why rebuild index

[steeve123]

Hi, could you tell me why rebuild index?
What are the common reasons?

[molonede]

Here is a note from MetaLink:
Doc ID: Note:77574.1
Subject: Guidelines on When to Rebuild an Index
Type: BULLETIN
Status: PUBLISHED
Content Type: TEXT/PLAIN
Creation Date: 23-NOV-1999
Last Revision Date: 18-JUL-2001


PURPOSE
Explains how to determine if an index is a good candidate for a rebuild.

SCOPE & APPLICATION
For DBAs requiring to know when to rebuild an index.

RELATED DOCUMENTS
[NOTE:33343.1] How to Find Out How Much Space an Index is Using


Guidelines on When to Rebuild an Index:
=======================================

You have to periodically check your indexes to see if they become skewed and,
therefore, good candidates for rebuild.

A skewed index has many records clumped close together on the index tree due to
their similar indexed values. When an index is skewed, parts of an index are
accessed more frequently than others. As a result, disk contention may occur,
creating a bottleneck in performance. It is important to periodically examine
your indexes to determine if they have become skewed and might need to be
rebuilt.

Here is a sample procedure on how to identify the skewed indexes:

1. Gather statistics on your indexes. For large indexes (over one hundred
thousand records in the underlying table), use ESTIMATE instead of COMPUTE
STATISTICS.

For example:

SQL> analyze index A1_PK compute statistics;

Index analyzed.

2. Run the script given below - index_check.sql - to find out how skewed
each index is.

This query checks on all indexes that belong to user SCOTT:

SQL> select index_name, blevel,
decode(blevel,0,'OK BLEVEL',1,'OK BLEVEL',
2,'OK BLEVEL',3,'OK BLEVEL',4,'OK BLEVEL','BLEVEL HIGH') OK
from dba_indexes
where owner='SCOTT';

INDEX_NAME BLEVEL OK
---------------------------------------- ------ ----
A1_PK BLEVEL HIGH
A1_UK BLEVEL HIGH
BUDVERPORT_BUDVERPORT2_UK 1 OK BLEVEL
BUDVERPORT_BV_FK_I 2 OK BLEVEL
BUDVERPORT_CHAR_CL_FK_I 1 OK BLEVEL
BUDVERPORT_DIRCTE_FK_I 3 OK BLEVEL
BUDVERPORT_FUND_TYPE_FK_I 1 OK BLEVEL
BUDVERPORT_OMB_SUBFCT_FK_I 1 OK BLEVEL
BUDVERPORT_ORG_FK_I 0 OK BLEVEL
BUDVERPORT_PL_TITLE_FK_I 1 OK BLEVEL
BUDVERPORT_RDC_FK_I 1 OK BLEVEL
S_INVENTORY_PRODID_WARID_PK BLEVEL HIGH
S_ITEM_ORDID_ITEMID_PK BLEVEL HIGH
S_ITEM_ORDID_PRODID_UK BLEVEL HIGH
S_LONGTEXT_ID_PK BLEVEL HIGH
S_ORD_ID_PK BLEVEL HIGH
S_PRODUCT_ID_PK BLEVEL HIGH
S_PRODUCT_NAME_UK BLEVEL HIGH
S_REGION_ID_PK BLEVEL HIGH
S_REGION_NAME_UK BLEVEL HIGH
S_TITLE_TITLE_PK BLEVEL HIGH
S_WAREHOUSE_ID_PK BLEVEL HIGH


3. The BLEVEL (or branch level) is part of the B-tree index format and relates
to the number of times Oracle has to narrow its search on the index while
searching for a particular record. In some cases, a separate disk hit is
requested for each BLEVEL. If the BLEVEL were to be more than 4, it is
recommended to rebuild the index.

Note: If you do not analyze the index, the index_check.sql script will
show "BLEVEL HIGH" for such an index.

4. Gather more index statistics using the VALIDATE STRUCTURE option of the
ANALYZE command to populate the INDEX_STATS virtual table. This table does
not contain an OWNER column and assumes you are looking for statistics for
indexes created by your active session only.

SQL> analyze index SCOTT.ORG_PK validate structure;

Index analyzed.

SQL> select DEL_LF_ROWS*100/decode(LF_ROWS, 0, 1, LF_ROWS) PCT_DELETED,
(LF_ROWS-DISTINCT_KEYS)*100/ decode(LF_ROWS,0,1,LF_ROWS) DISTINCTIVENESS
from index_stats
where NAME='&index_name';

Enter value for index_name: ORG_PK

PCT_DELETED DISTINCTIVENESS
----------- ---------------
0 0

The PCT_DELETED column shows what percent of leaf entries (index entries)
have been deleted and remain unfilled. The more deleted entries exist on an
index, the more unbalanced the index becomes. If the PCT_DELETED is 20% or
higher, the index is candidate for rebuilding. If you can afford to rebuild
indexes more frequently, then do so if the value is higher than 10%. Leaving
indexes with high PCT_DELETED without rebuild might cause excessive redo
allocation on some systems.
The DISTINCTIVENESS column shows how often a value for the column(s) of the
index is repeated on average. For example, if a table has 10000 records and
9000 distinct SSN values, the formula would result in
(10000-9000) x 100 / 10000 = 10. This shows a good distribution of values.
If, however, the table has 10000 records and only 2 distinct SSN values, the
formula would result in (10000-2) x 100 /10000 = 99.98. This shows that
there are very few distinct values as a percentage of total records in the
column. Such columns are not candidates for a rebuild but good candidates
for bitmapped indexes.


ADDITIONAL SEARCH WORDS:
========================

fragmented; rebuilt; fragmentation;