Index cardinality is actually a metric that is based on MySQL analyzing
the table contents, so its value here has more to do with whether the
tests are running against a new table, an old table, or whether it has
any data in it.
The Sub_part field is similarly unimportant for the purposes of these
tests since it refers to indexing internals based on data type and
partial indexing.
See: https://dev.mysql.com/doc/refman/5.5/en/show-index.html
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