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    Summary

    Good schema design is pretty universal, but of course MySQL has special implementation details to consider. In a nutshell, it’s a good idea to keep things as small and simple as you can. MySQL likes simplicity, and so will the people who have to work with your database:

    • Try to avoid extremes in your design, such as a schema that will force enormously complex queries, or tables with oodles and oodles of columns. (An oodle is somewhere between a scad and a gazillion.)

    • Use small, simple, appropriate data types, and avoid NULL unless it’s actually the right way to model your data’s reality.

    • Try to use the same data types to store similar or related values, especially if they’ll be used in a join condition.

    • Watch out for variable-length strings, which might cause pessimistic full-length memory allocation for temporary tables and sorting.

    • Try to use integers for identifiers if you can.

    • Avoid the legacy MySQL-isms such as specifying precisions for floating-point numbers or display widths for integers.

    • Be careful with ENUM and SET. They’re handy, but they can be abused, and they’re tricky sometimes. BIT is best avoided.

    Normalization is good, but denormalization (duplication of data, in most cases) is sometimes actually necessary and beneficial. We’ll see more examples of that in the next chapter. And precomputing, caching, or generating summary tables can also be a big win. Justin Swanhart’s Flexviews tool can help maintain summary tables.

    Finally, ALTER TABLE can be painful because in most cases, it locks and rebuilds the whole table. We showed a number of workarounds for specific cases; for the general case, you’ll have to use other techniques, such as performing the ALTER on a replica and then promoting it to master. There’s more about this later in the book.