MySQL 8.0 Reference Manual（读书笔记82节-- InnoDB and Online DDL (2)）

1. Column Operations

The following table provides an overview of online DDL support for column operations. An asterisk indicates additional information, an exception, or a dependency.

• Adding a column

ALTER TABLE tbl_name ADD COLUMN column_name column_definition, ALGORITHM=INSTANT;

INSTANT is the default algorithm as of MySQL 8.0.12, and INPLACE before that.

The following limitations apply when the INSTANT algorithm adds a column:---场景限制

• A statement cannot combine【kəmˈbaɪn 结合;联合;合并;混合;使融合；兼做;兼办;】 the addition of a column with other ALTER TABLE actions that do not support the INSTANT algorithm.

• The INSTANT algorithm can add a column at any position in the table. Before MySQL 8.0.29, the INSTANT algorithm could only add a column as the last column of the table.--因版本不同，指定位置（befor、after）会影响算法

• Columns cannot be added to tables that use ROW_FORMAT=COMPRESSED, tables with a FULLTEXT index, tables that reside【rɪˈzaɪd 居住在;定居于;】 in the data dictionary tablespace, or temporary tables. Temporary tables only support ALGORITHM=COPY.---这个共关键

• MySQL checks the row size when the INSTANT algorithm adds a column, and throws the following error if the addition exceeds the limit.--不能超过长度限制

ERROR 4092 (HY000): Column can't be added with ALGORITHM=INSTANT as
after this max possible row size crosses max permissible row size. Try
ALGORITHM=INPLACE/COPY.

Before MySQL 8.0.29, MySQL does not check the row size when the INSTANT algorithm adds a column. However, MySQL does check the row size during DML operations that insert and update rows in the table.【意思是DDL的时候不检查，DML的时候检查】

• The maximum number of columns in the internal【ɪnˈtɜːrnl 里面的;本身的;内政的;体内的;内心的;(机构)内部的;】 representation【ˌreprɪzenˈteɪʃn 代表;陈述;表现;描述;支持;描绘;表现形式;维护;抗议;有代理人;】 of the table cannot exceed 1022 after column addition with the INSTANT algorithm. The error message is:--不能超过列（字段）数限制

ERROR 4158 (HY000): Column can't be added to tbl_name with
ALGORITHM=INSTANT anymore. Please try ALGORITHM=INPLACE/COPY

Multiple columns may be added in the same ALTER TABLE statement. For example: --支持一次添加多列（字段）

ALTER TABLE t1 ADD COLUMN c2 INT, ADD COLUMN c3 INT, ALGORITHM=INSTANT;

A new row version is created after each ALTER TABLE ... ALGORITHM=INSTANT operation that adds one or more columns, drops one or more columns, or adds and drops one or more columns in the same operation. The INFORMATION_SCHEMA.INNODB_TABLES.TOTAL_ROW_VERSIONS column tracks the number of row versions for a table. The value is incremented each time a column is instantly added or dropped. The initial value is 0.

mysql> SELECT NAME, TOTAL_ROW_VERSIONS FROM INFORMATION_SCHEMA.INNODB_TABLES 
 WHERE NAME LIKE 'test/t1';
+---------+--------------------+
| NAME | TOTAL_ROW_VERSIONS |
+---------+--------------------+
| test/t1 | 0 |
+---------+--------------------+

When a table with instantly added or dropped columns is rebuilt by table-rebuilding ALTER TABLE or OPTIMIZE TABLE operation, the TOTAL_ROW_VERSIONS value is reset to 0. The maximum number of row versions permitted is 64, as each row version requires additional space for table metadata. When the row version limit is reached, ADD COLUMN and DROP COLUMN operations using ALGORITHM=INSTANT are rejected with an error message that recommends rebuilding the table using the COPY or INPLACE algorithm.---64是临界值

ERROR 4080 (HY000): Maximum row versions reached for table test/t1. No more
columns can be added or dropped instantly. Please use COPY/INPLACE.

Concurrent DML is not permitted when adding an auto-increment column. Data is reorganized substantially【[səbˈstænʃəli 基本上;大体上;非常;大大地;总的来说;】, making it an expensive operation. At a minimum, ALGORITHM=INPLACE, LOCK=SHARED is required.

 The table is rebuilt if ALGORITHM=INPLACE is used to add a column.

• Dropping a column

ALTER TABLE tbl_name DROP COLUMN column_name, ALGORITHM=INSTANT;

INSTANT is the default algorithm as of MySQL 8.0.29, and INPLACE before that.

The following limitations apply when the INSTANT algorithm is used to drop a column:

• Dropping a column cannot be combined in the same statement with other ALTER TABLE actions that do not support ALGORITHM=INSTANT.

• Columns cannot be dropped from tables that use ROW_FORMAT=COMPRESSED, tables with a FULLTEXT index, tables that reside in the data dictionary tablespace, or temporary tables. Temporary tables only support ALGORITHM=COPY.

Multiple columns may be dropped in the same ALTER TABLE statement; for example:

ALTER TABLE t1 DROP COLUMN c4, DROP COLUMN c5, ALGORITHM=INSTANT;

Each time a column is added or dropped using ALGORITHM=INSTANT, a new row version is created. The INFORMATION_SCHEMA.INNODB_TABLES.TOTAL_ROW_VERSIONS column tracks the number of row versions for a table. The value is incremented each time a column is instantly added or dropped. The initial value is 0.

mysql> SELECT NAME, TOTAL_ROW_VERSIONS FROM INFORMATION_SCHEMA.INNODB_TABLES 
 WHERE NAME LIKE 'test/t1';
+---------+--------------------+
| NAME | TOTAL_ROW_VERSIONS |
+---------+--------------------+
| test/t1 | 0 |
+---------+--------------------+

When a table with instantly added or dropped columns is rebuilt by table-rebuilding ALTER TABLE or OPTIMIZE TABLE operation, the TOTAL_ROW_VERSIONS value is reset to 0. The maximum number of row versions permitted is 64, as each row version requires additional space for table metadata. When the row version limit is reached, ADD COLUMN and DROP COLUMN operations using ALGORITHM=INSTANT are rejected with an error message that recommends rebuilding the table using the COPY or INPLACE algorithm.

ERROR 4080 (HY000): Maximum row versions reached for table test/t1. No more
columns can be added or dropped instantly. Please use COPY/INPLACE.

If an algorithm other than ALGORITHM=INSTANT is used, data is reorganized substantially, making it an expensive operation.

• Renaming a column

ALTER TABLE tbl CHANGE old_col_name new_col_name data_type, ALGORITHM=INSTANT, LOCK=NONE;

ALGORITHM=INSTANT support for renaming a column was added in MySQL 8.0.28. Earlier MySQL Server releases support only ALGORITHM=INPLACE and ALGORITHM=COPY when renaming a column.

To permit concurrent DML, keep the same data type and only change the column name.---保持字段类型不变，更新的只是名字

When you keep the same data type and [NOT] NULL attribute, only changing the column name, the operation can always be performed online.---并且NULL属性值也没有调整

Renaming a column referenced from another table is only permitted with ALGORITHM=INPLACE.【此时，只能是INPLACE】 If you use ALGORITHM=INSTANT, ALGORITHM=COPY, or some other condition that causes the operation to use those algorithms, the ALTER TABLE statement fails.

ALGORITHM=INSTANT supports renaming a virtual column; ALGORITHM=INPLACE does not.

ALGORITHM=INSTANT and ALGORITHM=INPLACE do not support renaming a column when adding or dropping a virtual column in the same statement. In this case, only ALGORITHM=COPY is supported.

• Reordering columns

To reorder columns, use FIRST or AFTER in CHANGE or MODIFY operations.

ALTER TABLE tbl_name MODIFY COLUMN col_name column_definition FIRST, ALGORITHM=INPLACE, LOCK=NONE;

Data is reorganized substantially, making it an expensive operation.

• Changing the column data type

ALTER TABLE tbl_name CHANGE c1 c1 BIGINT, ALGORITHM=COPY;

 Changing the column data type is only supported with ALGORITHM=COPY.

• Extending VARCHAR column size

ALTER TABLE tbl_name CHANGE COLUMN c1 c1 VARCHAR(255), ALGORITHM=INPLACE, LOCK=NONE;

The number of length bytes required by a VARCHAR column must remain【rɪˈmeɪn 保持不变；仍然存在，继续存在；】 the same. For VARCHAR columns of 0 to 255 bytes in size, one length byte is required to encode the value. For VARCHAR columns of 256 bytes in size or more, two length bytes are required. As a result, in-place ALTER TABLE only supports increasing VARCHAR column size from 0 to 255 bytes, or from 256 bytes to a greater size. In-place ALTER TABLE does not support increasing the size of a VARCHAR column from less than 256 bytes to a size equal to or greater than 256 bytes. In this case, the number of required length bytes changes from 1 to 2, which is only supported by a table copy (ALGORITHM=COPY). For example, attempting to change VARCHAR column size for a single byte character set from VARCHAR(255) to VARCHAR(256) using in-place ALTER TABLE returns this error:

ALTER TABLE tbl_name ALGORITHM=INPLACE, CHANGE COLUMN c1 c1 VARCHAR(256);
ERROR 0A000: ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.

Decreasing VARCHAR size using in-place ALTER TABLE is not supported. Decreasing VARCHAR size requires a table copy (ALGORITHM=COPY).

• Setting a column default value

ALTER TABLE tbl_name ALTER COLUMN col SET DEFAULT literal, ALGORITHM=INSTANT;

Only modifies table metadata. Default column values are stored in the data dictionary.

• Dropping a column default value

ALTER TABLE tbl ALTER COLUMN col DROP DEFAULT, ALGORITHM=INSTANT;

• Changing the auto-increment value

ALTER TABLE table AUTO_INCREMENT=next_value, ALGORITHM=INPLACE, LOCK=NONE;

Modifies a value stored in memory, not the data file.

In a distributed system using replication or sharding, you sometimes reset the auto-increment counter for a table to a specific value. The next row inserted into the table uses the specified value for its auto-increment column. You might also use this technique in a data warehousing environment where you periodically【ˌpiriˈɑdɪkəli】 empty all the tables and reload them, and restart the auto-increment sequence from 1.

• Making a column NULL

ALTER TABLE tbl_name MODIFY COLUMN column_name data_type NULL, ALGORITHM=INPLACE, LOCK=NONE;

Rebuilds the table in place. Data is reorganized substantially, making it an expensive operation.

• Making a column NOT NULL

ALTER TABLE tbl_name MODIFY COLUMN column_name data_type NOT NULL, ALGORITHM=INPLACE, LOCK=NONE;

Rebuilds the table in place. STRICT_ALL_TABLES or STRICT_TRANS_TABLES SQL_MODE is required for the operation to succeed. The operation fails if the column contains NULL values. The server prohibits【prəˈhɪbɪts (尤指以法令)禁止;阻止;使不可能;】 changes to foreign key columns that have the potential【pəˈtenʃl 潜在的;可能的;】 to cause loss of referential【refəˈrenʃl】 integrity【参照完整性】.Data is reorganized substantially, making it an expensive operation.

• Modifying the definition of an ENUM or SET column

CREATE TABLE t1 (c1 ENUM('a', 'b', 'c'));
ALTER TABLE t1 MODIFY COLUMN c1 ENUM('a', 'b', 'c', 'd'), ALGORITHM=INSTANT;

Modifying the definition of an ENUM or SET column by adding new enumeration【ɪˌnuːməˈreɪʃn】 or set members to the end of the list of valid member values may be performed instantly or in place, as long as the storage size of the data type does not change. For example, adding a member to a SET column that has 8 members changes the required storage per value from 1 byte to 2 bytes; this requires a table copy. Adding members in the middle of the list causes renumbering of existing members, which requires a table copy【个数和类型长度很重要】.

2. Generated Column Operations

The following table provides an overview of online DDL support for generated column operations.

• Adding a STORED column

ALTER TABLE t1 ADD COLUMN (c2 INT GENERATED ALWAYS AS (c1 + 1) STORED), ALGORITHM=COPY;

ADD COLUMN is not an in-place operation for stored columns (done without using a temporary table) because the expression must be evaluated by the server.

• Modifying STORED column order

ALTER TABLE t1 MODIFY COLUMN c2 INT GENERATED ALWAYS AS (c1 + 1) STORED FIRST, ALGORITHM=COPY;

Rebuilds the table in place.

• Dropping a STORED column

ALTER TABLE t1 DROP COLUMN c2, ALGORITHM=INPLACE, LOCK=NONE;

Rebuilds the table in place.

• Adding a VIRTUAL column

ALTER TABLE t1 ADD COLUMN (c2 INT GENERATED ALWAYS AS (c1 + 1) VIRTUAL), ALGORITHM=INSTANT;

Adding a virtual column can be performed instantly or in place for non-partitioned tables.

Adding a VIRTUAL is not an in-place operation for partitioned tables.

• Modifying VIRTUAL column order

ALTER TABLE t1 MODIFY COLUMN c2 INT GENERATED ALWAYS AS (c1 + 1) VIRTUAL FIRST, ALGORITHM=COPY;

• Dropping a VIRTUAL column

ALTER TABLE t1 DROP COLUMN c2, ALGORITHM=INSTANT;

Dropping a VIRTUAL column can be performed instantly or in place for non-partitioned tables.

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