Why are these two queries having such different executions? I'm trying to get all users that haven't been sent a message in a given day. I have two tables, one with the messages that have a reporterid indexed column, and a reporter table.

Let me walk you through it:

This inner query returns reporters that have received messages in a given day. It takes tenths of a second on my local dev machine:

select  m.reporterid
    from  nubamessage m
    where  m.messagetypeenum =7
      and  m.createdOn&gt;='2016-06-18 00:00:00'
      and  m.createdOn&lt;='2016-06-18 09:30:00';


There is no difference in the execution time of the above query if I do select(distinct) or not, and it returns the same rows.

But if I used that in an outer join query, it basically never completes:

select  *
    from  reporter r
    left outer join  
    (
        SELECT  m.reporterid
            from  nubamessage m
            where  m.messagetypeenum =7
              and  m.createdOn&gt;='2016-06-18 00:00:00'
              and  m.createdOn&lt;='2016-06-18 09:30:00'
    ) as mm  ON mm.reporterid=r.id
    where  r.enabled=1 and m.reporterid is null;


However, if I change the inner query to distinct as the below, it takes a fraction of a second:

select  *
    from  reporter r
    left outer join  
    (
        SELECT  distinct(m.reporterid)   -- the only difference
            from  nubamessage m
            where  m.messagetypeenum =7
              and  m.createdOn&gt;='2016-06-18 00:00:00'
              and  m.createdOn&lt;='2016-06-18 09:30:00'
    ) as mm  ON mm.reporterid=r.id
    where  r.enabled=1 and m.reporterid is null;


The execution plan for the non-completing query:

id  select_type table   partitions  type    possible_keys   key key_len ref rows    filtered    Extra
1   SIMPLE  r   NULL    ALL NULL    NULL    NULL    NULL    11538   50.00   "Using where"
1   SIMPLE  m   NULL    ALL NULL    NULL    NULL    NULL    968388  100.00  "Using where; Using join buffer (Block Nested Loop)"


The execution plan for the query where I just add "distinct", it adds some auto-key:

id  select_type table   partitions  type    possible_keys   key key_len ref rows    filtered    Extra
1   PRIMARY r   NULL    ALL NULL    NULL    NULL    NULL    11538   50.00   "Using where"
1   PRIMARY &lt;derived2&gt;  NULL    ref &lt;auto_key0&gt; &lt;auto_key0&gt; 5   r.id    10  100.00  "Using index"
2   DERIVED m   NULL    ALL NULL    NULL    NULL    NULL    968388  1.11    "Using where; Using temporary"


If someone could find it in their heart to explain to me why 1. the first query never completes/is so slow and 2. why the distinct makes it run fast, even though the inner query returns the exact same rows in both cases, I'd be most grateful.

Thanks

Edit: Notice that I write "all users that haven't been sent a message. This is why I have the left join - later in the query I have "where m.reporterid is null", in order to only get out the reporters that don't already have a message in the messagetable. I have edited the questions above to reflect that.

I could have used a "not in" instead but from what I've read and in my tests it was a bit slower than left join.
LEFT is killing performance; remove it unless you have a good reason for keeping it.  To elaborate...

((but first... The purpose for the LEFT was missing from the original query; this answer assumes the LEFT was not necessary.))

LEFT JOIN says that you want data from the right table, whether or not there was a match with the left table.

JOIN says to display only the rows that match (via ON) both tables.

(OUTER is optional and adds no semantics.)

If you are running a version before 5.6, the derived table (subquery in LEFT JOIN) will have no index, so it must be scanned repeatedly.  This is a big reason to get rid of LEFT.

Without the LEFT, the Optimizer is likely to evaluate the subquery once, then efficiently JOIN to reporter to finish the query.

For JOIN, the inner query needs this composite (and covering) index INDEX(messagetypeenum, createdOn, reporterid).

There is another technique (I think)...  Get rid of the inner SELECT, simply JOIN (or LEFT JOIN) to the table:

select  r.*
    from  reporter r
    left join  nubamessage m  ON m.reporterid = r.id
      AND  m.messagetypeenum = 7
      and  m.createdOn &gt;= '2016-06-18 00:00:00'
      and  m.createdOn &lt;= '2016-06-18 09:30:00'
    WHERE  r.enabled = 1
      AND  m.reporterid is null;


In this case, it may needs INDEX(reporterid, messagetypeenum, createdOn).

Yet another variant would use EXISTS and, I think, provides the equivalent of the LEFT

select  r.*
    from  reporter r
    WHERE  r.enabled=1
      AND NOT EXISTS 
    (
        SELECT  *
            from  nubamessage m
            where  m.messagetypeenum =7
              and  m.createdOn&gt;='2016-06-18 00:00:00'
              and  m.createdOn&lt;='2016-06-18 09:30:00'
              AND  m.reporterid=r.id 
    ) 


I can't predict which variant will be fastest.  It partially depends on how "many" in the many:1 mapping of m.reporterid : r.id.