Query: Read Avg/Min/Max as nullable from database to throw exception …

[smitpatel]

…only when it is empty

Resolves #18955

[smitpatel]

Description
Since 3.0, we read value from database according to the type. In Avg/Min/Max database returns null if there are no elements. We read this as default value of non-nullable type and throw exception if we hit default. If the actual result of above methods ends up being same as default value, we incorrect identify it is empty sequence.

Customer Impact
Query failure when result of Avg/Min/Max in database is 0.

How found
Customer-reported (#18955)

Test coverage
We did not have tests when Avg/Min/Max returned 0 result.

Regression?
Yes, from 2.2.

Risk
Risk is very small - the fix is reading database value as nullable for those functions and disambiguate null from 0 in result to throw exception only in correct case.

[ajcvickers]

@Pilchie FYI, we would like to patch this in 3.1.1.

[Pilchie]

What workaround is available to the customer, if any?

[ajcvickers]

@Pilchie Having a workaround may not help much in this case. The issue is that if you are calculating the max of a set of values, then your application may work fine until the result happens to be zero at which point the app will crash. To use a workaround you would have to know that the issue exists, which is very easily missed here, even with reasonable testing.

[maumar]

keep as lambda body

[ajcvickers]

@Pilchie @wtgodbe @dougbu Can this now be merged for 3.1.1?