Is the MySQL index tree built before each search or when the index is built?

MySQL The index tree is built before each search , It was generated when the index was built ？

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Take the answer 1：

Generated when indexing , Index and data are the same , It's all real , Every update insert delete will maintain the index , If established before each search , How slow is the search

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Other answers 1：

Generated when indexing , Index and data are the same , It's all real , Every update insert delete will maintain the index , If established before each search , How slow is the search