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When this variable is enabled, the server does not use the optimized method of processing an ALTER TABLE  operation. It reverts to using a temporary table, copying over the data, and then renaming the temporary table to the original, as used by MySQL 5.0 and earlier. For more information on the operation of ALTER TABLE , see Section 13.1.9, “ALTER TABLE Statement” . 

ALTER TABLE ... DROP PARTITION  with old_alter_table=ON  rebuilds the partitioned table and attempts to move data from the dropped partition to another partition with a compatible PARTITION ... VALUES  definition. Data that cannot be moved to another partition is deleted. In earlier releases, ALTER TABLE ... DROP PARTITION  with old_alter_table=ON  deletes data stored in the partition and drops the partition. 

• open_files_limit 

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The number of file descriptors available to mysqld  from the operating system: 

•At startup, mysqld  reserves descriptors with setrlimit() , using the value requested at by setting this variable directly or by using the --open-files-limit  option to mysqld_safe . If mysqld  produces the error Too many open files , try increasing the open_files_limit  value. Internally, the maximum value for this variable is the maximum unsigned integer value, but the actual maximum is platform dependent. 

•At runtime, the value of open_files_limit  indicates the number of file descriptors actually permitted to mysqld  by the operating system, which might differ from the value requested at startup. If the number of file descriptors requested during startup cannot be allocated, mysqld  writes a warning to the error log. 

The effective open_files_limit  value is based on the value specified at system startup (if any) and the values of max_connections  and table_open_cache , using these formulas: 

• 10 + max_connections + (table_open_cache * 2) 

• max_connections * 5 

•MySQL 8.0.19 and higher: The operating system limit. 

