16.1.2.3. Determination of Safe and Unsafe Statements in Binary Logging

When speaking of the "safeness" of a statement in MySQL Replication, we are referring to whether a statement and its effects can be replicated correctly using statement-based format. If this is true of the statement, we refer to the statement as safe; otherwise, we refer to it as unsafe.

In general, a statement is safe if it deterministic, and unsafe if it is not. However, certain nondeterministic functions are not considered unsafe (see Nondeterministic functions not considered unsafe, later in this section). In addition, statements using results from floating-point math functions—which are hardware-dependent—are always considered safe (see Section 16.4.1.12, "Replication and Floating-Point Values").

Handling of safe and unsafe statements. A statement is treated differently depending on whether the statement is considered safe, and with respect to the binary logging format (that is, the current value of binlog_format).

No distinction is made in the treatment of safe and unsafe statements when the binary logging mode is ROW.
If the binary logging format is MIXED, statements flagged as unsafe are logged using the row-based format; statements regarded as safe are logged using the statement-based format.
If the binary logging format is STATEMENT, statements flagged as being unsafe generate a warning to this effect. (Safe statements are logged normally.)

Each statement flagged as unsafe generates a warning. Formerly, in cases where a great many such statements were executed on the master, this could lead to very large error log files, sometimes even filling up an entire disk unexpectedly. To guard against this, MySQL 5.6.7 introduced a warning suppression mechanism, which behaves as follows: Whenever the 50 most recent ER_BINLOG_UNSAFE_STATEMENT warnings have been generated more than 50 times in any 50-second period, warning suppression is enabled. When activated, this causes such warnings not to be written to the error log; instead, for each 50 warnings of this type, a note The last warning was repeated N times in last S seconds is written to the error log. This continues as long as the 50 most recent such warnings were issued in 50 seconds or less; once the rate has decreased below this threshold, the warnings are once again logged normally. Warning suppression has no effect on how the safety of statements for statement-based logging is determined, nor on how warnings are sent to the client (MySQL clients still receive one warning for each such statement).

For more information, see Section 16.1.2, "Replication Formats".

Statements considered unsafe. Statements having the following characteristics are considered unsafe:

Statements containing system functions that may return a different value on slave. These functions include FOUND_ROWS(), GET_LOCK(), IS_FREE_LOCK(), IS_USED_LOCK(), LOAD_FILE(), MASTER_POS_WAIT(), PASSWORD(), RAND(), RELEASE_LOCK(), ROW_COUNT(), SESSION_USER(), SLEEP(), SYSDATE(), SYSTEM_USER(), USER(), UUID(), and UUID_SHORT().

Nondeterministic functions not considered unsafe. Although these functions are not deterministic, they are treated as safe for purposes of logging and replication: CONNECTION_ID(), CURDATE(), CURRENT_DATE(), CURRENT_TIME(), CURRENT_TIMESTAMP(), CURTIME(),, LAST_INSERT_ID(), LOCALTIME(), LOCALTIMESTAMP(), NOW(), UNIX_TIMESTAMP(), UTC_DATE(), UTC_TIME(), and UTC_TIMESTAMP().

For more information, see Section 16.4.1.15, "Replication and System Functions".
References to system variables. Most system variables are not replicated correctly using the statement-based format. For exceptions, see Section 5.2.4.3, "Mixed Binary Logging Format".

See Section 16.4.1.33, "Replication and Variables".
UDFs. Since we have no control over what a UDF does, we must assume that it is executing unsafe statements.
Updates a table having an AUTO_INCREMENT column. This is unsafe because the order in which the rows are updated may differ on the master and the slave.

In addition, an INSERT into a table that has a composite primary key containing an AUTO_INCREMENT column that is not the first column of this composite key is unsafe.

For more information, see Section 16.4.1.1, "Replication and AUTO_INCREMENT".
INSERT DELAYED statement. This statement is considered unsafe because the insertion of the rows may interleave with concurrently executing statements.
INSERT ... ON DUPLICATE KEY UPDATE statements on tables with multiple primary or unique keys. When executed against a table that contains more than one primary or unique key, this statement is considered unsafe, being sensitive to the order in which the storage engine checks the keys, which is not deterministic, and on which the choice of rows updated by the MySQL Server depends.

An INSERT ... ON DUPLICATE KEY UPDATE statement against a table having more than one unique or primary key is marked as unsafe for statement-based replication beginning with MySQL 5.6.6. (Bug #11765650, Bug #58637)
Updates using LIMIT. The order in which rows are retrieved is not specified.

See Section 16.4.1.16, "Replication and LIMIT".
Accesses or references log tables. The contents of the system log table may differ between master and slave.
Nontransactional operations after transactional operations. Within a transaction, allowing any nontransactional reads or writes to execute after any transactional reads or writes is considered unsafe.

For more information, see Section 16.4.1.30, "Replication and Transactions".
Accesses or references self-logging tables. All reads and writes to self-logging tables are considered unsafe. Within a transaction, any statement following a read or write to self-logging tables is also considered unsafe.
LOAD DATA INFILE statements. Beginning with MySQL 5.6, LOAD DATA INFILE is considered unsafe, it causes a warning in statement-based mode, and a switch to row-based format when using mixed-format logging. See Section 16.4.1.17, "Replication and LOAD DATA INFILE".

For additional information, see Section 16.4.1, "Replication Features and Issues".