If you’re getting an error that reads “could not determine polymorphic type because input has type unknown” when using the generate_subscripts() function in PostgreSQL, it’s probably because your first argument is of the wrong type.

The generate_subscripts() function requires an array as its first argument. Passing a non-array value will result in an error. The actual error can vary, depending on the argument you pass, but regardless, the error is usually due to a non-array argument being passed.

To fix this error, be sure to pass an array as the argument when calling generate_subscripts().

Example of Error

Here’s an example of code that produces the error:

SELECT generate_subscripts(NULL, 1);

Result:

ERROR:  could not determine polymorphic type because input has type unknown

I got the error because my argument wasn’t an array. In this case it was NULL. The “polymorphic type” part refers to the fact that the generate_subscripts() function is a polymorphic function. That is, it accepts a polymorphic type as an argument. In this case the first argument can be any array type.

Polymorphic types enable a single function definition to operate on many different data types, with the specific data type(s) being determined by the data types actually passed to it in a particular call.

Solution

We can fix the problem by passing an array to the function:

SELECT generate_subscripts('{"Flower","Leaf","Trunk"}'::text[], 1);

Result:

 generate_subscripts 
---------------------
                   1
                   2
                   3

This time the function worked without error. That’s because we passed an array this time.