Certain functions do not replicate well under some conditions:
The
USER(),CURRENT_USER()(orCURRENT_USER),UUID(),VERSION(), andLOAD_FILE()functions are replicated without change and thus do not work reliably on the slave unless row-based replication is enabled. (See Section 17.1.2, “Replication Formats”.)USER()andCURRENT_USER()are automatically replicated using row-based replication when usingMIXEDmode, and generate a warning inSTATEMENTmode. (Bug#28086) Beginning with MySQL 5.5.1, the same is true forVERSION(). (Bug#47995)For
NOW(), the binary log includes the timestamp. This means that the value as returned by the call to this function on the master is replicated to the slave. This can lead to a possibly unexpected result when replicating between MySQL servers in different time zones. Suppose that the master is located in New York, the slave is located in Stockholm, and both servers are using local time. Suppose further that, on the master, you create a tablemytable, perform anINSERTstatement on this table, and then select from the table, as shown here:mysql>
CREATE TABLE mytable (mycol TEXT);Query OK, 0 rows affected (0.06 sec) mysql>INSERT INTO mytable VALUES ( NOW() );Query OK, 1 row affected (0.00 sec) mysql>SELECT * FROM mytable;+---------------------+ | mycol | +---------------------+ | 2009-09-01 12:00:00 | +---------------------+ 1 row in set (0.00 sec)Local time in Stockholm is 6 hours later than in New York; so, if you issue
SELECT NOW()on the slave at that exact same instant, the value2009-09-01 18:00:00is returned. For this reason, if you select from the slave's copy ofmytableafter theCREATE TABLEandINSERTstatements just shown have been replicated, you might expectmycolto contain the value2009-09-01 18:00:00. However, this is not the case; when you select from the slave's copy ofmytable, you obtain exactly the same result as on the master:mysql>
SELECT * FROM mytable;+---------------------+ | mycol | +---------------------+ | 2009-09-01 12:00:00 | +---------------------+ 1 row in set (0.00 sec)Unlike
NOW(), theSYSDATE()function is not replication-safe because it is not affected bySET TIMESTAMPstatements in the binary log and is nondeterministic if statement-based logging is used. This is not a problem if row-based logging is used.An alternative is to use the
--sysdate-is-nowoption to causeSYSDATE()to be an alias forNOW(). This must be done on the master and the slave to work correctly. In such cases, a warning is still issued by this function, but can safely be ignored as long as--sysdate-is-nowis used on both the master and the slave.Beginning with MySQL 5.5.1,
SYSDATE()is automatically replicated using row-based replication when usingMIXEDmode, and generates a warning inSTATEMENTmode. (Bug#47995)The following restriction applies to statement-based replication only, not to row-based replication. The
GET_LOCK(),RELEASE_LOCK(),IS_FREE_LOCK(), andIS_USED_LOCK()functions that handle user-level locks are replicated without the slave knowing the concurrency context on the master. Therefore, these functions should not be used to insert into a master table because the content on the slave would differ. For example, do not issue a statement such asINSERT INTO mytable VALUES(GET_LOCK(...)).Beginning with MySQL 5.5.1, these functions are automatically replicated using row-based replication when using
MIXEDmode, and generate a warning inSTATEMENTmode. (Bug#47995)
As a workaround for the preceding limitations when
statement-based replication is in effect, you can use the
strategy of saving the problematic function result in a user
variable and referring to the variable in a later statement. For
example, the following single-row
INSERT is problematic due to the
reference to the UUID() function:
INSERT INTO t VALUES(UUID());
To work around the problem, do this instead:
SET @my_uuid = UUID(); INSERT INTO t VALUES(@my_uuid);
That sequence of statements replicates because the value of
@my_uuid is stored in the binary log as a
user-variable event prior to the
INSERT statement and is available
for use in the INSERT.
The same idea applies to multiple-row inserts, but is more cumbersome to use. For a two-row insert, you can do this:
SET @my_uuid1 = UUID(); @my_uuid2 = UUID(); INSERT INTO t VALUES(@my_uuid1),(@my_uuid2);
However, if the number of rows is large or unknown, the workaround is difficult or impracticable. For example, you cannot convert the following statement to one in which a given individual user variable is associated with each row:
INSERT INTO t2 SELECT UUID(), * FROM t1;
Within a stored function, RAND()
replicates correctly as long as it is invoked only once during
the execution of the function. (You can consider the function
execution timestamp and random number seed as implicit inputs
that are identical on the master and slave.)
The FOUND_ROWS() and
ROW_COUNT() functions are not
replicated reliably using statement-based replication. A
workaround is to store the result of the function call in a user
variable, and then use that in the
INSERT statement. For example, if
you wish to store the result in a table named
mytable, you might normally do so like this:
SELECT SQL_CALC_FOUND_ROWS FROM mytable LIMIT 1; INSERT INTO mytable VALUES( FOUND_ROWS() );
However, if you are replicating mytable, you
should use SELECT INTO, and then store the
variable in the table, like this:
SELECT SQL_CALC_FOUND_ROWS INTO @found_rows FROM mytable LIMIT 1; INSERT INTO mytable VALUES(@found_rows);
In this way, the user variable is replicated as part of the context, and applied on the slave correctly.
These functions are automatically replicated using row-based
replication when using MIXED mode, and
generate a warning in STATEMENT mode.
(Bug#12092, Bug#30244)