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Prefixes are a neat trick. One nice consequence is that an index for the column pair (a, b) is also an index for (a).

• masklinn 5y ago
  Yup, also that `like ‘foo%’` can use the index.
• branko_d 5y ago
  And if the cardinality of a is low, it can even be an index for b! This is called “skip scan”. Oracle has supported this for a long time, and apparently newer versions of MySQL as well.