Each binary logging format has advantages and disadvantages. For most users, the mixed replication format should provide the best combination of data integrity and performance. If, however, you want to take advantage of the features specific to the statement-based or row-based replication format when performing certain tasks, you can use the information in this section, which provides a summary of their relative advantages and disadvantages, to determine which is best for your needs.
Advantages of statement-based replication:
Proven technology that has existed in MySQL since 3.23.
Less data written to log files. When updates or deletes affect many rows, this results in much less storage space required for log files. This also means that taking and restoring from backups can be accomplished more quickly.
Log files contain all statements that made any changes, so they can be used to audit the database.
Disadvantages of statement-based replication:
Statements that are unsafe for SBR. Not all statements which modify data (such as
INSERT
DELETE
,UPDATE
, andREPLACE
statements) can be replicated using statement-based replication. Any nondeterministic behavior is difficult to replicate when using statement-based replication. Examples of such DML (Data Modification Language) statements include the following:A statement that depends on a UDF or stored program that is nondeterministic, since the value returned by such a UDF or stored program or depends on factors other than the parameters supplied to it. (Row-based replication, however, simply replicates the value returned by the UDF or stored program, so its effect on table rows and data is the same on both the master and slave.) See Section 17.4.1.8, “Replication of Invoked Features”, for more information.
DELETE
andUPDATE
statements that use aLIMIT
clause without anORDER BY
are nondeterministic. See Section 17.4.1.12, “Replication andLIMIT
”.Statements using any of the following functions cannot be replicated properly using statement-based replication:
SYSDATE()
(unless both the master and the slave are started with the--sysdate-is-now
option)
However, all other functions are replicated correctly using statement-based replication, including
RAND()
,NOW()
, and so forth.For more information, see Section 17.4.1.11, “Replication and System Functions”.
Statements that cannot be replicated correctly using statement-based replication are logged with a warning like the one shown here:
090213 16:58:54 [Warning] Statement is not safe to log in statement format.
A similar warning is also issued to the client in such cases. The client can display it using
SHOW WARNINGS
.INSERT ... SELECT
requires a greater number of row-level locks than with row-based replication.UPDATE
statements that require a table scan (because no index is used in theWHERE
clause) must lock a greater number of rows than with row-based replication.For
InnoDB
: AnINSERT
statement that usesAUTO_INCREMENT
blocks other nonconflictingINSERT
statements.For complex statements, the statement must be evaluated and executed on the slave before the rows are updated or inserted. With row-based replication, the slave only has to modify the affected rows, not execute the full statement.
If there is an error in evaluation on the slave, particularly when executing complex statements, statement-based replication may slowly increase the margin of error across the affected rows over time. See Section 17.4.1.24, “Slave Errors During Replication”.
Stored functions execute with the same
NOW()
value as the calling statement. However, this is not true of stored procedures.Deterministic UDFs must be applied on the slaves.
Table definitions must be (nearly) identical on master and slave. See Section 17.4.1.6, “Replication with Differing Table Definitions on Master and Slave”, for more information.
Advantages of row-based replication:
All changes can be replicated. This is the safest form of replication.
The
mysql
database is not replicated. Themysql
database is instead seen as a node-specific database. Row-based replication is not supported on tables in this database. Instead, statements that would normally update this information—such asGRANT
,REVOKE
and the manipulation of triggers, stored routines (including stored procedures), and views—are all replicated to slaves using statement-based replication.For statements such as
CREATE TABLE ... SELECT
, aCREATE
statement is generated from the table definition and replicated using statement-based format, while the row insertions are replicated using row-based format.The technology is the same as in most other database management systems; knowledge about other systems transfers to MySQL.
Fewer row locks are required on the master, which thus achieves higher concurrency, for the following types of statements:
Fewer row locks are required on the slave for any
INSERT
,UPDATE
, orDELETE
statement.
Disadvantages of row-based replication:
RBR tends to generate more data that must be logged. To replicate a DML statement (such as an
UPDATE
orDELETE
statement), statement-based replication writes only the statement to the binary log. By contrast, row-based replication writes each changed row to the binary log. If the statement changes many rows, row-based replication may write significantly more data to the binary log; this is true even for statements that are rolled back. This also means that taking and restoring from backup can require more time. In addition, the binary log is locked for a longer time to write the data, which may cause concurrency problems.Deterministic UDFs that generate large
BLOB
values take longer to replicate with row-based replication than with statement-based replication. This is because theBLOB
column value is logged, rather than the statement generating the data.You cannot examine the logs to see what statements were executed, nor can you see on the slave what statements were received from the master and executed.
However, you can see what data was changed using mysqlbinlog with the options
--base64-output=DECODE-ROWS
and--verbose
.