RowExpression duplicate isNull on same variable issue

[chliang71]

Part of simplifyRowExpressionOptimizer execution is eliminates duplicates. Part of execution is the the code below

    public RowExpression minimalNormalForm(RowExpression expression)
    {
        RowExpression conjunctiveNormalForm = convertToConjunctiveNormalForm(expression);
        RowExpression disjunctiveNormalForm = convertToDisjunctiveNormalForm(expression);
        return numOfClauses(conjunctiveNormalForm) > numOfClauses(disjunctiveNormalForm) ? disjunctiveNormalForm : conjunctiveNormalForm;
    }

One scenario is that there can be OR(IS_NULL(a), IS_NULL(a)), i.e. two duplicate IS_NULL on same variable. In this case, the two convertTo*NormalForm call will both simplify it to just one IS_NULL(a) instance, and with form set to IS_NULL (replacing original form = OR). However, this causes trouble for the numOfClauses call later, because current code assumes specialFormExpression must have 2 arguments, while here it is just IS_NULL(a) with only one argument.

It is quite tricky to get to this corner case scenario though, just having a = null or a = null is not enough. I only got one query hitting this, and have been trying to hand craft a query that can cause this. Still have little clue so far, but the query that hit this issue has the core part below hitting the issue:

with x as (
  select a from xx
), z as (
  select case when (x.a is null or y.a is null) then 'AAA' else 'BBB' as col
  from y left join x on x.a = y.a
)
select * from z
where col != 'AAA'

Interestingly, even slightly tweaking the conditions (e.g. change to col = 'AAA' instead) this issue would not happen.

Test plan - (Please fill in how you tested your changes)

Added unit test.

== RELEASE NOTES ==

General Changes
* Add support for expressions with multiple ``IS NULL`` on the same variable.

[chliang71]

cc @vkorukanti @zhenxiao any thoughts?

[zhenxiao]

looks good. one minor issue

[zhenxiao]

how about having meaningful names:
s/aIsNull/isNullExpression/g
s/args/arguments/g
s/xx/duplicateIsNullExpression/g

[chliang71]

Updated, thanks!

[chliang71]

@zhenxiao I think the checks are passing, mind taking another look? Thanks!

[ajaygeorge]

Hi @zhenxiao / @chliang71
Release notes are missing for this PR.
Please add them according to the guidelines mentioned here

[zhenxiao]

Support expressions with multiple IS NULL on the same variable @chliang71 what do you think?

[chliang71]

Thanks @zhenxiao. Seems @ajaygeorge already edited (thanks!)