GROUP BY also means SORT

[mmmm]

Hi to all,

DB2 seems to do a SORT before every GROUP. At least if I look at the query plan of

-- connected to the sample db
SELECT WORKDEPT, SUM(SALARY)
FROM EMPLOYEE
GROUP BY WORKDEPT
 

a SORT is performed before GRPBY. I can imagine that it's necessary, but why?

Greetings, Martina.

[r035198x]

Hi to all,

DB2 seems to do a SORT before every GROUP. At least if I look at the query plan of

-- connected to the sample db
SELECT WORKDEPT, SUM(SALARY)
FROM EMPLOYEE
GROUP BY WORKDEPT
 

a SORT is performed before GRPBY. I can imagine that it's necessary, but why?

Greetings, Martina.

Hi, how did you check that a sort is performed before the groupby?

[mmmm]

I use Visual Explain within the Command Center.

[NeoPa]

This is quite standard in SQL generally.
The engine has to process through the input somehow.
If it can process through by a relevant key then it will - otherwise it has to sort before it can tell if the next record is part of 'this' group or the beginning of the next.