Ever wanted to look at a query’s actual execution plan (execution plan with runtime information) without waiting for the query to finish? Now you can.
Enable that and run a query, and you get an execution plan immediately, one with a few more details in it than we’re used to.
I’m not sure what the times on the operators show, because a constant scan wouldn’t take over 4 seconds, it generates a single number. I suspect it’s the time between the first row request and the operator returning ‘no more rows’, but I’d have to test more to be sure.
The percentage done is probably based on the estimated row count, because on this plan all the operators went to 99% done instantly. The two numbers underneath the % done are the number of rows currently processed and below that the estimated number of rows.
It’s clear there’s a severe row estimation problem here. The last nested loop join estimated 97 rows and, at the point the screenshot was taken, had processed 170500 rows. It’s a pity that the estimated executions isn’t shown as well, as it makes identifying row estimation errors slightly harder for operators that execute multiple times (eg the key lookup). http://www.sqlinthewild.co.za/index.php/2009/09/22/estimated-rows-actual-rows-and-execution-count/
Identifying this kind of problem is, I think, one of the primary uses for Live Query Stats (other that using it to see how the QP works). The old example of ‘it ran fast yesterday and slow today, why?’ can be at least partially answered with Live Query Stats. Run the query with it turned on and see what plan is used and what row counts are flowing through them. Combine that with Query Store’s history of what plans were used yesterday, and we have a very powerful way of identifying why something is running unusually slow.
However is it something that should be used sparsely in production, as it does add some overhead.
In most ways the Live Query Stats behaves like a normal execution plan, it can be saved and if saved part way through a query’s execution, the resultant file is a normal .sqlplan file and has actual row counts of the point in execution where it was saved.
It’s not a ground breaking new feature, but it’s a fun way to see how queries execute and it does have some uses in debugging sudden performance problems.