Mysql Order By after Group By causes very slow query - mysql

Question

I can't figure out why the 'order by' clauses in Query2 below causes it to take over a minute while the first one returns results instantly. Is there a better way to do this 'order by'
Fast:
select c.id, max(date(a.sent)) as sent,
if(c.id in (select id from bin where (num=1 or num=2)),1,0) as done
from test c, test2 a
where c.id=a.id
group by c.id
limit 1;
Slow
select c.id, max(date(a.sent)) as sent,
if(c.id in (select id from bin where (num=1 or num=2)),1,0) as done
from test c, test2 a
where c.id=a.id
group by c.id
order by done, sent
limit 1;

Answer 1

It's because the "columns" in the order by clause are not real columns, but aliases for calculations elsewhere in the query. Thus, they aren't indexed, and the server has to order them on the fly. Using a join for the calculation of done, rather than a subquery, would likely speed this up a lot.

Answer 2

If you were bringing back all records, the sorting should not take much time, even though they are computed / non-indexed fields. However, you are using "Limit 1". This changes the approach of the optimizer.
In the first case, you are ordering by an ID. Since you have "limit 1" and the ID probably has an index, the optimizer can go ID by ID and when it gets one records that matches the WHERE clause it can return.
However, in the second query, even though you only want 1 record, the optimizer does not know which one that will be unless it computes the entire set (as if you did not have "limit 1") and then returns only the first one.
Take off the "LIMIT 1" and compare the two queries. If the difference remains, it may be a different problem.
It is difficult to say what would work best with your volumes. Try this query:
select id, max(date(sent)) as sent, 0 As done
from test2
where exists (select 1 from bin where bin.id=test2.id and num not in (1,2))
group by id
union all
select id, max(date(sent)) as sent, 1 As done
from test2
where exists (select 1 from bin where bin.id=test2.id and num in (1,2))
group by id
order by done, sent
limit 1
SQL Fiddle is here if you want to tweak it.
I left out the test table, because you were not bringing back any field except ID, which is already on test2. If you need other fields from test, you will have to tweak it.