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I have created one varray of varchar2 of length 30 and size 5.. so according to that the total size of the column should be 30 * 5=150 but it shows 172.

It should be 150. Is Varray allocating some extra space?

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Varray stores the total length of the varray and length of each element, so yes, it needs some extra bytes.

I am not aware of any public documentation on this topic, and I do not know the complete internal representation, so below is a simple example.

create or replace type jobs_done as varray(5) of varchar2(30);
/
create table emp_job_his (empno number(6), jobhistory jobs_done);
insert into emp_job_his values (1, jobs_done(
  'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA', 
  'BBBBBBBBBBBBBBBBBBBBBBBBBBBBBB', 
  'CCCCCCCCCCCCCCCCCCCCCCCCCCCCCC', 
  'DDDDDDDDDDDDDDDDDDDDDDDDDDDDDD', 
  'EEEEEEEEEEEEEEEEEEEEEEEEEEEEEE')
);
commit;
alter system checkpoint;

select
  dbms_rowid.rowid_relative_fno(rowid) as fno,
  dbms_rowid.rowid_block_number(rowid) as block_number
from emp_job_his;

       FNO BLOCK_NUMBER
---------- ------------
         4          134

alter system dump datafile 4 block 134;

The relevant part from the dump:

col  1: [162]
 88 01 a2 01 01 00 05 1e 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41
 41 41 41 41 41 41 41 41 41 41 41 41 41 1e 42 42 42 42 42 42 42 42 42 42 42
 42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 42 1e 43 43 43 43 43
 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43 43
 1e 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44
 44 44 44 44 44 44 1e 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45
 45 45 45 45 45 45 45 45 45 45 45 45

The 3rd byte, a2, is the length of the column in bytes = 162 (col 1: [162]).

The 7th byte, 05, is the number of elements in the array = 5.

Starting after that, each element is stored as length + data. Length is 1e = 30, and data is 30 times the letter A = 0x41, and so on.

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