19.5.1.14 Replication and System Functions (original) (raw)

19.5.1.14 Replication and System Functions

Certain functions do not replicate well under some conditions:

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 replica at that exact same instant, the value2009-09-01 18:00:00 is returned. For this reason, if you select from the replica's copy ofmytable after theCREATE TABLE andINSERT statements just shown have been replicated, you might expectmycol to contain the value2009-09-01 18:00:00. However, this is not the case; when you select from the replica's copy ofmytable, you obtain exactly the same result as on the source: 

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 by SET TIMESTAMP statements 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-now option to cause SYSDATE() to be an alias for NOW(). This must be done on the source and the replica to work correctly. In such cases, a warning is still issued by this function, but can safely be ignored as long as--sysdate-is-now is used on both the source and the replica.
SYSDATE() is automatically replicated using row-based replication when usingMIXED mode, and generates a warning inSTATEMENT mode.
See also Section 19.5.1.33, “Replication and Time Zones”.

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-rowINSERT 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 theINSERT 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 source and replica.)

The FOUND_ROWS() andROW_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 theINSERT statement. For example, if you wish to store the result in a table namedmytable, 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 replica 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)