• For the server, the default value of the default_authentication_plugin system variable changes from mysql_native_password to caching_sha2_password.

  This change applies only to new accounts created after installing or upgrading to MySQL 8.0 or higher. For accounts already existing in an upgraded installation, their authentication plugin remains unchanged. Existing users who wish to switch to caching_sha2_password can do so using the ALTER USER statement:

  ALTER USER user
    IDENTIFIED WITH caching_sha2_password
    BY 'password';

• The libmysqlclient library treats caching_sha2_password as the default authentication plugin rather than mysql_native_password.

• caching_sha2_password Compatibility Issues and Solutions

• caching_sha2_password-Compatible Clients and Connectors

• caching_sha2_password and the root Administrative Account

• caching_sha2_password and Replication

• Clients and connectors that have not been updated to know about caching_sha2_password may have trouble connecting to a MySQL 8.0 server configured with caching_sha2_password as the default authentication plugin, even to use accounts that do not authenticate with caching_sha2_password. This issue occurs because the server specifies the name of its default authentication plugin to clients. If a client or connector is based on a client/server protocol implementation that does not gracefully handle an unrecognized default authentication plugin, it may fail with an error such as one of these:

  Authentication plugin 'caching_sha2_password' is not supported

  Authentication plugin 'caching_sha2_password' cannot be loaded:
  dlopen(/usr/local/mysql/lib/plugin/caching_sha2_password.so, 2):
  image not found

  Warning: mysqli_connect(): The server requested authentication
  method unknown to the client [caching_sha2_password]

  For information about writing connectors to gracefully handle requests from the server for unknown default authentication plugins, see Authentication Plugin Connector-Writing Considerations.

• Clients that use an account that authenticates with caching_sha2_password must use either a secure connection (made using TCP using TLS/SSL credentials, a Unix socket file, or shared memory), or an unencrypted connection that supports password exchange using an RSA key pair. This security requirement does not apply to mysql_native_passsword, so the switch to caching_sha2_password may require additional configuration (see Section 6.4.1.2, “Caching SHA-2 Pluggable Authentication”). However, client connections in MySQL 8.0 prefer use of TLS/SSL by default, so clients that already conform to that preference may need no additional configuration.

• Clients and connectors that have not been updated to know about caching_sha2_password cannot connect to accounts that authenticate with caching_sha2_password because they do not recognize this plugin as valid. (This is a particular instance of how client/server authentication plugin compatibility requirements apply, as discussed at Authentication Plugin Client/Server Compatibility.) To work around this issue, relink clients against libmysqlclient from MySQL 8.0 or higher, or obtain an updated connector that recognizes caching_sha2_password.

• Because caching_sha2_password is also now the default authentication plugin in the libmysqlclient client library, authentication requires an extra round trip in the client/server protocol for connections from MySQL 8.0 clients to accounts that use mysql_native_password (the previous default authentication plugin), unless the client program is invoked with a --default-auth=mysql_native_password option.

• Standard MySQL clients such as mysql and mysqladmin are libmysqlclient-based.

• The DBD::mysql driver for Perl DBI is libmysqlclient-based.

• MySQL Connector/Python has a C Extension module that is libmysqlclient-based. To use it, include the use_pure=False option at connect time.

• Prior to the upgrade, DBI scripts that use DBD::mysql can connect to a MySQL 8.0 server, except for accounts that authenticate with caching_sha2_password.

• After the upgrade, the same scripts become able to use caching_sha2_password accounts as well.

• Some MySQL clients support a --default-auth option. (Standard MySQL clients such as mysql and mysqladmin support this option but can successfully connect to 8.0 servers without it. However, other clients may support a similar option. If so, it is worth trying it.)

• Programs that use the libmysqlclient C API can call the mysql_options() function with the MYSQL_DEFAULT_AUTH option.

• MySQL Connector/Python scripts that use the native Python implementation of the client/server protocol can specify the auth_plugin connection option. (Alternatively, use the Connector/Python C Extension, which is able to connect to MySQL 8.0 servers without the need for auth_plugin.)

• The libmysqlclient client library in MySQL 8.0 (8.0.4 or higher). Standard MySQL clients such as mysql and mysqladmin are libmysqlclient-based, so they are compatible as well.

• The libmysqlclient client library in MySQL 5.7 (5.7.23 or higher). Standard MySQL clients such as mysql and mysqladmin are libmysqlclient-based, so they are compatible as well.

• MySQL Connector/C++ 1.1.11 or higher or 8.0.7 or higher.

• MySQL Connector/J 8.0.9 or higher.

• MySQL Connector/NET 8.0.10 or higher (through the classic MySQL protocol).

• MySQL Connector/Node.js 8.0.9 or higher.

• PHP: the X DevAPI PHP extension (mysql_xdevapi) supports caching_sha2_password.

  PHP: the PDO_MySQL and ext/mysqli extensions do not support caching_sha2_password. In addition, when used with PHP versions before 7.1.16 and PHP 7.2 before 7.2.4, they fail to connect with default_authentication_plugin=caching_sha2_password even if caching_sha2_password is not used.

• Supply a --default-authentication-plugin=mysql_native_password option along with --initialize or --initialize-insecure.

• Set default_authentication_plugin to mysql_native_password in an option file, and name that option file using a --defaults-file option along with --initialize or --initialize-insecure. (In this case, if you continue to use that option file for subsequent server startups, new accounts are created with mysql_native_password rather than caching_sha2_password unless you remove the default_authentication_plugin setting from the option file.)

• Use any of the following CHANGE MASTER TO options:

  MASTER_SSL = 1
  GET_MASTER_PUBLIC_KEY = 1
  MASTER_PUBLIC_KEY_PATH='path to RSA public key file'

• Alternatively, you can use the RSA public key-related options if the required keys are supplied at server startup.

• For MySQL built using OpenSSL, set any of the following system variables:

  SET GLOBAL group_replication_recovery_use_ssl = ON;
  SET GLOBAL group_replication_recovery_get_public_key = 1;
  SET GLOBAL group_replication_recovery_public_key_path = 'path to RSA public key file';

• Alternatively, you can use the RSA public key-related options if the required keys are supplied at server startup.

• Incompatible change: A MySQL storage engine is now responsible for providing its own partitioning handler, and the MySQL server no longer provides generic partitioning support. InnoDB and NDB are the only storage engines that provide a native partitioning handler that is supported in MySQL 8.0. A partitioned table using any other storage engine must be altered—either to convert it to InnoDB or NDB, or to remove its partitioning—before upgrading the server, else it cannot be used afterwards.

  For information about converting MyISAM tables to InnoDB, see Section 15.6.1.5, “Converting Tables from MyISAM to InnoDB”.

  A table creation statement that would result in a partitioned table using a storage engine without such support fails with an error (ER_CHECK_NOT_IMPLEMENTED) in MySQL 8.0. If you import databases from a dump file created in MySQL 5.7 (or earlier) using mysqldump into a MySQL 8.0 server, you must make sure that any statements creating partitioned tables do not also specify an unsupported storage engine, either by removing any references to partitioning, or by specifying the storage engine as InnoDB or allowing it to be set as InnoDB by default.

  Note注意

  The procedure given at Section 2.11.5, “Preparing Your Installation for Upgrade”, describes how to identify partitioned tables that must be altered before upgrading to MySQL 8.0.

  See Section 24.6.2, “Partitioning Limitations Relating to Storage Engines”, for further information.

• Incompatible change: Several server error codes are not used and have been removed (for a list, see Features Removed in MySQL 8.0). Applications that test specifically for any of them should be updated.

• Important change: The default character set has changed from latin1 to utf8mb4. These system variables are affected:

  • The default value of the character_set_server and character_set_database system variables has changed from latin1 to utf8mb4.

  • The default value of the collation_server and collation_database system variables has changed from latin1_swedish_ci to utf8mb4_0900_ai_ci.

  As a result, the default character set and collation for new objects differ from previously unless an explicit character set and collation are specified. This includes databases and objects within them, such as tables, views, and stored programs. Assuming that the previous defaults were used, one way to preserve them is to start the server with these lines in the my.cnf file:

  [mysqld]
  character_set_server=latin1
  collation_server=latin1_swedish_ci

  In a replicated setting, when upgrading from MySQL 5.7 to 8.0, it is advisable to change the default character set back to the character set used in MySQL 5.7 before upgrading. After the upgrade is completed, the default character set can be changed to utf8mb4.

• Incompatible change: As of MySQL 8.0.11, it is prohibited to start the server with a lower_case_table_names setting that is different from the setting used when the server was initialized. The restriction is necessary because collations used by various data dictionary table fields are based on the lower_case_table_names setting that was defined when the server was initialized, and restarting the server with a different setting would introduce inconsistencies with respect to how identifiers are ordered and compared.

• In MySQL 8.0.11, several deprecated features related to account management have been removed, such as use of the GRANT statement to modify nonprivilege characteristics of user accounts, the NO_AUTO_CREATE_USER SQL mode, the PASSWORD() function, and the old_passwords system variable.

  Replication from MySQL 5.7 to 8.0 of statements that refer to these removed features can cause replication failure. Applications that use any of the removed features should be revised to avoid them and use alternatives when possible, as described in Features Removed in MySQL 8.0.

  To avoid a startup failure on MySQL 8.0, remove any instance of NO_AUTO_CREATE_USER from sql_mode system variable settings in MySQL option files.

  Loading a dump file that includes the NO_AUTO_CREATE_USER SQL mode in stored program definitions into a MySQL 8.0 server causes a failure. As of MySQL 5.7.24 and MySQL 8.0.13, mysqldump removes NO_AUTO_CREATE_USER from stored program definitions. Dump files created with an earlier version of mysqldump must be modified manually to remove instances of NO_AUTO_CREATE_USER.

• In MySQL 8.0.11, these deprecated compatibility SQL modes were removed: DB2, MAXDB, MSSQL, MYSQL323, MYSQL40, ORACLE, POSTGRESQL, NO_FIELD_OPTIONS, NO_KEY_OPTIONS, NO_TABLE_OPTIONS. They can no longer be assigned to the sql_mode system variable or used as permitted values for the mysqldump --compatible option.

  Removal of MAXDB means that the TIMESTAMP data type for CREATE TABLE or ALTER TABLE is no longer treated as DATETIME.

  Replication from MySQL 5.7 to 8.0 of statements that refer to the removed SQL modes can cause replication failure. This includes replication of CREATE statements for stored programs (stored procedures and functions, triggers, and events) that are executed while the current sql_mode value includes any of the removed modes. Applications that use any of the removed modes should be revised to avoid them.

• As of MySQL 8.0.3, spatial data types permit an SRID attribute, to explicitly indicate the spatial reference system (SRS) for values stored in the column. See Section 11.4.1, “Spatial Data Types”.

  A spatial column with an explicit SRID attribute is SRID-restricted: The column takes only values with that ID, and SPATIAL indexes on the column become subject to use by the optimizer. The optimizer ignores SPATIAL indexes on spatial columns with no SRID attribute. See Section 8.3.3, “SPATIAL Index Optimization”. If you want the optimizer to consider SPATIAL indexes on spatial columns that are not SRID-restricted, each such column should be modified:

  • Verify that all values within the column have the same SRID. To determine the SRIDs contained in a geometry column col_name, use the following query:

    SELECT DISTINCT ST_SRID(col_name) FROM tbl_name;

    If the query returns more than one row, the column contains a mix of SRIDs. In that case, modify its contents so all values have the same SRID.

  • Redefine the column to have an explicit SRID attribute.

  • Recreate the SPATIAL index.

• Several spatial functions were removed in MySQL 8.0.0 due to a spatial function namespace change that implemented an ST_ prefix for functions that perform an exact operation, or an MBR prefix for functions that perform an operation based on minimum bounding rectangles. The use of removed spatial functions in generated column definitions could cause an upgrade failure. Before upgrading, run mysqlcheck --check-upgrade for removed spatial functions and replace any that you find with their ST_ or MBR named replacements. For a list of removed spatial functions, refer to Features Removed in MySQL 8.0.

• The BACKUP_ADMIN privilege is automatically granted to users with the RELOAD privilege when performing an in-place upgrade to MySQL 8.0.3 or higher.

• From MySQL 8.0.13, because of differences between row-based or mixed replication mode and statement-based replication mode in the way that temporary tables are handled, there are new restrictions on switching the binary logging format at runtime.

  • SET @@SESSION.binlog_format cannot be used if the session has any open temporary tables.

  • SET @@global.binlog_format and SET @@persist.binlog_format cannot be used if any replication channel has any open temporary tables. SET @@persist_only.binlog_format is allowed if replication channels have open temporary tables, because unlike PERSIST, PERSIST_ONLY does not modify the runtime global system variable value.

  • SET @@global.binlog_format and SET @@persist.binlog_format cannot be used if any replication channel applier is running. This is because the change only takes effect on a replication channel when its applier is restarted, at which time the replication channel might have open temporary tables. This behavior is more restrictive than before. SET @@persist_only.binlog_format is allowed if any replication channel applier is running.

• INFORMATION_SCHEMA views based on InnoDB system tables were replaced by internal system views on data dictionary tables. Affected InnoDB INFORMATION_SCHEMA views were renamed:

  Table 2.16 Renamed InnoDB Information Schema Views

  Old Name	New Name
  INNODB_SYS_COLUMNS	INNODB_COLUMNS
  INNODB_SYS_DATAFILES	INNODB_DATAFILES
  INNODB_SYS_FIELDS	INNODB_FIELDS
  INNODB_SYS_FOREIGN	INNODB_FOREIGN
  INNODB_SYS_FOREIGN_COLS	INNODB_FOREIGN_COLS
  INNODB_SYS_INDEXES	INNODB_INDEXES
  INNODB_SYS_TABLES	INNODB_TABLES
  INNODB_SYS_TABLESPACES	INNODB_TABLESPACES
  INNODB_SYS_TABLESTATS	INNODB_TABLESTATS
  INNODB_SYS_VIRTUAL	INNODB_VIRTUAL

  
  

  After upgrading to MySQL 8.0.3 or higher, update any scripts that reference previous InnoDB INFORMATION_SCHEMA view names.

• The zlib library version bundled with MySQL was raised from version 1.2.3 to version 1.2.11.

  The zlib compressBound() function in zlib 1.2.11 returns a slightly higher estimate of the buffer size required to compress a given length of bytes than it did in zlib version 1.2.3. The compressBound() function is called by InnoDB functions that determine the maximum row size permitted when creating compressed InnoDB tables or inserting and updating rows in compressed InnoDB tables. As a result, CREATE TABLE ... ROW_FORMAT=COMPRESSED, INSERT, and UPDATE operations with row sizes very close to the maximum row size that were successful in earlier releases could now fail. To avoid this issue, test CREATE TABLE statements for compressed InnoDB tables with large rows on a MySQL 8.0 test instance prior to upgrading.

• With the introduction of the --innodb-directories feature, the location of file-per-table and general tablespace files created with an absolute path or in a location outside of the data directory should be added to the innodb_directories argument value. Otherwise, InnoDB is not able to locate these files during recovery. To view tablespace file locations, query the INFORMATION_SCHEMA.FILES table:

  SELECT TABLESPACE_NAME, FILE_NAME FROM INFORMATION_SCHEMA.FILES \G

• Undo logs can no longer reside in the system tablespace. In MySQL 8.0, undo logs reside in two undo tablespaces by default. For more information, see Section 15.6.3.4, “Undo Tablespaces”.

  When upgrading from MySQL 5.7 to MySQL 8.0, any undo tablespaces that exist in the MySQL 5.7 instance are removed and replaced by two new default undo tablespaces. Default undo tablespaces are created in the location defined by the innodb_undo_directory variable. If the innodb_undo_directory variable is undefined, undo tablespaces are created in the data directory. Upgrade from MySQL 5.7 to MySQL 8.0 requires a slow shutdown which ensures that undo tablespaces in the MySQL 5.7 instance are empty, permitting them to be removed safely.

  When upgrading to MySQL 8.0.14 or later from an earlier MySQL 8.0 release, undo tablespaces that exist in the pre-upgrade instance as a result of an innodb_undo_tablespaces setting greater than 2 are treated as user-defined undo tablespaces, which can be deactivated and dropped using ALTER UNDO TABLESPACE and DROP UNDO TABLESPACE syntax, respectively, after upgrading. Upgrade within the MySQL 8.0 release series may not always require a slow shutdown which means that existing undo tablespaces could contain undo logs. Therefore, existing undo tablespaces are not removed by the upgrade process.

• Incompatible change: As of MySQL 8.0.17, the CREATE TABLESPACE ... ADD DATAFILE clause does not permit circular directory references. For example, the circular directory reference (/../) in the following statement is not permitted:

  CREATE TABLESPACE ts1 ADD DATAFILE ts1.ibd 'any_directory/../ts1.ibd';

  An exception to the restriction exists on Linux, where a circular directory reference is permitted if the preceding directory is a symbolic link. For example, the data file path in the example above is permitted if any_directory is a symbolic link. (It is still permitted for data file paths to begin with '../'.)

  To avoid upgrade issues, remove any circular directory references from tablespace data file paths before upgrading to MySQL 8.0.17 or higher. To inspect tablespace paths, query the INFORMATION_SCHEMA.INNODB_DATAFILES table.

• Due to a regression introduced in MySQL 8.0.14, in-place upgrade on a case-sensitive file system from MySQL 5.7 or a MySQL 8.0 release prior to MySQL 8.0.14 to MySQL 8.0.16 failed for instances with partitioned tables and lower_case_table_names=1. The failure was caused by a case mismatch issue related to partitioned table file names. The fix that introduced the regression was reverted, which permits upgrades to MySQL 8.0.17 from MySQL 5.7 or MySQL 8.0 releases prior to MySQL 8.0.14 to function as normal. However, the regression is still present in the MySQL 8.0.14, 8.0.15, and 8.0.16 releases.

  In-place upgrade on a case-sensitive file system from MySQL 8.0.14, 8.0.15, or 8.0.16 to MySQL 8.0.17 fails with the following error when starting the server after upgrading binaries or packages to MySQL 8.0.17 if partitioned tables are present and lower_case_table_names=1:

  Upgrading from server version version_number with
  partitioned tables and lower_case_table_names == 1 on a case sensitive file
  system may cause issues, and is therefore prohibited. To upgrade anyway, restart
  the new server version with the command line option 'upgrade=FORCE'. When
  upgrade is completed, please execute 'RENAME TABLE part_table_name
  TO new_table_name; RENAME TABLE new_table_name
  TO part_table_name;' for each of the partitioned tables.
  Please see the documentation for further information.

  If you encounter this error when upgrading to MySQL 8.0.17, perform the following workaround:

  1. Restart the server with --upgrade=force to force the upgrade operation to proceed.

  2. Identify partitioned table file names with lowercase partition name delimiters (#p# or #sp#):

     mysql> SELECT FILE_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_NAME LIKE '%#p#%' OR FILE_NAME LIKE '%#sp#%';

  3. For each file identified, rename the associated table using a temporary name, then rename the table back to its original name.

     mysql> RENAME TABLE table_name TO temporary_table_name;
     mysql> RENAME TABLE temporary_table_name TO table_name;

  4. Verify that there are no partitioned table file names lowercase partition name delimiters (an empty result set should be returned).

     mysql> SELECT FILE_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_NAME LIKE '%#p#%' OR FILE_NAME LIKE '%#sp#%';
     Empty set (0.00 sec)

  5. Run ANALYZE TABLE on each renamed table to update the optimizer statistics in the mysql.innodb_index_stats and mysql.innodb_table_stats tables.

  Because of the regression still present in the MySQL 8.0.14, 8.0.15, and 8.0.16 releases, importing partitioned tables from MySQL 8.0.14, 8.0.15, or 8.0.16 to MySQL 8.0.17 is not supported on case-sensitive file systems where lower_case_table_names=1. Attempting to do so results in a “Tablespace is missing for table” error.

• MySQL uses delimiter strings when constructing tablespace names and file names for table partitions. A “ #p# ” delimiter string precedes partition names, and an “ #sp# ” delimiter string precedes subpartition names, as shown:

        schema_name.table_name#p#partition_name#sp#subpartition_name
  table_name#p#partition_name#sp#subpartition_name.ibd

  Historically, delimiter strings have been uppercase (#P# and #SP#) on case-sensitive file systems such as Linux, and lowercase (#p# and #sp#) on case-insensitive file systems such as Windows. As of MySQL 8.0.19, delimiter strings are lowercase on all file systems. This change prevents issues when migrating data directories between case-sensitive and case-insensitive file systems. Uppercase delimiter strings are no longer used.

  Additionally, partition tablespace names and file names generated based on user-specified partition or subpartition names, which can be specified in uppercase or lowercase, are now generated (and stored internally) in lowercase regardless of the lower_case_table_names setting to ensure case-insensitivity. For example, if a table partition is created with the name PART_1, the tablespace name and file name are generated in lowercase:

        schema_name.table_name#p#part_1
  table_name#p#part_1.ibd

  During upgrade, MySQL checks and modifies if necessary:

  • Partition file names on disk and in the data dictionary to ensure lowercase delimiters and partition names.

  • Partition metadata in the data dictionary for related issues introduced by previous bug fixes.

  • InnoDB statistics data for related issues introduced by previous bug fixes.

  During tablespace import operations, partition tablespace file names on disk are checked and modified if necessary to ensure lowercase delimiters and partition names.

• As of MySQL 8.0.21, a warning is written to the error log at startup or when upgrading from MySQL 5.7 if tablespace data files are found to reside in unknown directories. Known directories are those defined by the datadir, innodb_data_home_dir, and innodb_directories variables. To make a directory known, add it to the innodb_directories setting. Making directories known ensures that data files can be found during recovery. For more information, see Tablespace Discovery During Crash Recovery.

• Incompatible change: As of MySQL 8.0.13, the deprecated ASC or DESC qualifiers for GROUP BY clauses have been removed. Queries that previously relied on GROUP BY sorting may produce results that differ from previous MySQL versions. To produce a given sort order, provide an ORDER BY clause.

  Queries and stored program definitions from MySQL 8.0.12 or lower that use ASC or DESC qualifiers for GROUP BY clauses should be amended. Otherwise, upgrading to MySQL 8.0.13 or higher may fail, as may replicating to MySQL 8.0.13 or higher replica servers.

• Some keywords may be reserved in MySQL 8.0 that were not reserved in MySQL 5.7. See Section 9.3, “Keywords and Reserved Words”. This can cause words previously used as identifiers to become illegal. To fix affected statements, use identifier quoting. See Section 9.2, “Schema Object Names”.

• After upgrading, it is recommended that you test optimizer hints specified in application code to ensure that the hints are still required to achieve the desired optimization strategy. Optimizer enhancements can sometimes render certain optimizer hints unnecessary. In some cases, an unnecessary optimizer hint may even be counterproductive.

• Incompatible change: In MySQL 5.7, specifying a FOREIGN KEY definition for an InnoDB table without a CONSTRAINT symbol clause, or specifying the CONSTRAINT keyword without a symbol, causes InnoDB to use a generated constraint name. That behavior changed in MySQL 8.0, with InnoDB using the FOREIGN KEY index_name value instead of a generated name. Because constraint names must be unique per schema (database), the change caused errors due to foreign key index names that were not unique per schema. To avoid such errors, the new constraint naming behavior has been reverted in MySQL 8.0.16, and InnoDB once again uses a generated constraint name.

  For consistency with InnoDB, NDB releases based on MySQL 8.0.16 or higher use a generated constraint name if the CONSTRAINT symbol clause is not specified, or the CONSTRAINT keyword is specified without a symbol. NDB releases based on MySQL 5.7 and earlier MySQL 8.0 releases used the FOREIGN KEY index_name value.

  The changes described above may introduce incompatibilities for applications that depend on the previous foreign key constraint naming behavior.

Server Defaults

• The default value of the character_set_server system variable and command line option --character-set-server  changed from latin1 to utf8mb4. This is the server’s default character set. At this time, UTF8MB4 is the dominant character encoding for the web, and this change makes life easier for the vast majority of MySQL users. The upgrade from 5.7 to 8.0 does not change any character set for any existing database objects. But unless you specify character_set_server back to your previous default or explicitly set the character set then a new schema, table, or column by default uses utf8mb4. We recommend you move to utf8mb4 whenever possible.

• The default value of the  collation_server system variable and command line argument --collation-server changed from latin1_swedish_ci to utf8mb4_0900_ai_ci. This is the server’s default collation, the ordering of characters in a character set. There is a link between collations and character sets as each character set comes with a list of possible collations. The upgrade from 5.7 to 8.0 does not change any collation for any existing database objects, but takes effect for new objects.

• The default value of the explicit_defaults_for_timestamp system variable changed from OFF (MySQL legacy behavior) to ON (SQL standard behavior). This option was originally introduced in 5.6 and was OFF in 5.6 and 5.7. 

• The default value of the optimizer_trace_max_mem_size system variable changed from 16KB to 1MB. The old default caused the optimizer trace to be truncated for any non-trivial query. This change ensures useful optimizer traces for most queries.

• The default value of the validate_password_check_user_name system variable changed from OFF to ON. This means that when the validate_password plugin is enabled, by default it now rejects passwords that match the current session user name.

• The autosize algorithm for the back_log system variable has changed.  The value for autosize (-1) is now set to the value of max_connections, which is bigger than the calculated by 50 + (max_connections / 5). The back_log queues up incoming IP connect requests in situations where the server is not able to keep up with incoming requests. In the worst case, with max_connections number of clients trying to reconnect at the same time, for example after a network failure, they can all be buffered and reject-retry loops are avoided.

• The default value of the max_allowed_packet system variable changed from 4194304 (4M) to 67108864 (64M). The main advantage with this larger default is less chance of receiving errors about an insert or query being larger than max_allowed_packet. It should be as big as the largest Section 11.3.4, “The BLOB and TEXT Types” you want to use. To revert to the previous behavior, set max_allowed_packet=4194304.

• The default value of the max_error_count system variable changed from 64 to 1024. This ensures that MySQL handles a larger number of warnings, such as an UPDATE statement that touches 1000s of rows and many of them give conversion warnings. It is common for many tools to batch updates, to help reduce replication lag. External tools such as pt-online-schema-change defaults to 1000, and gh-ost defaults to 100. MySQL 8.0 covers full error history for these two use cases. There are no static allocations, so this change only affects memory consumption for statements that generate lots of warnings.

• The default value of the event_scheduler system variable changed from OFF to ON. In other words, the event scheduler is enabled by default. This is an enabler for new features in SYS, for example “kill idle transactions”.

• The default value of the table_open_cache system variable changed from 2000 to 4000. This is a minor change which increases session concurrency on table access.

• The default value of the log_error_verbosity system variable changed from 3 (Notes) to 2 (Warning). The purpose is to make the MySQL 8.0 error log less verbose by default.

InnoDB Defaults

• Incompatible change The default value of the innodb_undo_tablespaces system variable changed from 0 to 2. The configures the number of undo tablespaces used by InnoDB. In MySQL 8.0 the minimum value for innodb_undo_tablespaces is 2 and rollback segments cannot be created in the system tablespace anymore. Thus, this is a case where you cannot revert back to 5.7 behavior. The purpose of this change is to be able to auto-truncate Undo logs (see next item), reclaiming disk space used by (occasional) long transactions such as a mysqldump.

• The default value of the innodb_undo_log_truncate system variable  changed from OFF to ON. When enabled, undo tablespaces that exceed the threshold value defined by innodb_max_undo_log_size are marked for truncation. Only undo tablespaces can be truncated. Truncating undo logs that reside in the system tablespace is not supported. An upgrade from 5.7 to 8.0 automatically converts your system to use  undo tablespaces, using the system tablespace is not an option in 8.0.

• The default value of the innodb_flush_method system variable changed from NULL to fsync on Unix-like systems and from NULL to unbuffered on Windows systems. This is more of a terminology and option cleanup without any tangible impact. For Unix this is just a documentation change as the default was fsync also in 5.7 (the default NULL meant fsync). Similarly on Windows, innodb_flush_method default NULL meant async_unbuffered in 5.7, and is replaced by default unbuffered in 8.0, which in combination with the existing default innodb_use_native_aio=ON has the same effect.

• Incompatible change The default value of the innodb_autoinc_lock_mode system variable changed from 1 (consecutive) to 2 (interleaved). The change to interleaved lock mode as the default setting reflects the change from statement-based to row-based replication as the default replication type, which occurred in MySQL 5.7. Statement-based replication requires the consecutive auto-increment lock mode to ensure that auto-increment values are assigned in a predictable and repeatable order for a given sequence of SQL statements, whereas row-based replication is not sensitive to the execution order of SQL statements. Thus, this change is known to be incompatible with statement based replication, and may break some applications or user-generated test suites that depend on sequential auto increment. The previous default can be restored by setting innodb_autoinc_lock_mode=1;

• The default value of the innodb_flush_neighbors system variable changes from 1 (enable) to 0 (disable). This is done because fast IO (SSDs) is now the default for deployment. We expect that for the majority of users, this results in a small performance gain. Users who are using slower hard drives may see a performance loss, and are encouraged to revert to the previous defaults by setting innodb_flush_neighbors=1.

• The default value of the innodb_max_dirty_pages_pct_lwm system variable changed from 0 (%) to 10 (%). With innodb_max_dirty_pages_pct_lwm=10, InnoDB increases its flushing activity when >10% of the buffer pool contains modified (‘dirty’) pages. The purpose of this change is to trade off peak throughput slightly, in exchange for more consistent performance.

• The default value of the innodb_max_dirty_pages_pct system variable changed from 75 (%) to 90 (%). This change combines with the change to innodb_max_dirty_pages_pct_lwm and together they ensure a smooth InnoDB flushing behavior, avoiding flushing bursts. To revert to the previous behavior, set innodb_max_dirty_pages_pct=75 and innodb_max_dirty_pages_pct_lwm=0.

Performance Schema Defaults

• Performance Schema Meta Data Locking  (MDL) instrumentation is turned on by default. The compiled default  for performance-schema-instrument='wait/lock/metadata/sql/%=ON' changed from OFF to ON.  This is an enabler for adding MDL oriented views in SYS.

• Performance Schema Memory instrumentation is turned on by default. The compiled default  for performance-schema-instrument='memory/%=COUNTED' changed from OFF to COUNTED. This is important because the accounting is incorrect if instrumentation is enabled after server start, and you could get a negative balance from missing an allocation, but catching a free.

• Performance Schema Transaction instrumentation is turned on by default. The compiled default  for  performance-schema-consumer-events-transactions-current=ON, performance-schema-consumer-events-transactions-history=ON, and performance-schema-instrument='transaction%=ON' changed from OFF to ON.

Replication Defaults

• The default value of the log_bin system variable changed from OFF to ON. In other words, binary logging is enabled by default. Nearly all production installations have the binary log enabled as it is used for replication and point-in-time recovery. Thus, by enabling binary log by default we eliminate one configuration step, enabling it later requires a mysqld restart. Enabling it by default also provides better test coverage and it becomes easier to spot performance regressions. Remember to also set server_id (see following change). The 8.0 default behavior is as if you issued ./mysqld --log-bin --server-id=1. If you are on 8.0 and want 5.7 behavior you can issue ./mysqld --skip-log-bin --server-id=0.

• The default value of the server_id system variable changed from 0 to 1 (combines with the change to log_bin=ON). The server can be started with this default ID, but in practice you must set the server-id according to the replication infrastructure being deployed, to avoid having duplicate server ids.

• The default value of the log-slave-updates system variable changed from OFF to ON.  This causes a replica to log replicated events into its own binary log. This option is required for Group Replication, and also ensures correct behavior in various replication chain setups, which have become the norm today.

• The default value of the expire_logs_days system variable changed from 0 to 30. The new default 30 causes mysqld to periodically purge unused binary logs that are older than 30 days. This change helps prevent excessive amounts of disk space being wasted on binary logs that are no longer needed for replication or recovery purposes. The old value of 0 disables any automatic binary log purges.

• The default value of the master_info_repository and relay_log_info_repository system variables change from FILE to TABLE. Thus in 8.0, replication metadata is stored in InnoDB by default. This increases reliability to try and achieve crash safe replication by default.

• The default value of the transaction-write-set-extraction system variable changed from OFF to XXHASH64. This change enables transaction write sets by default. By using Transaction Write Sets, the source has to do slightly more work to generate the write sets, but the result is helpful in conflict detection. This is a requirement for Group Replication and the new default makes it easy to enable binary log writeset parallelization on the source to speed up replication.

• The default value of the slave_rows_search_algorithms system variable changed from INDEX_SCAN,TABLE_SCAN to INDEX_SCAN,HASH_SCAN. This change speeds up row-based replication by reducing the number of table scans the replica applier has to do to apply the changes to a table without a primary key.

• The default value of the slave_pending_jobs_size_max system variable changed from 16M to 128M. This change increases the amount of memory available to multithreaded replicas.

• The default value of the gtid_executed_compression_period system variable changed from 1000 to 0. This change ensures that compression of the mysql.gtid_executed table only occurs implicitly as required.

Group Replication Defaults

• The default value of group_replication_autorejoin_tries changed from 0 to 3, which means that automatic rejoin is enabled by default. This system variable specifies the number of tries that a member makes to automatically rejoin the group if it is expelled, or if it is unable to contact a majority of the group before the group_replication_unreachable_majority_timeout setting is reached.

• The default value of group_replication_exit_state_action changed from ABORT_SERVER to READ_ONLY. This means that when a member exits the group, for example after a network failure, the instance becomes read-only, rather than being shut down.

• The default value of group_replication_member_expel_timeout changed from 0 to 5, meaning that a member suspected of having lost contact with the group is liable for expulsion 5 seconds after the 5-second detection period.