Simplify SimplifyRedundantCase rule

[Praveen2112]

Description

Instead of handling the transformed expression as Optional<Expression> and replace them with FALSE if it doesn't exist - we could simplify them.

Additional context and related issues

#23796

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## Section
* Fix some things. ({issue}`issuenumber`)

[martint]

Returning Optional<Expression> was intentional handle the base case. Otherwise you end up with a spurious OR expression for any CASE that has a default clause that returns false, as you've been able to observe in the tests you had to modify.

E.g., for

CASE a 
    WHEN x THEN true
    ELSE false
END

Turns into:

(x ≡ true) OR false

instead of:

x ≡ true

[Praveen2112]

Yes - But we do have a different rule EvaluateLogical which would remove this redundant
(x ≡ true) OR false => (x ≡ true) right ? Won't that rule would help us here ?

For my understanding, when we write a new rule should it focus on one optimization which it tries to achieve or try to come up with a best plan ?

[martint]

That's correct. But in this case, the OR with false would be spurious, and only an artifact of how the code is structured. There's no reason for the rule to produce that structure.