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This article mainly introduces MySQL innodb how to choose a cluster index, has a certain reference value, interested friends can refer to, I hope you read this article after a lot of gains, the following let Xiaobian take you to understand.

For innodb, the primary key is undoubtedly a clustered index. But what does innodb do when a table doesn't have a primary key or an index? See the following rules:

If a primary key is defined, it is used as a clustered index.

If no primary key is defined, the first unique non-null index of the table is treated as a clustered index.

If there is no primary key and no suitable unique index, innodb internally generates a hidden primary key as a clustered index. This hidden primary key is a 6-byte column whose value increases as the data is inserted.

Places to watch:

The leaf nodes of the secondary index do not store the physical address of the row data. It is the primary key value of the row stored.

So: the secondary index consists of two lookups. Once to find the secondary index itself. Then look up the primary key (clustered index)

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