Attached is my work in progress to implement the changes to the CAST() function as proposed by Vik Fearing.
CAST(expr AS typename NULL ON ERROR)
will use error-safe functions to do the cast of expr, and will return NULL if the cast fails.
CAST(expr AS typename DEFAULT expr2 ON ERROR) will use error-safe functions to do the cast of expr, and will return expr2 if the cast fails.
Is there any difference between NULL and DEFAULT NULL?
What I think you're asking is: is there a difference between these two statements:
SELECT CAST(my_string AS integer NULL ON ERROR) FROM my_table;
SELECT CAST(my_string AS integer DEFAULT NULL ON ERROR) FROM my_table;
And as I understand it, the answer would be no, there is no practical difference. The first case is just a convenient shorthand, whereas the second case tees you up for a potentially complex expression. Before you ask, I believe the ON ERROR syntax could be made optional. As I implemented it, both cases create a default expression which then typecast to integer, and in both cases that expression would be a const-null, so the optimizer steps would very quickly collapse those steps into a plain old constant.
