Vladimir Sitnikov created CALCITE-4387:
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Summary: Document that RexSimplify.simplifyUnknownAs could narrow
or widen nullness of the given expression
Key: CALCITE-4387
URL: https://issues.apache.org/jira/browse/CALCITE-4387
Project: Calcite
Issue Type: Bug
Components: core
Affects Versions: 1.26.0
Environment: It is expected that {{RexSimplify.simplifyUnknownAs}}
would make the expression "simpler" (e.g. less RexNodes or something like
that), however, there's no explicit contract on the resulting expression.
For instance the following expression has non-nullable type
{noformat}
case_(isUnknown(vBool(1)), trueLiteral, vBool(1), falseLiteral, trueLiteral)
{noformat}
however, it might be simplified to the following **nullable** expression:
{noformat}
or(isUnknown(vBool(1)), not(vBool(1)))
{noformat}
Note: the expression never returns null, however, it might be not that obvious
that the simplified expression never returns null.
I suggest we add explicit note to {{simplifyUnknownAs}} method that says the
method is allowed to narrow or widen the nullness of the given expression type
{{simplifyPreservingType(RexNode, RexUnknownAs, boolean matchNullability)}}
could be used to keep nullness intact.
PS. It woud be nice to simplify {{or(isUnknown(vBool(1)), not(vBool(1)))}} to
{{isNotTrue(vBool(1))}}, however, that is another story. I guess there might be
different cases (e.g. involving more complicated cases) that would result in
nullability "widening".
Reporter: Vladimir Sitnikov
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