Avg_row_len always 1 byte extra

Question

Hi
Anyone can help to explain the different in vsize and avg_row_len in the following example? Thanks
create table t (n1 varchar2(10));
INSERT INTO t values('aaaaaaaaaa')
SELECT vsize(n1) FROM t;  -- 10 bytes
SELECT avg_row_len FROM user_tables
WHERE table_name = 'T';  -- 11 bytes
Thanks. I found the answer in Note 1301462, 231214.1
Edited by: Kok Aik on Mar 16, 2009 6:02 PM

Answer 1

The OP's comparison is invalid to begin with. The vsize function returns the interanl size of the column value while the avg_row_len is supposed to be the size of the row. Even for a one column row these are not the same thing as every row also includes a three byte row header. The dbms_stats module currently fails to include the length of the row header in the average row length. This can result in a significant error for high row count tables when trying to estimate space usage requirements. The analyze command does count the row header.
mtu1 > !vi analvsstat.txt
"analvsstat.txt" 45 lines, 935 characters
mtu1 > desc marktest
Name Null? Type
C1 NUMBER
C2 VARCHAR2(10)
mtu1 > select * from marktest;
no rows selected
mtu1 > insert into marktest values (1,'xxxxxxxxxx');
1 row created.
mtu1 > analyze table marktest compute statistics;
Table analyzed.
mtu1 > c /lena/le_na/
1* select avg_row_len from dba_tables where table_name = 'MARKTEST'
mtu1 > /
AVG_ROW_LEN
17
mtu1 > analyze table marktest delete statistics;
Table analyzed.
mtu1 > @gts
Enter Table Owner mpowel01
Enter Table Name marktest
Enter Est % 100
Enter Degree % 1
PL/SQL procedure successfully completed.
mtu1 > select avg_row_len from dba_tables where table_name = 'MARKTEST';
AVG_ROW_LEN
14
:1
Mtu1 > desc marktest
Name Null? Type
C1 NUMBER
C2 VARCHAR2(10)
mtu1 > select * from marktest;
no rows selected
mtu1 > insert into marktest values (1,'xxxxxxxxxx');
1 row created.
mtu1 > analyze table marktest compute statistics;
Table analyzed.
mtu1 > c /lena/le_na/
1* select avg_row_len from dba_tables where table_name = 'MARKTEST'
mtu1 > /
AVG_ROW_LEN
8 substitutions were made
Table analyzed.
> @gts
Enter Table Owner mpowel01
Enter Table Name marktest
Enter Est % 100
Enter Degree % 1
PL/SQL procedure successfully completed.
> select avg_row_len from dba_tables where table_name = 'MARKTEST';
AVG_ROW_LEN
14
HTH -- Mark D Powell --