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MySQL 5.7 Release Notes  /  Changes in MySQL 5.7.33 (2021-01-18, General Availability)

Changes in MySQL 5.7.33 (2021-01-18, General Availability)

Optimizer Notes

  • MySQL attempts to use an ordered index for any ORDER BY or GROUP BY query that has a LIMIT clause, overriding any other choices made by the optimizer, whenever it determines that this would result in faster execution. Because the algorithm for making this determination makes certain assumptions about data distribution and other conditions, it may not always be completely correct, and it is possible in some cases that choosing a different optimization for such queries can provide better performance. To handle such occurrences, it is now possible to disable this optimization by setting the optimizer_switch system variable's prefer_ordering_index flag to off.

    For more information about this flag and examples of its use, see Switchable Optimizations, and LIMIT Query Optimization.

    Our thanks to Jeremy Cole for the contribution. (Bug #31686878)

    References: See also: Bug #97001, Bug #30348211.

Security Notes

Functionality Added or Changed

  • When invoked with the --all-databases option, mysqldump now dumps the mysql database first, so that when the dump file is reloaded, any accounts named in the DEFINER clause of other objects will already have been created. (Bug #32141046)

Bugs Fixed

  • InnoDB: The full-text search synchronization thread attempted to read a previously-freed word from the index cache. (Bug #31310404)

  • InnoDB: Calls to numa_all_nodes_ptr were replaced by the numa_get_mems_allowed() function. Thanks to Daniel Black for the contribution. (Bug #24693086, Bug #83044)

  • Replication: When the system variable transaction_write_set_extraction=XXHASH64 is set, which is the default in MySQL 8.0 and a requirement for Group Replication, the collection of writes for a transaction previously had no upper size limit. Now, for standard source to replica replication, the numeric limit on write sets specified by binlog_transaction_dependency_history_size is applied, after which the write set information is discarded but the transaction continues to execute. Because the write set information is then unavailable for the dependency calculation, the transaction is marked as non-concurrent, and is processed sequentially on the replica. For Group Replication, the process of extracting the writes from a transaction is required for conflict detection and certification on all group members, so the write set information cannot be discarded if the transaction is to complete. The byte limit set by group_replication_transaction_size_limit is applied instead of the numeric limit, and if the limit is exceeded, the transaction fails to execute. (Bug #32019842)

  • Replication: As the number of replicas replicating from a semisynchronous source server increased, locking contention could result in a performance degradation. The locking mechanisms used by the plugins have been changed to use shared locks where possible, avoid unnecessary lock acquisitions, and limit callbacks. The new behaviors can be implemented by enabling the following system variables:

    • replication_sender_observe_commit_only=1 limits callbacks.

    • replication_optimize_for_static_plugin_config=1 adds shared locks and avoids unnecessary lock acquisitions. This system variable must be disabled if you want to uninstall the plugin.

    Both system variables can be enabled before or after installing the semisynchronous replication plugin, and can be enabled while replication is running. Semisynchronous replication source servers can also get performance benefits from enabling these system variables, because they use the same locking mechanisms as the replicas. (Bug #30519928)

  • Replication: On a multi-threaded replica where the commit order is preserved, worker threads must wait for all transactions that occur earlier in the relay log to commit before committing their own transactions. If a deadlock occurs because a thread waiting to commit a transaction later in the commit order has locked rows needed by a transaction earlier in the commit order, a deadlock detection algorithm signals the waiting thread to roll back its transaction. Previously, if transaction retries were not available, the worker thread that rolled back its transaction would exit immediately without signalling other worker threads in the commit order, which could stall replication. A worker thread in this situation now waits for its turn to call the rollback function, which means it signals the other threads correctly. (Bug #26883680, Bug #87796)

  • Replication: GTIDs are only available on a server instance up to the number of non-negative values for a signed 64-bit integer (2 to the power of 63 minus 1). If you set the value of gtid_purged to a number that approaches this limit, subsequent commits can cause the server to run out of GTIDs and take the action specified by binlog_error_action. From MySQL 8.0.23, a warning message is issued when the server instance is approaching the limit. (Bug #26035544)

  • Microsoft Windows: On Windows, running the MySQL server as a service caused shared-memory connections to fail. (Bug #32009251)

  • The MRR iterator normally filters out NULL keys by checking impossible_null_ref(), but when a join condition either contained an IS NULL predicate, or used the NULL-safe equals operator ≪=>, the optimizer had to check whether the join conditition used the predicate terms as part of its join condition, and not set the internal flag HA_MRR_NO_NULL_ENDPOINTS in such cases. Now we check, using a bitmask, whether the each column in the key rejects NULL, in wich case we can set HA_MRR_NO_NULL_ENDPOINTS without further checks. (Bug #32774281)

  • The server did not handle all cases of the WHERE_CONDITION optimization correctly. (Bug #31905199)

  • For the engines which support primary key extension, when the total key length exceeded MAX_KEY_LENGTH or the number of key parts exceeded MAX_REF_PARTS, key parts of primary keys which did not fit within these limits were not added to the secondary key, but key parts of primary keys were unconditionally marked as part of secondary keys.

    This led to a situation in which the secondary key was treated as a covering index, which meant sometimes the wrong access method was chosen.

    This is fixed by modifying the way in which key parts of primary keys are added to secondary keys so that those which do not fit within which do not fit within the limits mentioned previously mentioned are cleared. (Bug #31617858)

  • Privileges for some INFORMATION_SCHEMA tables were checked incorrectly. (Bug #31553323)

  • In certain cases, the server did not handle multiply-nested subqueries correctly. (Bug #31472704)

  • Certain accounts could cause server startup failure if the skip_name_resolve system variable was enabled. (Bug #31018510)

  • Client programs could unexpectedly exit if communication packets contained bad data. (Bug #30890850)

  • A buffer overflow in the client library was fixed. (Bug #30885987)

  • mysql_config_editor incorrectly treated # in password values as a comment character. (Bug #29861961, Bug #95597)