14.7 Data Dictionary Usage Differences

ANALYZE TABLE fails because it updates table statistics, which are stored in the data dictionary.

ALTER TABLE tbl_name ENGINE=engine_name fails because it updates the storage engine designation, which is stored in the data dictionary.

Previously, INFORMATION_SCHEMA queries for table statistics in the STATISTICS and TABLES tables retrieved statistics directly from storage engines. As of MySQL 8.0, cached table statistics are used by default. The information_schema_stats_expiry system variable defines the period of time before cached table statistics expire. The default is 86400 seconds (24 hours). (To update the cached values at any time for a given table, use ANALYZE TABLE.) If there are no cached statistics or statistics have expired, statistics are retrieved from storage engines when querying table statistics columns. To always retrieve the latest statistics directly from storage engines, set information_schema_stats_expiry to 0. For more information, see Section 8.2.3, “Optimizing INFORMATION_SCHEMA Queries”.

Several INFORMATION_SCHEMA tables are views on data dictionary tables, which enables the optimizer to use indexes on those underlying tables. Consequently, depending on optimizer choices, the row order of results for INFORMATION_SCHEMA queries might differ from previous results. If a query result must have specific row ordering characteristics, include an ORDER BY clause.

Queries on INFORMATION_SCHEMA tables may return column names in a different lettercase than in earlier MySQL series. Applications should test result set column names in case-insensitive fashion. If that is not feasible, a workaround is to use column aliases in the select list that return column names in the required lettercase. For example:

mysqldump and mysqlpump no longer dump the INFORMATION_SCHEMA database, even if explicitly named on the command line.

CREATE TABLE dst_tbl LIKE src_tbl requires that src_tbl be a base table and fails if it is an INFORMATION_SCHEMA table that is a view on data dictionary tables.

Previously, result set headers of columns selected from INFORMATION_SCHEMA tables used the capitalization specified in the query. This query produces a result set with a header of table_name:

As of MySQL 8.0, these headers are capitalized; the preceding query produces a result set with a header of TABLE_NAME. If necessary, a column alias can be used to achieve a different lettercase. For example:

Previously, it was possible to dump all tables in the mysql system database. As of MySQL 8.0, mysqldump and mysqlpump dump only non-data dictionary tables in that database.

Previously, the --routines and --events options were not required to include stored routines and events when using the --all-databases option: The dump included the mysql system database, and therefore also the proc and event tables containing stored routine and event definitions. As of MySQL 8.0, the event and proc tables are not used. Definitions for the corresponding objects are stored in data dictionary tables, but those tables are not dumped. To include stored routines and events in a dump made using --all-databases, use the --routines and --events options explicitly.

Previously, the --routines option required the SELECT privilege for the proc table. As of MySQL 8.0, that table is not used; --routines requires the global SELECT privilege instead.

Previously, it was possible to dump stored routine and event definitions together with their creation and modification timestamps, by dumping the proc and event tables. As of MySQL 8.0, those tables are not used, so it is not possible to dump timestamps.