The result of the concatenation operation is a new 1-D ARRAY value of the same type as the data type of the two arguments, where all of the elements present in expr1 are followed by all the elements that are present in expr2. Elements that are present include NULL elements, but do not include elements that are in an uninitialized state.
Since the result value and the argument values are of the same 1-D ARRAY type, they have the same maximum size n. Therefore, if the number of elements that results from the concatenation of expr1 and expr2 is greater than the maximum size n of the defined 1-D ARRAY type, the operation will abort with an error.
If either argument is NULL, the result of the operation is NULL.