Notice that unlike a data row, index rows do not contain the status byte B or the 2 bytes to hold the length of fixed-length data fields. Instead of storing the length of the fixed-length data, which also indicates where the fixed-length portion of a row ends and the variable-length portion begins, the page header
pminlen value is used to help describe an index row. The
pminlen value is the minimum length of the index row, which is essentially the sum of the size of all fixed-width fields and overhead. Therefore, if no variable-length or nullable fields are in the index key,
pminlen also indicates the width of each index row.
more on why simple keys are better
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