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Group by is slow with Order by

3154057Aug 18 2017 — edited Aug 22 2017

Hi All,

I am running following SQL, which it taking almost 4-5 seconds and returning only 18 records.

SELECT

SUM(TOTAL_AMOUNT) C1,

a.LEVEL2_ENAME AS c2,

c.CURR_YEARMO AS c3

FROM TERRITORY_PRESET_MSNP a,

CV_RESTATED_MSNP b

LEFT OUTER JOIN

MONTH_D c

ON b.YEARMO = c.CURR_YEARMO,

PRODUCT_OFFERING d

WHERE ( b.PO_ID = d.ROW_ID

AND b.DATASOURCE_ID = 10

AND b.YEARMO = 201704

AND b.OWNER_TERR_ID = a.TERR_ID

AND c.CURR_YEARMO = 201704

AND a.YEARMO = 201706

GROUP BY c.CURR_YEARMO, a.LEVEL2_ENAME

ORDER BY C3, C2;

If I remove the ORDER BY clause it is returning results in 1 second.

Can you guide me what is the issue?

~Shalini

This post has been answered by Nimish Garg on Aug 18 2017

Jump to Answer

Nimish Garg

Can you please post the explain plan of your SQL with Order by and without Order by and the Database version?

HOW TO: Post a SQL statement tuning request - template posting

1 person found this helpful

Nimish Garg

Answer

Another issue with Group by and Sort is, if you see your SQL using "SORT GROUP BY" in execution plan.

Then to separate GROUP and SORT operations., you might want to re-write and test your SQL as

with t as
(
SELECT /*+ no_merge */
   SUM(TOTAL_AMOUNT) C1,
   a.LEVEL2_ENAME AS c2,
   c.CURR_YEARMO AS c3
FROM TERRITORY_PRESET_MSNP a,
   CV_RESTATED_MSNP b
   LEFT OUTER JOIN
   MONTH_D c
   ON b.YEARMO = c.CURR_YEARMO,
   PRODUCT_OFFERING d
WHERE b.PO_ID = d.ROW_ID
    AND b.DATASOURCE_ID = 10
    AND b.YEARMO = 201704
    AND b.OWNER_TERR_ID = a.TERR_ID
    AND c.CURR_YEARMO = 201704
    AND a.YEARMO = 201706
GROUP BY c.CURR_YEARMO, a.LEVEL2_ENAME
)
select * from t 
ORDER BY C3, C2

Marked as Answer by 3154057 · Sep 27 2020

2 people found this helpful

BEDE

What about the structure of those tables?

What does the explain plan show? I mean what does the explain-plan look in with order by and without order by? Maybe order by determines the use of some index that is not so good. If that be the case, an alter index rebuild may help. There must be a sort group by in the select, then another sort for the order. Still, does the access path for the joined table change in case you add the order by? In case it does change, then try what Nimish suggested - I'd try that anyway.

Sven W.

How much data is processed to get this result?

It is a difference whether the grouping is done over 20 records or over 20 trillion records.

I guess that some relevant part has missing statistics. Because of that a wrong execution plan might be choosen when the ORDER BY is added.

Mostly this happens when the optimizer thinks there is only very little data in one of the relevant parts, while there in reality is a huge number.

Also you are mixing ANSI SQL and tranditional oracle syntax. This also can easily be confusing for the optimizer.

Try to write ansi SQL only. And make sure the joins are done via indexed FKs.

Then make sure the statistics are up to date for the tables and the indexes.

Paul Horth

3154057 wrote:
Hi All,
I am running following SQL, which it taking almost 4-5 seconds and returning only 18 records.
SELECT 
 SUM(TOTAL_AMOUNT) C1,
 a.LEVEL2_ENAME AS c2,
 c.CURR_YEARMO AS c3
 FROM TERRITORY_PRESET_MSNP a,
 CV_RESTATED_MSNP b
 LEFT OUTER JOIN
 MONTH_D c
 ON b.YEARMO = c.CURR_YEARMO,
 PRODUCT_OFFERING d
WHERE ( b.PO_ID = d.ROW_ID
 AND b.DATASOURCE_ID = 10
 AND b.YEARMO = 201704
 AND b.OWNER_TERR_ID = a.TERR_ID
 AND c.CURR_YEARMO = 201704
 AND a.YEARMO = 201706
GROUP BY c.CURR_YEARMO, a.LEVEL2_ENAME
ORDER BY C3, C2;
If I remove the ORDER BY clause it is returning results in 1 second. 
Can you guide me what is the issue?
~Shalini

Post the EXACT SQL you are running (The above won't even compile due to an unclosed parenthesis).

Post the explain plan with and without the ORDER BY.

It looks like you are stroing dates as numbers - please don't do that. Dates should be in DATE columns.

It is not too surprising that an ORDER BY increases the time taken - you are asking it to sort, but we will

know more when you post the plans.

1 person found this helpful

Cookiemonster76

It might help if you didn't mix and match ANSI and non-ANSI syntax - use JOIN for all the tables, not just one.

Also your LEFT JOIN isn't because you've specified c.CURR_YEARMO = 201704 in the where clause, which forces it to act like an inner join.

Jonathan Lewis

Nimish Garg has picked up an important general point. A query with "group by X,Y order by X,Y" will probably do a SORT GROUP BY operation to eliminate a sort for the "order by", while a query with just "group by X,Y" will probably do "HASH GROUP BY" which returns the data in an arbitrary order.

Given the optimizer can find lots of ways of getting the arithmetic wrong it's then possible for a switch from SORT to HASH to result in other parts of the plan changing, and this may affect the performance dramatically even when it's obvious to you that it shouldn't. In a case like yours, if there's no other "pure" code alternative than the suggested code rewrite to force a separate HASH GROUP BY followed by SORT GROUP BY is a suitable alternative. (You could force a HASH GROUP BY with the use_hash_aggregation() hint, though that's potentially harder for other people to manage later if the code has to change.)

THere are a couple of problems with your SQL, though:

a) It's not executable - there's a missing ")".

b) The mix of older (Oracle) and newer (ANSI) syntax is highly undesirable

c) The left outer join should disappear because the subsequent "AND c.CURR_YEARMO = 201704" will eliminate the preserved rows.

You may want to review the code; and, as others have said, the first step to understanding the difference in performance is looking at the execution plans.

Regards

Jonathan Lewis

2 people found this helpful

3154057

( is a typing error. I have sent a mail to my dba to provide me execution plan from production.

3154057

Thank You. I will test this and will update you.

~Shalini

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SQL & PL/SQL

Group by is slow with Order by

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