MySQL-官方文档摘抄

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There are three phases to an index build.
In the first phase, the clustered index is scanned, and index entries are generated and added to the sort buffer. When the sort buffer becomes full, entries are sortedand writtenout to a temporary intermediate file. This process is also known as a “run”.
In the second phase, with oneor more runs written to the temporary intermediate file, a merge sort is performed on all entriesin the file.
In the thirdand final phase, the sorted entries are insertedinto the B-tree.

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When running an online DDL operation, the thread that runs the ALTER TABLE statement applies an “online log” of DML operations that were run concurrently on the same table from other connection threads. When the DML operations are applied, it is possible to encounter a duplicate key entry error(ERROR 1062(23000): Duplicate entry), evenif the duplicate entry is only temporaryand would be reverted by a later entryin the “online log”.This is similar to the ideaof a foreign key constraint checkin InnoDBin which constraints must hold during a transaction.

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Semisynchronous Replication
The slave acknowledges receipt of a transaction's events only after the events have been written to its relay log and flushed to disk.

半同步只负责relaylog的落盘, 并不关心Slave是否真的有应用这些events, 所以Master的双1+半同步正常可以确认binlog到了Slave端, Slave在不出问题的情况下能做到最终一致性;

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innodb_lock_wait_timeout applies to InnoDB row locks only. A MySQL table lock does not happen inside InnoDB and this timeout does not apply to waits for table locks.
The lock wait timeout value does not apply to deadlocks when innodb_deadlock_detect is enabled (the default) because InnoDB detects deadlocks immediately and rolls back one of the deadlocked transactions. When innodb_deadlock_detect is disabled, InnoDB relies

on innodb_lock_wait_timeout for transaction rollback when a deadlock occurs.

For example, metadata lock

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