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MySQL 5.5 Reference Manual  /  ...  /  Optimizing for Character and String Types Optimizing for Character and String Types

For character and string columns, follow these guidelines:

  • Use binary collation order for fast comparison and sort operations, when you do not need language-specific collation features. You can use the BINARY operator to use binary collation within a particular query.

  • When comparing values from different columns, declare those columns with the same character set and collation wherever possible, to avoid string conversions while running the query.

  • For column values less than 8KB in size, use binary VARCHAR instead of BLOB. The GROUP BY and ORDER BY clauses can generate temporary tables, and these temporary tables can use the MEMORY storage engine if the original table does not contain any BLOB columns.

  • If a table contains string columns such as name and address, but many queries do not retrieve those columns, consider splitting the string columns into a separate table and using join queries with a foreign key when necessary. When MySQL retrieves any value from a row, it reads a data block containing all the columns of that row (and possibly other adjacent rows). Keeping each row small, with only the most frequently used columns, allows more rows to fit in each data block. Such compact tables reduce disk I/O and memory usage for common queries.

  • When you use a randomly generated value as a primary key in an InnoDB table, prefix it with an ascending value such as the current date and time if possible. When consecutive primary values are physically stored near each other, InnoDB can insert and retrieve them faster.

  • See Section, “Optimizing for Numeric Data” for reasons why a numeric column is usually preferable to an equivalent string column.

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