kgyrtkirk commented on issue #1044: [CALCITE-2838] Simplification: Remove redundant IS TRUE/IS NOT FALSE … URL: https://github.com/apache/calcite/pull/1044#issuecomment-463818974 >> By the way, why don't you consider IS_TRUE+UnknownAs.TRUE and other Unknown.As.TRUE cases? > we are switching from `UnknownAs.TRUE` to `UnknownAs.FALSE` ; so it can't be removed. An alternate way to think about `UnknownAs` modes is by `IS X` operators. x | x IS TRUE | [UAF] x | x IS NOT FALSE | [UAT] x -|-|-|-|- -1|-1|-1|-1|-1 0|-1|-1|1|1 1|1|1|1|1 Let's consider `[UAT] x IS TRUE`; to move to proper 3-valued logic, I would instead write: `(x IS TRUE) IS NOT FALSE` x | x IS TRUE | (x IS TRUE) IS NOT FALSE | F(X) IS NOT FALSE -|-|-|- -1|-1|-1|-1 0|-1|-1|-1 1|1|1|1 Question is: is there an `F` which could satisfy the truth table? What could be used to construct `F`? * `IS X` operators are most probably out of scope; since we are trying to replace one of them * `NOT` is good; but has no effect on "UNKNOWN" * `CASE`, `AND`, `OR`, `COALESCE` .... adds much more complexity I think it's not possible to give a an `F` in this case; which might be better. So I think the existing 4 is all what we can do right now; I'm considering adding something like the following as a comment: `UnknownAs.FALSE` corresponds to `x IS TRUE` `UnknownAs.TRUE` to `x IS NOT FALSE` Note that both `UnknownAs.TRUE` and `UnknownAs.FALSE` only changes the meaning of `Unknown` * if we are already in `UnknownAs.FALSE` mode; `x IS TRUE` can be simiplified to `x` without any side effects * similarily in `UnknownAs.TRUE` mode ; `x IS NOT FALSE` can be simplified to `x` * `x IS FALSE` could be rewritten to `(NOT x) IS TRUE` and from there the 1. rule applies * `x IS NOT TRUE` can be rewritten to `(NOT x) IS NOT FALSE` and from there the 2. rule applies
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