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This article mainly explains "what is the operation principle of pt-online-schema-change". Interested friends may wish to have a look at it. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "what is the operation principle of pt-online-schema-change"?

(1)。 Create an empty temporary table first

(2)。 Define according to the structure after the original table alter table

(3)。 Add a trigger to the original table to synchronize the data changes (insert, update, and delete) generated by the original table during the subsequent data copy process into the temporary table

(4)。 Copy the data of the original table to the temporary table, and the data modification during the copying process will be synchronized by the trigger to the temporary table, thus ensuring that the original table will not be locked during the alter table operation.

(5)。 After synchronizing the data, rename the operation, change the temporary table name to the original table name and delete the original table

(6)。 Delete the trigger.

There are some holes to be aware of when using pt-osc:

1. If there is duplicate data in the table, adding a unique key will result in data loss

two。 In row format, using pt-osc only in the slave library will also lose data.

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