Temp tables vs subqueries in inner join - mysql

Question

Both SQL, return the same results. The first my joins are on the subqueries the second the final queryis a join with a temporary that previously I create/populate them
SELECT COUNT(*) totalCollegiates, SUM(getFee(c.collegiate_id, dateS)) totalMoney
FROM collegiates c
LEFT JOIN (
SELECT collegiate_id FROM collegiateRemittances r
INNER JOIN remittances r1 USING(remittance_id)
WHERE r1.type_id = 1 AND r1.name = remesa
) hasRemittance ON hasRemittance.collegiate_id = c.collegiate_id
WHERE hasRemittance.collegiate_id IS NULL AND c.typePayment = 1 AND c.active = 1 AND c.exentFee = 0 AND c.approvedBoard = 1 AND IF(notCollegiate, c.collegiate_id NOT IN (notCollegiate), '1=1');
DROP TEMPORARY TABLE IF EXISTS hasRemittance;
CREATE TEMPORARY TABLE hasRemittance
SELECT collegiate_id FROM collegiateRemittances r
INNER JOIN remittances r1 USING(remittance_id)
WHERE r1.type_id = 1 AND r1.name = remesa;
SELECT COUNT(*) totalCollegiates, SUM(getFee(c.collegiate_id, dateS)) totalMoney
FROM collegiates c
LEFT JOIN hasRemittance ON hasRemittance.collegiate_id = c.collegiate_id
WHERE hasRemittance.collegiate_id IS NULL AND c.typePayment = 1 AND c.active = 1 AND c.exentFee = 0 AND c.approvedBoard = 1 AND IF(notCollegiate, c.collegiate_id NOT IN (notCollegiate), '1=1');
Which will have better performance for a few thousand records?

Answer 1

The two formulations are identical except that your explicit temp table version is 3 sql statements instead of just 1. That is, the overhead of the back and forth to the server makes it slower. But...
Since the implicit temp table is in a LEFT JOIN, that subquery may be evaluated in one of two ways...
Older versions of MySQL were 'dump' and re-evaluated it. Hence slow.
Newer versions automatically create an index. Hence fast.
Meanwhile, you could speed up the explicit temp table version by adding a suitable index. It would be PRIMARY KEY(collegiate_id). If there is a chance of that INNER JOIN producing dups, then say SELECT DISTINCT.
For "a few thousand" rows, you usually don't need to worry about performance.
Oracle has a zillion options for everything. MySQL has very few, with the default being (usually) the best. So ignore the answer that discussed various options that you could use in MySQL.
There are issues with
AND IF(notCollegiate,
c.collegiate_id NOT IN (notCollegiate),
'1=1')
I can't tell which table notCollegiate is in. notCollegiate cannot be a list, so why use IN? Instead simply use !=. Finally, '1=1' is a 3-character string; did you really want that?
For performance (of either version)
remittances needs INDEX(type_id, name, remittance_id) with remittance_id specifically last.
collegiateRemittances needs INDEX(remittance_id) (unless it is the PK).
collegiates needs INDEX(typePayment, active, exentFee , approvedBoard) in any order.
Bottom line: Worry more about indexes than how you formulate the query.
Ouch. Another wrinkle. What is getFee()? If it is a Stored Function, maybe we need to worry about optimizing it?? And what is dateS?

Answer 2

It depends actually. You'll have to test performance of every option. On my website I had 2 tables with articles and comments to them. It turned out it's faster to call comment counts 20 times for each article, than using a single union query. MySQL (like other DBs) caches queries, so small simple queries can run amazingly fast.

Answer 3

I did not saw that you have tagged the question as mysql so I initialy aswered for Oracle. Here is what I think about mySQL.
MySQL
There are two options when it comes to temporary tables Memory or Disk. And for Disk you can have MyIsam - non transactional and InnoDB transactional. Of course you can expect better performance for non transactional type of storage.
Additionaly you need to figure out how big resultset are you dealing with. For small resultset the memory option would be faster for large resultset the disk option would be faster.
Again at the end as in my original answer you need to figure out what performance is good enough and go for the most descriptive and easy to read option.
Oracle
It depends on what kind of temporary table you are dealing with.
You can have session based temporary tables - data is held until logout, or transaction based - data is held until commit . On top of this they can support transaction logging or not support it. Depending on configuration you can get better performance from a temporary table.
As everything in the world performance is relative therm. Most probably for few thousand records it will not do significant difference between the two queries. In which case I would go not for the most performant on but for the most easier to read and understand one.