I have a big, ugly query which is quadratic in the input size and therefore want to display an approximate completion time prior to sending it off:
select concat('Ready by ', time(now() + pow(select count(*) as datapoints from data, 2)));
Syntactically this is wrong and is merely illustrative. There is no FROM clause, so I cannot use a traditional subquery. How would this be programmed in MySQL 5.7?
A question: can you not place your data temporarily in a file and use command line tools to get your answer, since the query runs for potentially a very long time? What table contains "data"? Then you can use a subquery.
Plus, having been an admin and system programmer for a long time I would commend code like this to help from a very senior techie. Quadratic time queries may have system wide impacts that affect other processes and users. This is a polite way of me saying if you did this you would incur the wrath of a lot of people. Just saying. If it is your desktop, okay, maybe.
I don't see the link between what you want and the query and like you have said it's not correct. So, to analyze the query you have the explain command to get information about the plan that will be used and from there you can estimate quick query and slow query.
See the official web site for "EXPLAIN"
The other option is to do it on the application side.