that use those indexes.

Anything more than that we need specifics.

Answer 3

另请注意，有时您无法摆脱全表扫描，即当您需要表中的所有行...或者扫描索引的成本是扫描整个表的成本时。

Answer 4

Use a LIMIT clause when you know how many rows you are expecting to return, for example if you are looking for a record with a known ID field that is unique, limit your select to 1, that way mysql will stop searching after it finds the first record. The same goes for updates and deletes.

SELECT * FROM `yourTable` WHERE `idField` = 123 LIMIT 1