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Index Organized Tables – Overflow Segment (Shadow Man) January 13, 2012

Posted by Richard Foote in Block Dumps, Index Internals, Index Organized Tables, IOT, Oracle Indexes, Overflow Segment.
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In my previous introductory IOT post, I illustrated how an Index Organized Table (IOT) might be worth consideration if most or all columns in a table were to be included within an index.

I’m going to use a slightly different demo this time, replacing one of the columns with a much larger DESCRIPTION column, one which is rarely accessed by the application:

SQL> CREATE TABLE album_sales_details_iot(album_id NUMBER, country_id NUMBER, total_sales NUMBER, description VARCHAR2(1000), CONSTRAINT album_sales_det_pk PRIMARY KEY(album_id, country_id)) ORGANIZATION INDEX;

Table created.

SQL> BEGIN
  2    FOR i in 1..5000 LOOP
  3      FOR c in 1..100 LOOP
  4         INSERT INTO album_sales_details_iot VALUES(i, c, ceil(dbms_random.value(1,5000000)), 'A really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really rlly really really really really really long description');
  5       END LOOP;
  6    END LOOP;
  7    COMMIT;
  8  END;
  9  /

PL/SQL procedure successfully completed.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats(ownname=>'BOWIE', tabname=> 'ALBUM_SALES_DETAILS_IOT', cascade=> true, estimate_percent=> null, method_opt=> 'FOR ALL COLUMNS SIZE 1');

PL/SQL procedure successfully completed.

Sorry for the unimaginative manner of loading the description field but you get the point :)

OK, let’s have a look at the size of the IOT:

SQL> ANALYZE INDEX album_sales_det_pk VALIDATE STRUCTURE;

Index analyzed.

SQL> SELECT blocks, height, br_blks, lf_blks FROM index_stats;

    BLOCKS     HEIGHT    BR_BLKS    LF_BLKS
---------- ---------- ---------- ----------
     71680          3        116      71429

As expected, the IOT is quite large as it has to accommodate the very large Description field within the IOT index structure. At 71,429 leaf blocks for the 500,000 rows in the table, that’s just 7 rows on average per leaf block.

The application doesn’t generally access the Description column with the following query typical (Note: to make fetching data as efficient as possible, I’ve set the arraysize to 100):

SQL> set arraysize 100
SQL> SELECT album_id, country_id, total_sales FROM album_sales_details_iot WHERE album_id = 42;

100 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 521866300

---------------------------------------------------------------------------------------
| Id  | Operation        | Name               | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |                    |   100 |  1300 |    17   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| ALBUM_SALES_DET_PK |   100 |  1300 |    17   (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("ALBUM_ID"=42)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         19  consistent gets
          0  physical reads
          0  redo size
       2387  bytes sent via SQL*Net to client
        524  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        100  rows processed

The query requires 19 consistent gets to retrieve the 100 rows because even though the data is extremely well clustered, there are very few rows per leaf block.

If we look at a partial block dump of one of these IOT leaf blocks:

Leaf block dump
===============
header address 548373084=0x20af825c
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x90: opcode=0: iot flags=I– is converted=Y
kdxconco 2
kdxcosdc 1
kdxconro 7
kdxcofbo 50=0x32
kdxcofeo 1011=0x3f3
kdxcoavs 961
kdxlespl 0
kdxlende 0
kdxlenxt 20978307=0x1401a83
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[1011] flag: K—–, lock: 0, len=1004
col 0; len 2; (2):  c1 02
col 1; len 2; (2):  c1 02
tl: 996 fb: –H-FL– lb: 0x0  cc: 2
col  0: [ 5]  c4 04 05 3b 03
col  1: [984]
 41 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 6c 6f 6e 67 20 64 65
 73 63 72 69 70 74 69 6f 6e

We can see the leaf block only has 7 rows, with the vast majority of space taken up by the very large Description column.

Considering the Description column is so large and/or that it’s rarely accessed, wouldn’t it be nice if we didn’t have to store this column directly within the IOT index structure itself.

Enter the IOT Overflow segment. The IOT Overflow segment enables us to store in another physical location those columns that we don’t necessarily want to store directly within the IOT index structure. So those columns that might be particularly large (or just the occurrences of those columns when the specific values might be too large to store within the IOT index structure) or those columns that are rarely accessed can be stored elsewhere. Effectively, we’re back to having a separate “table” like structure, but the Overflow segment will only hold those columns that we don’t necessarily want to store within the index structure. Unlike a normal Heap table, in which all columns are stored within the table segment.

There are a number of different methods we could use (to be explored further in future posts), for now I’ll use the INCLUDING clause:

SQL> CREATE TABLE album_sales_details_iot2(album_id NUMBER, country_id NUMBER, total_sales NUMBER, description VARCHAR2(1000), CONSTRAINT album_sales_det_pk2 PRIMARY KEY(album_id, country_id)) ORGANIZATION INDEX INCLUDING total_sales OVERFLOW TABLESPACE bowie2;

Table created.

So in the above example, all columns up to and “including” the total_sales column will be included in the IOT index structure. All the following columns listed in the table definition (in this case the Description column) will be store in the Overflow segment, which in the above example will be created within the BOWIE2 tablespace.

If we now populate this table with the identical data as before:

SQL> BEGIN
  2    FOR i in 1..5000 LOOP
  3      FOR c in 1..100 LOOP
  4         INSERT INTO album_sales_details_iot2 VALUES(i, c, ceil(dbms_random.value(1,5000000)), 'A really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really rlly really really really really really long description');
  5       END LOOP;
  6    END LOOP;
  7    COMMIT;
  8  END;
  9  /

PL/SQL procedure successfully completed.

SQL> exec dbms_stats.gather_table_stats(ownname=>'BOWIE', tabname=> 'ALBUM_SALES_DETAILS_IOT2', cascade=> true, estimate_percent=> null, method_opt=> 'FOR ALL COLUMNS SIZE 1');

PL/SQL procedure successfully completed.

SQL> ANALYZE INDEX album_sales_det_pk2 VALIDATE STRUCTURE;

Index analyzed.

SQL> SELECT blocks, height, br_blks, lf_blks FROM index_stats;

    BLOCKS     HEIGHT    BR_BLKS    LF_BLKS
---------- ---------- ---------- ----------
      1664          3          4       1613

We notice the IOT index structure is now significantly smaller, down from 71,429 to just 1,613 leaf blocks. All the “clutter” has now been removed and is stored elsewhere.

If we now re-run our query:

SQL> SELECT album_id, country_id, total_sales FROM album_sales_details_iot2 WHERE album_id = 42;

100 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2379894191

----------------------------------------------------------------------------------------
| Id  | Operation        | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |                     |   100 |  1300 |    18   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| ALBUM_SALES_DET_PK2 |   100 |  1300 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("ALBUM_ID"=42)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          5  consistent gets
          0  physical reads
          0  redo size
       2390  bytes sent via SQL*Net to client
        524  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        100  rows processed

Things are now much more efficient, having reduced the required consistent gets down from 19 to just 5 consistent gets.

If we now look at a partial block dump of an IOT leaf block:

Leaf block dump
===============
header address 441197148=0x1a4c225c
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x90: opcode=0: iot flags=I– is converted=Y
kdxconco 2
kdxcosdc 1
kdxconro 322
kdxcofbo 680=0x2a8
kdxcofeo 703=0x2bf
kdxcoavs 23
kdxlespl 0
kdxlende 0
kdxlenxt 21049987=0x1413283
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[703] flag: K—–, lock: 0, len=23
col 0; len 2; (2):  c1 02
col 1; len 2; (2):  c1 02
tl: 15 fb: –H-F— lb: 0x0  cc: 1
nrid:  0x01800081.0
col  0: [ 5]  c4 02 5e 0d 25
row#1[726] flag: K—–, lock: 0, len=23
col 0; len 2; (2):  c1 02
col 1; len 2; (2):  c1 03
tl: 15 fb: –H-F— lb: 0x0  cc: 1
nrid:  0x01800081.1
col  0: [ 5]  c4 04 41 13 43

We can see the number of index entries in the leaf block has increased from 7 to 322, with the size of the index entry decreasing from 1004 to just 23 bytes. Instead of the Description column being stored within the leaf block, we now have a nrid entry consisting of a 6 byte relative block address and row directory number (0x01800081.0), which effectively points to the actual location of the remaining portion of the row within the Overflow segment. We only therefore have a table column count of 1 (cc:1).

To find out more about the corresponding Overflow segment, we first must determine the OBJECT_ID of the IOT:

SQL> SELECT object_id FROM user_objects WHERE object_name = 'ALBUM_SALES_DETAILS_IOT2';

 OBJECT_ID
----------
     74116

This OBJECT_ID is used to name the corresponding Overflow segment which we can determine from DBA_TABLES as it has a format of SYS_IOT_OVER_object_id:

SQL> SELECT table_name, iot_name, iot_type, blocks FROM user_tables WHERE table_name = 'SYS_IOT_OVER_74116';

TABLE_NAME         IOT_NAME                 IOT_TYPE      BLOCKS
------------------ ------------------------ ------------ -------
SYS_IOT_OVER_74116 ALBUM_SALES_DETAILS_IOT2 IOT_OVERFLOW   71430

We notice this Overflow segment (at 71,430 blocks) is where the majority of our storage has been allocated.

Although it’s listed as a table, the Overflow segment can’t be directly accessed or manipulated. Any attempt to do so will result in an error:

SQL> select * from SYS_IOT_OVER_74116;
select * from SYS_IOT_OVER_74116
              *
ERROR at line 1:
ORA-25191: cannot reference overflow table of an index-organized table

If we look at a partial block dump of the Overflow segment block referenced in the previous IOT block dump:

Block header dump:  0x01800081
 Object id on Block? Y
 seg/obj: 0x12185  csc: 0x00.17482cc  itc: 1  flg: –  typ: 1 – DATA
     fsl: 0  fnx: 0x0 ver: 0x01
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x000a.00b.0000a919  0x00c24a2e.03d2.2a  C—    0  scn 0x0000.01748279
bdba: 0x01800081
data_block_dump,data header at 0x1a4c2244
===============
tsiz: 0x1fb8
hsiz: 0x20
pbl: 0x1a4c2244
     76543210
flag=——–
ntab=1
nrow=7
frre=-1
fsbo=0x20
fseo=0x4a6
avsp=0x486
tosp=0x486
0xe:pti[0] nrow=7 offs=0
0x12:pri[0] offs=0x1bda
0x14:pri[1] offs=0x17fc
0x16:pri[2] offs=0x141e
0x18:pri[3] offs=0x1040
0x1a:pri[4] offs=0xc62
0x1c:pri[5] offs=0x884
0x1e:pri[6] offs=0x4a6
block_row_dump:
tab 0, row 0, @0x1bda
tl: 990 fb: —–L– lb: 0x0  cc: 1
col  0: [984]
 41 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20
 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c
 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72
 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c
 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65
 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79
 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61
 6c 6c 79 20 72 65 61 6c 6c 79 20 72 65 61 6c 6c 79 20 6c 6f 6e 67 20 64 65
 73 63 72 69 70 74 69 6f 6e

We notice the Overflow block contains 7 rows as we would expect, as this was all the IOT segment could previously manage when it had to store the large Description column values.

The table row directory contains 7 rows, with the first row (#0) having an offset at address 0x1bda, which is the actual location of the first row within the Overflow block.

Therefore, in order to find a specific Description column value of interest from the IOT, Oracle references the (say) nrid:  0x01800081.0 within the IOT index entry for the row. This in turns points to the relative block address (0x01800081) of the Overflow block containing the description and the corresponding row directory number (0), which in turn specifies the offset (say) 0x1bda to the actual location of the Description value within the Overflow block. Easy !!

If we Analyze the IOT table:

SQL> ANALYZE TABLE album_sales_details_iot2 COMPUTE STATISTICS;

Table analyzed.

SQL> SELECT table_name, num_rows, chain_cnt, blocks from user_tables WHERE table_name = 'ALBUM_SALES_DETAILS_IOT2';

TABLE_NAME                       NUM_ROWS  CHAIN_CNT     BLOCKS
------------------------------ ---------- ---------- ----------
ALBUM_SALES_DETAILS_IOT2           500000     500000

We notice all the rows are listed as “Chained Rows“. This is because all the rows have a corresponding Description value stored in the Overflow segment and so the rows are not stored within the one block. As the previous query illustrated, this is no bad thing if we don’t need to reference these additional columns stored in the Overflow segment. It makes the resultant IOT table more compact and efficient to access.

However, on those (hopefully) rarer occasions when we do need to access the columns in the Overflow segment, this will clearly require additional block accesses:

SQL> SELECT * FROM album_sales_details_iot2 WHERE album_id = 42;

100 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2379894191

----------------------------------------------------------------------------------------
| Id  | Operation        | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |                     |   100 | 99400 |    18   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| ALBUM_SALES_DET_PK2 |   100 | 99400 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("ALBUM_ID"=42)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         32  consistent gets
          0  physical reads
          0  redo size
       5541  bytes sent via SQL*Net to client
        590  bytes received via SQL*Net from client
          8  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        100  rows processed

The above query which returns the Description column results in the consistent gets increasing to 32 consistent gets, from the 5 consistent gets when the Description wasn’t accessed and from the 19 consistent gets from when the Description column was co-located within the IOT segment. But this is a price we might be willing to pay if this query isn’t frequently executed while the frequently executed queries which don’t access the Description column are more efficient.

The Overflow segment gives us in a manner “the best of both worlds”. The ability to store just those columns of interest within the IOT segment (although these must always include all the Primary Key columns) and those that are less often accessed or too large to be efficiently stored within the IOT can be stored elsewhere. Effectively, it’s an index and table relationship except the table doesn’t have to store again the columns that are already stored within the index.

It’s all good news so far for IOTs …

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Comments»

1. Brian Tkatch - January 14, 2012

Nice.

Richard Foote - January 18, 2012

:)

2. Scott Lynch - January 14, 2012

Very cool. And very similar to storing LOB data out of the row of a more standard table structure. I can certainly see myself using this when the right problem comes along.

Richard Foote - January 18, 2012

Hi Scott.

Indeed.

Excellent, glad to hear you’ll consider IOTs. Just note though I haven’t yet discussed some of their disadvantages ;)

3. Henish - January 18, 2012

Nice Post!!!

Just quick question when you define IOT with overflow segement in the block dump we see there is a nrid (relative block address) for remaining columns, my question is why Oracle does not store 1 byte for column length for nrid?

Thanks in Advance

Regard’s

Richard Foote - January 18, 2012

Hi Henish

Because the nrid is not a variable length column value but is fixed in length (a piece of overhead if you like), similar to say a rowid within a Unique Index or the various flags and lock bytes.

Henish - January 19, 2012

Thanks for clarification!!! :)

4. Index Organized Tables – Overflow Segment Part II (The Loneliest Guy) « Richard Foote’s Oracle Blog - January 18, 2012

[...] my previous post on Index Organized Tables (IOT), I introduced the concept of the IOT Overflow Segment, where we can store columns that we may not want to include within the actual IOT index structure. [...]

5. Index Organized Tables – PCTTHRESHOLD (The Wedding Song) « Richard Foote’s Oracle Blog - February 8, 2012

[...] my previous IOT examples, we had a very large column called Description which we didn’t really want to store within [...]

6. Marko Sutic (@MarkoSutic) - March 27, 2012

Great article Richard. Nicely explained benefits of Overflow segment through clear examples.

btw
You have small typo in insert statement where you’re populating “ALBUM_SALES_DETAILS_IOT2″ table.

Regards,
Marko

Richard Foote - April 5, 2012

Thanks Marko, fixed :)

7. IOTs | Oracle Scratchpad - February 10, 2014

[…] Overflow segments – pt. 1 […]

8. Pavan - July 6, 2014

Hey Richard,

Thanks for nice doc, how do we find the fragmentation in IOT overflow segment and how do we rectify the fragmentation in overflow segment.
Pavan.

9. Richard Foote - July 8, 2014

Hi Pavan

Usual rules apply with regard to determining approx. size of table/column based on average row/column sizes x by number of rows. The overflow can be defragmented if necessary by simply shrinking the table with the overflow clause (alter table bowie overflow shrink space).


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