When to Rebuild an Index?

When to Rebuild an Index?

It is important to periodically examine your indexes to determine if they have become skewed and might need to be rebuilt.

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. Here is a sample procedure on how to identify the such 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 emp_empno_pk compute statistics;
Index analyzed.

2. Run the query below to find out how skewed each index is. This query checks on all indexes that are on emp table.


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 user_indexes;

select * from dba_indexes where blevel > 4;



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.



SQL> analyze index emp_empno_pk validate structure;
Index analyzed.

5. Run the following query to find out PCT_DELETED ratio.
SQL> select DEL_LF_ROWS*100/decode(LF_ROWS, 0, 1, LF_ROWS) PCT_DELETED,
2 (LF_ROWS-DISTINCT_KEYS)*100/ decode(LF_ROWS,0,1,LF_ROWS) DISTINCTIVENESS
3 from index_stats
4 where NAME='EMP_EMPNO_PK';



The PCT_DELETED column shows the percent of leaf entries (i.e. index entries) that 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.




In general, indexes and tables should be rebuilt when they become too fragmented.
In practice, you probably will not have need to do it with Oracle 10g. Fragmentation occurs on tables and indexes with lots of changes to structure (adding/removing columns) and lots of data changes (insert, update, delete).

From v10, Oracle have number of automated processes that take care about database performance. One of them is "Segment advisor" that runs automatically.


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