MySQL 8.4 Reference Manual Including MySQL NDB Cluster 8.4
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 19.5.1.12, “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).
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 source, this
could lead to excessively large error log files. To prevent
this, MySQL has a warning suppression mechanism. 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 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
last S seconds
For more information, see Section 19.2.1, “Replication Formats”.
Statements considered unsafe. Statements with the following characteristics are considered unsafe:
Statements containing system functions that may return a different value
on the replica.
These functions include
FOUND_ROWS(),
GET_LOCK(),
IS_FREE_LOCK(),
IS_USED_LOCK(),
LOAD_FILE(),
RAND(),
RELEASE_LOCK(),
ROW_COUNT(),
SESSION_USER(),
SLEEP(),
SOURCE_POS_WAIT(),
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 19.5.1.14, “Replication and System Functions”.
References to system variables. Most system variables are not replicated correctly using the statement-based format. See Section 19.5.1.39, “Replication and Variables”. For exceptions, see Section 7.4.4.3, “Mixed Binary Logging Format”.
Loadable Functions. Since we have no control over what a loadable function 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 source and the replica.
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 19.5.1.1, “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.
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. (Bug #11765650,
Bug #58637)
Updates using LIMIT. The order in which rows are retrieved is not specified, and is therefore considered unsafe. See Section 19.5.1.18, “Replication and LIMIT”.
Accesses or references log tables. The contents of the system log table may differ between source and replica.
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 19.5.1.35, “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 DATA is treated as
unsafe and when
binlog_format=MIXED the
statement is logged in row-based format. When
binlog_format=STATEMENT
LOAD DATA does not generate
a warning, unlike other unsafe statements.
XA transactions.
If two XA transactions committed in parallel on the source
are being prepared on the replica 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
replica. When
binlog_format=STATEMENT
is set, DML statements inside XA transactions are flagged
as being unsafe and generate a warning. When
binlog_format=MIXED or
binlog_format=ROW is set,
DML statements inside XA transactions are logged using
row-based replication, and the potential issue is not
present.
DEFAULT clause 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
INSERT,
UPDATE, and
ALTER TABLE. Unlike most
other unsafe statements, this category of statement cannot
be replicated safely in row-based format. When
binlog_format is set to
STATEMENT, the statement is logged and
executed but a warning message is written to the error
log. When binlog_format
is set to MIXED or
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
Explicit Default Handling.
For additional information, see Section 19.5.1, “Replication Features and Issues”.