The FLUSH statement clears or reloads different internal caches. One variant acquires a lock.
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FLUSH
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flush_option [, flush_option] ...
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flush_option:
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| TABLES table_name [, table_name] ...
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(closes all specified tables, forces those tables in use to be closed, and flushes the query cache for the named tables)
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| TABLES WITH READ LOCK
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(closes all open tables and locks all tables for all databases with a global read lock*)
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| LOGS
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(closes and reopens all log files--if binary logging is enabled, the sequence number of the binary log file is incremented by one relative to the previous file)
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| STATUS
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(adds the current thread's session status variable values to the global values and resets the session values to zero)
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To release a FLUSH TABLES WITH READ LOCK, you must issue an UNLOCK TABLES.
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*To release a FLUSH TABLES WITH READ LOCK, you must issue an UNLOCK TABLES.