How expensive is it to rename column name on a big table?

Question

Say we have table in an Oracle DB with millions of rows - how expensive is it rename it? My guess (and hope) is that it's just a metadata change, so the size of the table has no impact, but I'd like to verify (couldn't find any useful results).

Answer 1

Yes, it is just a metadata update.

SQL> create table t1 as 
     with g as (select * from dual connect by level <= 1000)
     select 'HELLO' as c1 from g,g,g where rownum <= 1e7;

Table created.

SQL> select bytes/1024/1024 from user_segments where segment_name = 'T1';

BYTES/1024/1024
---------------
            128

SQL> select
       s.value/1024/1024 
     from v$mystat s 
     natural join v$statname sn 
     where sn.name = 'redo size';

S.VALUE/1024/1024
-----------------
       131.848621

SQL> alter table t1 rename column c1 to c2;

Table altered.

SQL> select
       s.value/1024/1024 
     from v$mystat s 
     natural join v$statname sn 
     where sn.name = 'redo size';

S.VALUE/1024/1024
-----------------
       131.851715

Above statement produced just a tiny amount of redo.

But there are other aspects: the required locks and invalidation of related PL/SQL objects.