Re: [I] [DISCUSSION] Potentially consolidate Expr::IsNotUnknown, `IsKnown`, etc [datafusion]

["jonahgao (via GitHub)" <gi...@apache.org>]

jonahgao commented on issue #11282:
URL: https://github.com/apache/datafusion/issues/11282#issuecomment-2210921179

   When the parameter is a boolean, `IS NULL` and `IS UNKNOWN` are equivalent. According to the SQL standard
   > The Boolean data type comprises the distinct truth values True and False. Unless prohibited by a NOT NULL constraint, the Boolean data type also supports the truth value Unknown as the null value. This specification does not make a distinction between the null value of the Boolean data type and the truth value Unknown; they may be used interchangeably to mean exactly the same thing.
   
   `UNKNOWN` is the third truth value. So the **difference** is that `UNKNOWN` can only be applied to boolean types. DataFusion and PostgreSQL adhere to this, but DuckDB does not.
   ```sh
   postgres=# select 1 is unknown;
   ERROR:  argument of IS UNKNOWN must be type boolean, not type integer
   LINE 1: select 1 is unknown;
   ```
   ```sh
   DataFusion CLI v39.0.0
   > select 1 is unknown;
   type_coercion
   caused by
   Error during planning: Cannot infer common argument type for comparison operation Int64 IS DISTINCT FROM Boolean
   ```
   In DuckDB:
   ```sh
   D select 1 is unknown;
   ┌─────────────┐
   │ (1 IS NULL) │
   │   boolean   │
   ├─────────────┤
   │ false       │
   └─────────────┘
   ```
   


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