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The problem was that some facilities (like CONVERT_TZ() function or server HELP statement) may require implicit access to some tables in 'mysql' database. This access was done by ordinary means of adding such tables to the list of tables the query is going to open. However, if we issued LOCK TABLES before that, we would get "table was not locked" error trying to open such implicit tables. The solution is to treat certain tables as MySQL system tables, like we already do for mysql.proc. Such tables may be opened for reading at any moment regardless of any locks in effect. The cost of this is that system table may be locked for writing only together with other system tables, it is disallowed to lock system tables for writing and have any other lock on any other table. After this patch the following tables are treated as MySQL system tables: mysql.help_category mysql.help_keyword mysql.help_relation mysql.help_topic mysql.proc (it already was) mysql.time_zone mysql.time_zone_leap_second mysql.time_zone_name mysql.time_zone_transition mysql.time_zone_transition_type These tables are now opened with open_system_tables_for_read() and closed with close_system_tables(), or one table may be opened with open_system_table_for_update() and closed with close_thread_tables() (the latter is used for mysql.proc table, which is updated as part of normal MySQL server operation). These functions may be used when some tables were opened and locked already. NOTE: online update of time zone tables is not possible during replication, because there's no time zone cache flush neither on LOCK TABLES, nor on FLUSH TABLES, so the master may serve stale time zone data from cache, while on slave updated data will be loaded from the time zone tables. |
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