Personal anecdotal evidence - with EXTERNAL (externally managed) tables pointing to text files we saw no performance differences whether string/varchar/char. With ORC files we always used String as the datatype, so can't speak to any potential performance differences (except for vectorization - see below).
I would recommend string if at all possible - You are correct that it is very handy to not be limited by a length specifier. Even if the data coming in is only Varchar(30) in length, your ELT/ETL processing will not fail if you send in 31 characters while using a string datatype.
Also note that in Hive versions older than 0.14 (AKA on HDP 2.1 and older) varchar/char will not use Vectorization with your ORC files, Whereas String can use vectorization. That can be a big difference in query performance. Use string if possible. 😉
** Edited to add more info about varchar vs. string **
Here is an example of what happens when you push too much data into a varchar(20) field.
create table if not exists test_tbl (id int, stringval string, varchar20_val varchar(20)) ;
# insert 20 characters of text into both fields
insert into test_tbl values (1, '01234567890123456789', '01234567890123456789');
# insert 25 characters of text into string field, 20 chars into varchar(20) field
insert into test_tbl values (2, '0123456789012345678901234', '01234567890123456789');
# insert 25 characters of text into both fields
insert into test_tbl values (3, '0123456789012345678901234', '0123456789012345678901234');
# Note - row 3 varchar field has the last 5 characters silently truncated. This is bad.
select id,
length(stringval) as `length_string`, stringval,
length(varchar20_val) as `length_varchar`, varchar20_val
from test_tbl; The output is as follows:
id length_string stringval length_varchar varchar20_val
1 20 01234567890123456789 20 01234567890123456789
2 25 0123456789012345678901234 20 01234567890123456789
3 25 0123456789012345678901234 20 01234567890123456789
Time taken: 1.584 seconds, Fetched: 3 row(s)
In row 3 you have lost the last 5 characters of data without warning. IMHO this is the most compelling reason to use String instead of Varchar.
