Binary logs and replication improvements

Due to continuous development, Percona Server for MySQL incorporated a number of improvements related to replication and binary logs handling. This resulted in replication specifics, which distinguishes it from MySQL.

Statements with a LIMIT clause

In MySQL 8.0, any UPDATE/DELETE/INSERT … SELECT statements that include a LIMIT clause are indeed considered unsafe for statement-based replication. These statements will cause MySQL to automatically switch from statement-based logging to row-based logging if binlog_format is set to MIXED.

Here’s why:

• The LIMIT clause without an ORDER BY makes the result set non-deterministic

• The same statement might affect different rows on the primary and replicas

mysql> UPDATE table1 LIMIT 10 SET col1 = 'value';
mysql> DELETE FROM table1 LIMIT 5;
mysql> INSERT INTO table2 SELECT * FROM table1 LIMIT 3;

To make these statements safe for statement-based replication, you should do one of the following:

• Remove the LIMIT clause

• Add an ORDER BY clause to make the result set deterministic

mysql> UPDATE table1 SET col1 = 'value' ORDER BY id LIMIT 10;
mysql> DELETE FROM table1 ORDER BY id LIMIT 5;
mysql> INSERT INTO table2 SELECT * FROM table1 ORDER BY id LIMIT 3;

The exception is when the LIMIT is used with an ORDER BY clause that uses a unique key - in this case, the statement becomes deterministic and safe for statement-based replication.

Percona Server for MySQL acknowledges statements as safe when they include either an ORDER BY PK or WHERE condition.

Relay log position fix

MySQL always updated relay log position in multi-source replications setups regardless of whether the committed transaction has already been executed or not. Percona Server omits relay log position updates for the already logged GTIDs.

Relay log position details

Particularly, such unconditional relay log position updates caused additional fsync operations in case of relay-log-info-repository=TABLE, and with the higher number of channels transmitting such duplicate (already executed) transactions the situation became proportionally worse. Bug fixed #1786, (upstream #85141).

Source and connection status update fix

Replica nodes configured to update source status and connection information only on log file rotation did not experience the expected reduction in load. MySQL was additionally updating this information in case of multi-source replication when replica had to skip the already executed GTID event.

Write FLUSH commands to the binary log

FLUSH commands, such as FLUSH SLOW LOGS, are not written to the binary log if the system variable binlog_skip_flush_commands is set to ON.

In the context of MySQL, the read_only and super_read_only system variables control the ability to modify data in the database. The following changes were implemented in the behavior of read_only and super_read_only modes:

• When read_only is set to ON, any FLUSH ... command executed by a normal user (without the SUPER privilege) are not written to the binary log regardless of the value of the binlog_skip_flush_command variable.

• When super_read_only is set to ON, any FLUSH ... command executed by any user (even by those with the SUPER privilege) are not written to the binary log regardless of the value of the binlog_skip_flush_commands variable.

An attempt to run a FLUSH command without either SUPER or RELOAD privileges results in the ER_SPECIFIC_ACCESS_DENIED_ERROR exception regardless of the value of the binlog_skip_flush_commands variable.

binlog_skip_flush_commands

Option	Description
Command-line	Yes
Config file	Yes
Scope	Global
Dynamic	Yes
Default	OFF

When binlog_skip_flush_commands is set to ON, FLUSH ... commands are not written to the binary log.

The binlog_skip_flush_commands setting does not impact the following commands because they are never recorded in the binary log:

• FLUSH LOGS

• FLUSH BINARY LOGS

• FLUSH TABLES WITH READ LOCK

• FLUSH TABLES ... FOR EXPORT

The FLUSH command is not recorded in the binary log and ignores the binlog_skip_flush_commands setting when executed with the NO_WRITE_TO_BINLOG keyword (or its alias LOCAL).

Keep comments with DDL commands

When you run a DDL command, such as DROP TABLE, the server does the following in the binary log.

Actions	Description
Removes Comments	The server deletes any comments in the original command. For example, if you use DROP TABLE my_table /* This is a comment */;, the binary log does not save the comment.
Adds Quotation Marks	The server puts quotation marks around the table name. So, if you run DROP TABLE my_table;, it logs it as DROP TABLE "my_table";.

These actions simplify the logging format, but sometimes, you want the original format.

binlog_ddl_skip_rewrite

Option	Description
Command-line	Yes
Config file	Yes
Scope	Global
Dynamic	Yes
Default	OFF

When disabled (default setting), the server removes comments and adds quotation marks to DDL statements.

When enabled, all single-table DROP TABLE DDL statements are logged in the binary log with the following characteristics:

• Comments are preserved, allowing any notes added to the command to remain in the binary log.

• Quotation marks are not added.

Enable binlog_ddl_skip_rewrite

You can enable binlog_ddl_skip_rewrite at runtime:

-- Check current setting
mysql> SHOW VARIABLES LIKE 'binlog_ddl_skip_rewrite';

-- Enable feature
mysql> SET GLOBAL binlog_ddl_skip_rewrite = ON;

-- Disable feature
mysql> SET GLOBAL binlog_ddl_skip_rewrite = OFF;

to enable the variable permanently, add the following line to the my.cnf configuration file:

[mysqld]
binlog_ddl_skip_rewrite = ON

After making this change, restart the MySQL service for it to take effect.

Note

Multi-table DROP TABLE DDL statements are not supported and return an error.

Example usage

The following code block demonstrates how to enable binlog_ddl_skip_rewrite and shows the feature’s effect on a DROP TABLE statement:

mysql> SET binlog_ddl_skip_rewrite = ON;
/*comment at start*/DROP TABLE t /*comment at end*/;

Point-in-Time Recovery with binlog_utils_udf

Use the binlog_utils_udf component to assist with Point-in-Time Recovery (PiTR). The component installs user-defined functions (UDFs) that help you map GTIDs to binary log files and inspect the contents and timestamps of binlog files.

Functions

Function	Returns	Description
get_binlog_by_gtid(gtid)	STRING (binlog name)	Returns the binlog file that contains the specified GTID.
get_last_gtid_from_binlog(binlog)	STRING (GTID)	Returns the last GTID found in the specified binlog.
get_gtid_set_by_binlog(binlog)	STRING (GTID set)	Returns all GTIDs found in the specified binlog.
get_binlog_by_gtid_set(gtid_set)	STRING (binlog name)	Returns the first binlog file that contains at least one GTID from the specified set.
get_first_record_timestamp_by_binlog(binlog)	INTEGER (timestamp)	Returns the timestamp of the first event in the specified binlog.
get_last_record_timestamp_by_binlog(binlog)	INTEGER (timestamp)	Returns the timestamp of the last event in the specified binlog.

Notes

• Timestamp-returning functions provide values with microsecond precision in UNIX time. Each value represents the number of microseconds since 1970-01-01 00:00:00 UTC.

• Functions that accept a binlog name require a short file name only. Do not include a path. If the input contains a path separator (/), the server returns an error.

• The server reads binlogs from the current binlog directory defined by the @@log_bin_basename system variable.

• Functions that return a binlog file name return the short name (no path).

Install the component

Install the component once on each server where you want to use these UDFs.

mysql> INSTALL COMPONENT 'file://component_binlog_utils_udf';

You can confirm installation by checking the list of registered functions:

mysql> SELECT ROUTINE_NAME FROM INFORMATION_SCHEMA.ROUTINES \G

Usage examples

Replace the sample arguments with values from your environment. The examples show the typical way to call each function. For clarity, results are aliased.

get_binlog_by_gtid()

Locate the binlog that contains a GTID:

mysql> SELECT get_binlog_by_gtid('UUID-GROUP:1') AS binlog;

get_last_gtid_from_binlog()

Return the last GTID in a binlog

mysql> SELECT get_last_gtid_from_binlog('binlog.000001') AS last_gtid;

get_gtid_set_by_binlog()

Return all GTIDs in a binlog

mysql> SELECT get_gtid_set_by_binlog('binlog.000001') AS gtid_set;

get_binlog_by_gtid_set()

Find a binlog that contains any GTID in a set

mysql> SELECT get_binlog_by_gtid_set('UUID1:7,UUID1:8') AS binlog;

get_first_record_timestamp_by_binlog() and get_last_record_timestamp_by_binlog(binlog)

Get the first event timestamp from a binlog. The function returns microseconds since the UNIX epoch. Use the tabs below to see the raw numeric value or a human-readable timestamp.

Raw TimestampHuman-Readable

mysql> SELECT get_first_record_timestamp_by_binlog('binlog.000001') AS raw_ts;

mysql> SELECT FROM_UNIXTIME(
    get_first_record_timestamp_by_binlog('binlog.000001') DIV 1000000
) AS first_event_ts;

Get the last event timestamp from a binlog

Raw TimestampHuman-Readable

mysql> SELECT get_last_record_timestamp_by_binlog('binlog.000001') AS raw_ts;

mysql> SELECT FROM_UNIXTIME(
    get_last_record_timestamp_by_binlog('binlog.000001') DIV 1000000
) AS last_event_ts;

Expected output

+---------------+
| binlog        |
+---------------+
| binlog.000001 |
+---------------+

Actual values depend on your server state and binlog contents.

Uninstall the component

Remove the component and all associated UDFs:

mysql> UNINSTALL COMPONENT 'file://component_binlog_utils_udf';

Limitations

For the following variables, do not define values with one or more dot (.) characters:

• log_bin

• log_bin_index

A value defined with the dot (.) character is handled differently in MySQL and Percona XtraBackup and can cause unpredictable behavior.