How to calcalate the maximum size occupied by a table/row

Question

Hi,
I am using the below query to find the maximum size occupied by a table per a row of data.
SQL> desc t2
Name Null? Type
NO NUMBER(1)
CH VARCHAR2(10)
SQL> select sum(data_length) from user_tab_columns where table_name='T2';
SUM(DATA_LENGTH)
32
Is this correct? Does oracle takes 22 bytes of space to store a column of Number(1) per a row as it shows below !?
1* select data_length from user_tab_columns where table_name='T2'
SQL> /
DATA_LENGTH
22
10
Please give your comments/suggestions
Thanks and Regards
Srikanth

Answer 1

If you are trying to do this for an existing table, you can get the actual number of bytes stored per column by:
SELECT VSIZE(column_name)
FROM tableSo, to get the largest row you could:
SELECT MAX(VSIZE(col1) + VSIZE(col2) ... + VSIZE(coln))
FROM tableIf you want to get the theoretical largest possible row size, then you can use something like:
SELECT SUM(DECODE(data_type,'NUMBER', ROUND((data_precision/2)+.1,0) +1,
                            'DATE',   7,
                            'LONG',   2147483648,
                            'BLOB',   4000,
                            'CLOB',   4000,
                            data_length)) max_length
FROM user_tab_columns
WHERE table_name = 'MY_TABLE'This works because:
For a number, Oracle stores two digits in each byte, and uses one byte for the mantissa. Note, if you are expecting negative numbers in your numeric columns then add one additional byte).
For a date, Oracle always requires 7 bytes.
A long is at most 2 GB in size.
Both Blobs and Clobs will be stored out-of-line when they are larger than about 4,000 bytes.
For other data types you specify bytes of storage when you define the column, and that is stored in data_length.
I'm not sure at this point what sort of sizes you would get for object type columns or nested tables.
HTH
John