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MySQL 5.7 Reference Manual
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B.4.5 Optimizer-Related Issues

MySQL uses a cost-based optimizer to determine the best way to resolve a query. In many cases, MySQL can calculate the best possible query plan, but sometimes MySQL does not have enough information about the data at hand and has to make educated guesses about the data.

For the cases when MySQL does not do the "right" thing, tools that you have available to help MySQL are: