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• offline_mode 

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Whether the server is in “offline mode” , which has these characteristics: 

•Connected client users who do not have the CONNECTION_ADMIN  privilege (or the deprecated SUPER privilege) are disconnected on the next request, with an appropriate error. Disconnection includes terminating running statements and releasing locks. Such clients also cannot initiate new connections, and receive an appropriate error. 

•Connected client users who have the CONNECTION_ADMIN  or SUPER  privilege are not disconnected, and can initiate new connections to manage the server. 

•Replication threads are permitted to keep applying data to the server. 

Only users who have the SYSTEM_VARIABLES_ADMIN  or SUPER  privilege can control offline mode. To put a server in offline mode, change the value of the offline_mode  system variable from OFF  to ON . To resume normal operations, change offline_mode  from ON  to OFF . In offline mode, clients that are refused access receive an ER_SERVER_OFFLINE_MODE  error. 

• old 

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old  is a compatibility variable. It is disabled by default, but can be enabled at startup to revert the server to behaviors present in older versions. 

When old  is enabled, it changes the default scope of index hints to that used prior to MySQL 5.1.17. That is, index hints with no FOR  clause apply only to how indexes are used for row retrieval and not to resolution of ORDER BY  or GROUP BY  clauses. (See Section 8.9.4, “Index Hints” .) Take care about enabling this in a replication setup. With statement-based binary logging, having different modes for the source and replicas might lead to replication errors. 

• old_alter_table 

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