What does mysql metadata lock mean?

This article will explain in detail what MySQL metadata lock refers to. The editor thinks it is very practical, so I share it with you as a reference. I hope you can get something after reading this article.

1. MDL does not need to be used explicitly and will be added automatically when it is accessed.

2. The function of MDL is to ensure the correctness of reading and writing.

You can imagine that if a query is traversing the data in a table and another thread changes the structure of the table and deletes a column during execution, the results obtained by the query thread must not match the structure of the table.

3. MDL is introduced in the MySQL5.5 version, adding a MDL read lock when adding, deleting, modifying and querying a table.

When making structural changes to the table, add a MDL write lock.

Example

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