如果我创建一个没有主键的 table 然后添加一个主键，主键是聚簇索引吗？

Question

来自 Getting Started with Indexes: Primary Key 它说：

In InnoDB tables, all indexes contain the primary key as a suffix. Thus, when using this storage engine, keeping the primary key as small as possible is particularly important. If a primary key does not exist and there are no UNIQUE indexes, InnoDB creates a 6-bytes clustered index which is invisible to the user.

因此，在没有主键的情况下，我们将创建一个带有隐藏聚集索引的 table。然后在同一部分后面的几段后面说：

You cannot create a primary key with the CREATE INDEX command. If you do want to add one after the table has already been created, use ALTER TABLE, for example:

但没有说明新主键是否会成为聚簇索引。

也ALTER TABLE: ADD PRIMARY KEY只说：

For PRIMARY KEY indexes, you can specify a name for the index, but it is silently ignored, and the name of the index is always PRIMARY.

但不评论聚集索引。

我的困惑导致了这些问题：

如果我以后添加主键，主键会聚簇吗？
如果是这样，存储布局会发生什么变化？是否必须将所有数据复制到新的聚簇索引，然后删除之前隐藏的聚簇索引？
主键总是聚簇吗？

最小复制示例：

create database test
use test
create table test_table ( id bigint(20) NULL );

describe test_table;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id    | bigint(20) | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
1 row in set (0.003 sec)

show index from test_table;
Empty set (0.000 sec)


alter table test_table add primary key(id);
Query OK, 0 rows affected (0.067 sec)
Records: 0  Duplicates: 0  Warnings: 0

describe test_table;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id    | bigint(20) | NO   | PRI | NULL    |       |
+-------+------------+------+-----+---------+-------+
1 row in set (0.007 sec)

show index from test_table;
+------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table      | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test_table |          0 | PRIMARY  |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.001 sec)

Answer 1

如果 table 有一个主键，那么它总是聚簇索引（假设它在 InnoDB 引擎中）。

如果您更改 table 以添加主键，那么它将成为聚簇索引。

添加（或更改）主键确实需要重构 table，因为布局将完全改变。

如果我创建一个没有主键的 table 然后添加一个主键，主键是聚簇索引吗？

If I create a table without a primary key then add a primary key, is the primary key a clustered index?

mysql

mariadb