[CALCITE-5953] AggregateCaseToFilterRule may make inaccurate SUM transformation

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.36.0
• Labels: pull-request-available
• Assignee: Zoltan Haindrich
• Reporter: Zoltan Haindrich
• Created: 23/Aug/23 16:27
• Updated: 10/Nov/23 11:34
• Resolved: 05/Sep/23 07:13

Description

consider: sum(case when x = 1 then 2 else 0 end) as b
notice that this expression may only be null if there are no rows in the table

AggregateCaseToFilterRule rewrites the above expression to sum(2) filter (where x=1) which broadens when it could be `null` to when there are no matches to the filter

• A is 0 correctly in this case; but I think it will be still 0 in case there are 0 input rows
• The result for B supposed to be 0 but since there are no matches by the filter it becomes null
• C is not touched

# Convert CASE to FILTER without matches
select  sum(case when x = 1 then 1 else 0 end) as a,
        sum(case when x = 1 then 2 else 0 end) as b,
        sum(case when x = 1 then 3 else -1 end) as c
from (values 0, null, 0, 2) as t(x);
+---+---+----+
| A | B | C  |
+---+---+----+
| 0 |   | -4 | # incorrect result for B
| 0 | 0 | -4 | # correct
+---+---+----+
(1 row)

!ok
EnumerableCalc(expr#0..2=[{inputs}], expr#3=[CAST($t0):INTEGER], A=[$t3], B=[$t1], C=[$t2])
  EnumerableAggregate(group=[{}], A=[COUNT() FILTER $1], B=[SUM($2) FILTER $3], C=[SUM($0)])
    EnumerableCalc(expr#0=[{inputs}], expr#1=[1], expr#2=[=($t0, $t1)], expr#3=[3], expr#4=[-1], expr#5=[CASE($t2, $t3, $t4)], expr#6=[IS TRUE($t2)], expr#7=[2], $f2=[$t5], $f3=[$t6], $f4=[$t7], $f5=[$t6])
      EnumerableValues(tuples=[[{ 0 }, { null }, { 0 }, { 2 }]])
!plan

# Convert CASE to FILTER with no input rows
select  sum(case when x = 1 then 1 else 0 end) as a,
        sum(case when x = 1 then 2 else 0 end) as b,
        sum(case when x = 1 then 3 else -1 end) as c
from (values 0, null, 0, 2) as t(x) where x*x=1;
+---+---+---+
| A | B | C |
+---+---+---+
| 0 |   |   | # incorrect results - for 0 input rows sum supposed to be null
|   |   |   | # correct result
+---+---+---+
(1 row)

!ok

Issue Links

• links to: GitHub Pull Request #3400