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 following CREATE 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 columns c1, c2, and c3 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 the ALTER TABLE statement just shown, because the new column cnew2 comes between columns common to both versions of t.
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 a DEFAULT 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 column c2:
source>CREATE TABLE t1 (c1 INT, c2 BIGINT);replica>CREATE TABLE t1 (c1 INT, c2 INT, c3 INT);