Support "IS TRUE/FALSE" syntax

[sarahyurick]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
A similar issue regarding "IS NOT TRUE/FALSE" syntax has already been opened (#2265), but it doesn't look like there is support for the "IS TRUE/FALSE" syntax either.

Describe the solution you'd like
Enum datafusion::logical_plan::Expr should include functionality for IsTrue and IsFalse.

Describe alternatives you've considered
None.

Additional context
None.

[alamb]

Looks like there is sqlparser support: https://docs.rs/sqlparser/latest/sqlparser/ast/enum.Expr.html#variant.IsTrue

Added in 0.18.0: https://github.com/sqlparser-rs/sqlparser-rs/blob/main/CHANGELOG.md#0180-2022-06-06

So this should be a fairly straightforward exercise -- note that IS and = are very subtlety differnt

Since we already have similar semantics for IS with the IS NOT DISTINCT FROM longer form syntax, I think this could potentially simply have the the SQL planner plan

IS <expr>

as

is not distinct from <expr> as 'IS <expr.

Here is what postgres does

psql (14.4)
Type "help" for help.

alamb=# select null = null;
 ?column? 
----------
 
(1 row)

alamb=# select null IS null;
 ?column? 
----------
 t
(1 row)

alamb=# select null IS DISTINCT FROM null;
 ?column? 
----------
 f
(1 row)

alamb=# select null IS NOT DISTINCT FROM null;
 ?column? 
----------
 t
(1 row)

[sarahyurick]

Thanks for the response! I'll start working on this.

[alamb]

@sarahyurick here is an example of doing something similar for array:

https://github.com/apache/arrow-datafusion/pull/3122/files#diff-c01a34949db6258aa1593f011ecf90f62cbde406acd5cdbf8b9b60b970ace1cfR2372-R2374

        let fun = BuiltinScalarFunction::MakeArray;
        // follow postgres convention and name result "array"
        Ok(Expr::ScalarFunction { fun, args }.alias("array"))

So in other words you can probably do something like (untested):

Ok(binary_expr(left, Operator::IsNotDistinctFrom, right).alias("..."))

https://github.com/apache/arrow-datafusion/blob/ee55d89cbf20f4a6d17fe399c72d60dca6d67912/datafusion/expr/src/operator.rs#L59