Question

I am currently working with a query in in MSSQL that looks like:

SELECT
   ...
FROM
(SELECT
   ...
)T1
JOIN
(SELECT
   ...
)T2
GROUP BY
...

The inner selects are relatively fast, but the outer select aggregates the inner selects and takes an incredibly long time to execute, often timing out. Removing the group by makes it run somewhat faster and changing the join to a LEFT OUTER JOIN speeds things up a bit as well.

Why would doing a group by on a select which aggregates two inner selects cause the query to run so slow? Why does an INNER JOIN run slower than a LEFT OUTER JOIN? What can I do to troubleshoot this further?

EDIT: What makes this even more perplexing is the two inner queries are date limited and the overall query only runs slow when looking at date ranges between the start of July and any other day in July, but if the date ranges are anytime before the the July 1 and Today then it runs fine.

Answer 1

The issue was with fragmented data. After the data was defragmented the query started running within reasonable time constraints.