support IS [NOT] UNKNOWN

[neverchanje]

Syntax

expression IS UNKNOWN

Description

The IS UNKNOWN predicate in Postgres is used to test if an expression
returns an unknown value, which is typically represented by a NULL.
This predicate can be used with various expressions, such as boolean
expressions, to check if the result is unknown due to the presence of
NULL values.

Parameters

• expression: The expression to be tested for an unknown value.

Returns

The predicate returns true if the expression result is unknown
(NULL), and false otherwise.

Examples

SELECT (NULL AND true) IS UNKNOWN; -- Returns true
SELECT (NULL AND false) IS UNKNOWN; -- Returns true
SELECT (true AND false) IS UNKNOWN; -- Returns false

In these examples, the IS UNKNOWN predicate tests if the result of
boolean expressions is unknown due to the presence of NULL values.

See Also

• IS NULL: Tests if an expression returns a NULL value.

• IS NOT UNKNOWN: Tests if an expression does not return an unknown
  value.

• COALESCE: Returns the first non-null argument or null if all
  arguments are null.

Compatibility

The IS UNKNOWN predicate is compatible with PostgreSQL and is part of
the SQL standard.

[xiangjinwu]

Notice that IS UNKNOWN and IS NOT UNKNOWN are effectively the same as IS NULL and IS NOT NULL, respectively, except that the input expression must be of Boolean type.

Given we already support IS [NOT] NULL, we just need to check argument type and then bind to the same expr type... and add the missing parser support.

[neverchanje]

@xiangjinwu Could you assign someone for this issue?