Question

Suppose we have a query that orders by a calculated value that is not selected, for example:

select * from table
order by col1 * col2 - col3

During typical sorting operations in most languages, it is common for the sort value to be used multiple times during the sort, as a given record is compared to potentially many other records. It is possible that mysql has such an implementation.

Can anyone say definitively if mysql calculates such values once per row and stores them temporarily while the sort completes, or if the values are recalculated whenever a comparison is made (which may be 1-n times)?

I have tagged this mysql, but I would welcome comments/answers regarding/including other popular databases

Answer 1

And the answer is… mysql executes the calculation once per row.

Due to lack of credible answers, I ran a definitive test on sqlfiddle that orders by the result of a not deterministic function (that must be called every time it’s compared) that also records in another table when it has been called. It shows that the number of times called = number of rows.