[HACKERS] Does Type Have = Operator?

[David E. Wheeler]

Hackers,

pgTAP has a function that compares two values of a given type, which it uses 
for comparing column defaults. It looks like this:

    CREATE OR REPLACE FUNCTION _def_is( TEXT, TEXT, anyelement, TEXT )
    RETURNS TEXT AS $$
    DECLARE
        thing text;
    BEGIN
        IF $1 ~ '^[^'']+[(]' THEN
            -- It's a functional default.
            RETURN is( $1, $3, $4 );
        END IF;

        EXECUTE 'SELECT is('
                 || COALESCE($1, 'NULL' || '::' || $2) || '::' || $2 || ', '
                 || COALESCE(quote_literal($3), 'NULL') || '::' || $2 || ', '
                 || COALESCE(quote_literal($4), 'NULL')
        || ')' INTO thing;
        RETURN thing;
    END;
    $$ LANGUAGE plpgsql;

The is() function does an IS DISTINCT FROM to compare the two values passed to 
it. This has been working pretty well for years, but one place it doesn’t work 
is with JSON values. I get:

    LINE 1: SELECT NOT $1 IS DISTINCT FROM $2
                          ^
    HINT:  No operator matches the given name and argument type(s). You might 
need to add explicit type casts.
    QUERY:  SELECT NOT $1 IS DISTINCT FROM $2

This makes sense, of course, and I could fix it by comparing text values 
instead of json values when the values are JSON. But of course the lack of a = 
operator is not limited to JSON. So I’m wondering if there’s an interface at 
the SQL level to tell me whether a type has an = operator? That way I could 
always use text values in those situations.

Thanks,

David

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