16.4.1.10 Replication with Differing Table Definitions on Source and Replica (original) (raw)

16.4.1.10 Replication with Differing Table Definitions on Source and Replica

Source and target tables for replication do not have to be identical. A table on the source can have more or fewer columns than the replica's copy of the table. In addition, corresponding table columns on the source and the replica can use different data types, subject to certain conditions.

In all cases where the source and target tables do not have identical definitions, the database and table names must be the same on both the source and the replica. Additional conditions are discussed, with examples, in the following two sections.

16.4.1.10.1 Replication with More Columns on Source or Replica

You can replicate a table from the source to the replica such that the source and replica copies of the table have differing numbers of columns, subject to the following conditions:

• Columns common to both versions of the table must be defined in the same order on the source and the replica.
  (This is true even if both tables have the same number of columns.)
• Columns common to both versions of the table must be defined before any additional columns.
  This means that executing an ALTER TABLE statement on the replica where a new column is inserted into the table within the range of columns common to both tables causes replication to fail, as shown in the following example:
  Suppose that a table t, existing on the source and the replica, is defined by the followingCREATE TABLE statement:

CREATE TABLE t (  
    c1 INT,  
    c2 INT,  
    c3 INT  
);  

Suppose that the ALTER TABLE statement shown here is executed on the replica:

ALTER TABLE t ADD COLUMN cnew1 INT AFTER c3;  

The previous ALTER TABLE is permitted on the replica because the columnsc1, c2, andc3 that are common to both versions of table t remain grouped together in both versions of the table, before any columns that differ.
However, the following ALTER TABLE statement cannot be executed on the replica without causing replication to break:

ALTER TABLE t ADD COLUMN cnew2 INT AFTER c2;  

Replication fails after execution on the replica of theALTER TABLE statement just shown, because the new column cnew2 comes between columns common to both versions oft.

• Each “extra” column in the version of the table having more columns must have a default value.
  A column's default value is determined by a number of factors, including its type, whether it is defined with aDEFAULT option, whether it is declared as NULL, and the server SQL mode in effect at the time of its creation; for more information, see Section 11.6, “Data Type Default Values”).

In addition, when the replica's copy of the table has more columns than the source's copy, each column common to the tables must use the same data type in both tables.

Examples. The following examples illustrate some valid and invalid table definitions:

More columns on the source. The following table definitions are valid and replicate correctly:

source> CREATE TABLE t1 (c1 INT, c2 INT, c3 INT);
replica>  CREATE TABLE t1 (c1 INT, c2 INT);

The following table definitions would raise an error because the definitions of the columns common to both versions of the table are in a different order on the replica than they are on the source:

source> CREATE TABLE t1 (c1 INT, c2 INT, c3 INT);
replica>  CREATE TABLE t1 (c2 INT, c1 INT);

The following table definitions would also raise an error because the definition of the extra column on the source appears before the definitions of the columns common to both versions of the table:

source> CREATE TABLE t1 (c3 INT, c1 INT, c2 INT);
replica>  CREATE TABLE t1 (c1 INT, c2 INT);

More columns on the replica. The following table definitions are valid and replicate correctly:

source> CREATE TABLE t1 (c1 INT, c2 INT);
replica>  CREATE TABLE t1 (c1 INT, c2 INT, c3 INT);

The following definitions raise an error because the columns common to both versions of the table are not defined in the same order on both the source and the replica:

source> CREATE TABLE t1 (c1 INT, c2 INT);
replica>  CREATE TABLE t1 (c2 INT, c1 INT, c3 INT);

The following table definitions also raise an error because the definition for the extra column in the replica's version of the table appears before the definitions for the columns which are common to both versions of the table:

source> CREATE TABLE t1 (c1 INT, c2 INT);
replica>  CREATE TABLE t1 (c3 INT, c1 INT, c2 INT);

The following table definitions fail because the replica's version of the table has additional columns compared to the source's version, and the two versions of the table use different data types for the common columnc2:

source> CREATE TABLE t1 (c1 INT, c2 BIGINT);
replica>  CREATE TABLE t1 (c1 INT, c2 INT, c3 INT);

16.4.1.10.2 Replication of Columns Having Different Data Types

Corresponding columns on the source's and the replica's copies of the same table ideally should have the same data type. However, this is not always strictly enforced, as long as certain conditions are met.

It is usually possible to replicate from a column of a given data type to another column of the same type and same size or width, where applicable, or larger. For example, you can replicate from a CHAR(10) column to anotherCHAR(10), or from aCHAR(10) column to aCHAR(25) column without any problems. In certain cases, it also possible to replicate from a column having one data type (on the source) to a column having a different data type (on the replica); when the data type of the source's version of the column is promoted to a type that is the same size or larger on the replica, this is known as attribute promotion.

Attribute promotion can be used with both statement-based and row-based replication, and is not dependent on the storage engine used by either the source or the replica. However, the choice of logging format does have an effect on the type conversions that are permitted; the particulars are discussed later in this section.

Important

Whether you use statement-based or row-based replication, the replica's copy of the table cannot contain more columns than the source's copy if you wish to employ attribute promotion.

Statement-based replication. When using statement-based replication, a simple rule of thumb to follow is, “If the statement run on the source would also execute successfully on the replica, it should also replicate successfully”. In other words, if the statement uses a value that is compatible with the type of a given column on the replica, the statement can be replicated. For example, you can insert any value that fits in a TINYINT column into aBIGINT column as well; it follows that, even if you change the type of a TINYINT column in the replica's copy of a table toBIGINT, any insert into that column on the source that succeeds should also succeed on the replica, since it is impossible to have a legalTINYINT value that is large enough to exceed a BIGINT column.

Prior to MySQL 5.7.1, when using statement-based replication,AUTO_INCREMENT columns were required to be the same on both the source and the replica; otherwise, updates could be applied to the wrong table on the replica. (Bug #12669186)

Row-based replication: attribute promotion and demotion. Row-based replication supports attribute promotion and demotion between smaller data types and larger types. It is also possible to specify whether or not to permit lossy (truncated) or non-lossy conversions of demoted column values, as explained later in this section.

Lossy and non-lossy conversions. In the event that the target type cannot represent the value being inserted, a decision must be made on how to handle the conversion. If we permit the conversion but truncate (or otherwise modify) the source value to achieve a“fit” in the target column, we make what is known as a lossy conversion. A conversion which does not require truncation or similar modifications to fit the source column value in the target column is anon-lossy conversion.

Type conversion modes (slave_type_conversions variable). The setting of the slave_type_conversions global server variable controls the type conversion mode used on the replica. This variable takes a set of values from the following table, which shows the effects of each mode on the replica's type-conversion behavior:

When an integer type is promoted, its signedness is not preserved. By default, the replica treats all such values as signed. Beginning with MySQL 5.7.2, you can control this behavior using ALL_SIGNED,ALL_UNSIGNED, or both. (Bug#15831300)ALL_SIGNED tells the replica to treat all promoted integer types as signed;ALL_UNSIGNED instructs it to treat these as unsigned. Specifying both causes the replica to treat the value as signed if possible, otherwise to treat it as unsigned; the order in which they are listed is not significant. Neither ALL_SIGNED norALL_UNSIGNED has any effect if at least one of ALL_LOSSY orALL_NONLOSSY is not also used.

Changing the type conversion mode requires restarting the replica with the new slave_type_conversions setting.

Supported conversions. Supported conversions between different but similar data types are shown in the following list:

• Between any of the integer typesTINYINT,SMALLINT,MEDIUMINT,INT, andBIGINT.
  This includes conversions between the signed and unsigned versions of these types.
  Lossy conversions are made by truncating the source value to the maximum (or minimum) permitted by the target column. For ensuring non-lossy conversions when going from unsigned to signed types, the target column must be large enough to accommodate the range of values in the source column. For example, you can demote TINYINT UNSIGNED non-lossily toSMALLINT, but not toTINYINT.
• Between any of the decimal typesDECIMAL,FLOAT,DOUBLE, andNUMERIC.
  FLOAT to DOUBLE is a non-lossy conversion; DOUBLE toFLOAT can only be handled lossily. A conversion fromDECIMAL(_`M`_,_`D`_) toDECIMAL(_`M'`_,_`D'`_) where _`D'`_ >=_`D`_ and(_`M'`_-_`D'`_) >= (_`M`_-_`D`_) is non-lossy; for any case where_`M'`_ <_`M`_,_`D'`_ <_`D`_, or both, only a lossy conversion can be made.
  For any of the decimal types, if a value to be stored cannot be fit in the target type, the value is rounded down according to the rounding rules defined for the server elsewhere in the documentation. SeeSection 12.21.4, “Rounding Behavior”, for information about how this is done for decimal types.
• Between any of the string typesCHAR,VARCHAR, andTEXT, including conversions between different widths.
  Conversion of a CHAR,VARCHAR, or TEXT to a CHAR, VARCHAR, orTEXT column the same size or larger is never lossy. Lossy conversion is handled by inserting only the first N characters of the string on the replica, where N is the width of the target column.
  Important
  Replication between columns using different character sets is not supported.
• Between any of the binary data typesBINARY,VARBINARY, andBLOB, including conversions between different widths.
  Conversion of a BINARY,VARBINARY, or BLOB to a BINARY,VARBINARY, or BLOB column the same size or larger is never lossy. Lossy conversion is handled by inserting only the first_N_ bytes of the string on the replica, where N is the width of the target column.
• Between any 2 BIT columns of any 2 sizes.
  When inserting a value from aBIT(_`M`_) column into aBIT(_`M'`_) column, where _`M'`_ >_`M`_, the most significant bits of theBIT(_`M'`_) columns are cleared (set to zero) and the_M_ bits of theBIT(_`M`_) value are set as the least significant bits of theBIT(_`M'`_) column.
  When inserting a value from a sourceBIT(_`M`_) column into a targetBIT(_`M'`_) column, where _`M'`_ <_`M`_, the maximum possible value for theBIT(_`M'`_) column is assigned; in other words, an“all-set” value is assigned to the target column.

Conversions between types not in the previous list are not permitted.