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MySQL queries do not use index summaries

As we all know, increasing the index is an effective way to improve the query speed, but very often, even if the index is added, the query still does not use the index, which seriously affects the performance. Here is a brief summary of several cases where MySQL does not use the index.

If MySQL estimates that using indexes is slower than full table scans, indexes are not used. For example, if the column key is evenly distributed between 1 and 100, the following query does not use the index very well: select * from table_name where key > 1 and key

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