In a the structure KEY, I see a field named rec_per_key that has the
Array of AVG(#records with the same field value) for 1st ... Nth key part.
0 means 'not known'.
For temporary heap tables this member is NULL.
Our storage engine does not ever fill these values. We set the values
to 0. We recently saw that not setting this for the primary key
resulted in a bad query plan. We can set it to 1 for unique keys and
the primary key. Are there other known bad side effects to not setting