The “safeness” of a statement in MySQL replication refers to whether the 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 unsafe (see Section 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
When using row-based logging, no distinction is made in the treatment of safe and unsafe statements.
When using mixed-format logging, statements flagged as unsafe are logged using the row-based format; statements regarded as safe are logged using the statement-based format.
When using statement-based logging, statements flagged as being unsafe generate a warning to this effect. Safe statements are logged normally.
Each statement flagged as unsafe generates a warning. If a large
number of such statements were executed on the master, this
could lead to excessively large error log files. To prevent
this, MySQL has a warning suppression mechanism. Whenever the 50
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
last warning was repeated 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.
N times in
For more information, see Section 5.1, “Replication Formats”.
Statements containing system functions that may return a different value on the slave. These functions include
Nondeterministic functions not considered unsafe. Although these functions are not deterministic, they are treated as safe for purposes of logging and replication:
For more information, see Section 4.1.14, “Replication and System Functions”.
References to system variables. Most system variables are not replicated correctly using the statement-based format. See Section 4.1.38, “Replication and Variables”. For exceptions, see Mixed Binary Logging Format.
UDFs. Since we have no control over what a UDF does, we must assume that it is executing unsafe statements.
Fulltext plugin. This plugin may behave differently on different MySQL servers; therefore, statements depending on it could have different results. For this reason, all statements relying on the fulltext plugin are treated as unsafe in MySQL.
Trigger or stored program 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
INSERTinto a table that has a composite primary key containing an
AUTO_INCREMENTcolumn that is not the first column of this composite key is unsafe.
For more information, see Section 4.1.1, “Replication and AUTO_INCREMENT”.
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.
INSERT ... ON DUPLICATE KEY UPDATEstatement against a table having more than one unique or primary key is marked as unsafe for statement-based replication. (Bug #11765650, Bug #58637)
Updates using LIMIT. The order in which rows are retrieved is not specified, and is therefore considered unsafe. See Section 4.1.18, “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 4.1.34, “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 statements.
LOAD DATAis treated as unsafe and when
binlog_format=MIXEDthe statement is logged in row-based format. When
LOAD DATAdoes not generate a warning, unlike other unsafe statements.
XA transactions. If two XA transactions committed in parallel on the master are being prepared on the slave in the inverse order, locking dependencies can occur with statement-based replication that cannot be safely resolved, and it is possible for replication to fail with deadlock on the slave. When
binlog_format=STATEMENTis set, DML statements inside XA transactions are flagged as being unsafe and generate a warning. When
binlog_format=ROWis set, DML statements inside XA transactions are logged using row-based replication, and the potential issue is not present.
DEFAULTclause that refers to a nondeterministic function. If an expression default value refers to a nondeterministic function, any statement that causes the expression to be evaluated is unsafe for statement-based replication. This includes statements such as
ALTER TABLE. Unlike most other unsafe statements, this category of statement cannot be replicated safely in row-based format. When
binlog_formatis set to
STATEMENT, the statement is logged and executed but a warning message is written to the error log. When
binlog_formatis set to
ROW, the statement is not executed and an error message is written to the error log. For more information on the handling of explicit defaults, see Handling of Explicit Defaults as of MySQL 8.0.13.
For additional information, see Section 4.1, “Replication Features and Issues”.