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How to find indexes which are candidate for rebuild?

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I know it’s a very arguable question, but let me put my words and then you can comment.

I follow different approaches to find out indexes for rebuild

– Find out indexes having height(blevel+1) > 4 i.e. Indexes having BLEVEL > 3
How:
SQL> select owner,index_name,table_name,blevel from dba_indexes where BLEVEL>3

– Analyze indexes and find out ratio of (DEL_LF_ROWS/LF_ROWS*100) is > 20
How:
First “Analyze the index with validate structure option” and then,

SQL> SELECT name,height,lf_rows,del_lf_rows,(del_lf_rows/lf_rows)*100 as ratio FROM INDEX_STATS;

But (a big but), the reason to rebuild should be because of poor performance of your queries using indexes. You should/must not rebuild indexes if you find both the above reason true for index if it is not coupled with poor SQL performance.

See this example:

SQL> analyze index TEST_INDX validate structure; — First analyze the suspect index

Index analyzed.

SQL> SELECT name,height,lf_rows,lf_blks,del_lf_rows FROM INDEX_STATS;

NAME HEIGHT LF_ROWS LF_BLKS DEL_LF_ROWS
———— ———- ———- ———– ————-
TEST_INDX 8 938752 29575 73342

You can see height of the index is 8 and also high number of DEL_LF_ROWS

SQL> set autotrace on
SQL> set timing on
SQL>
SQL> select count(*) from TEST_TABLE where TEST_COL like ‘http://www.hots%’;

COUNT(*)
———-
39700
Elapsed: 00:00:27.25

Execution Plan
———————————————————-
Plan hash value: 870163320

Id Operation Name Rows Bytes Cost (%CPU) Time

0 SELECT STATEMENT 1 117 10 (0) 00:00:01

1 SORT AGGREGATE 1 117

*2 INDEX RANGE SCAN TEST_INDX 115 13455 10 (0) 00:00:01

Statistics
———————————————————-
1 recursive calls
0 db block gets
764 consistent gets
757 physical reads
0 redo size
516 bytes sent via SQL*Net to client
468 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

Now you rebuild the indexes

SQL> alter index TEST_INDX rebuild;

Index altered.

SQL> select count(*) from TEST_TABLE where TEST_COL like ‘http://www.hots%’;

COUNT(*)
———-
39700

Elapsed: 00:00:06.18

Execution Plan
———————————————————-
Plan hash value: 870163320 – See here although it is using the same plan but still it is faster

Id Operation Name Rows Bytes Cost (%CPU) Time

———————————————————————————-

0 SELECT STATEMENT 1 117 6 (0) 00:00:01

1 SORT AGGREGATE 1 117

* 2 INDEX RANGE SCAN TEST_INDX 115 13455 6 (0) 00:00:01

Statistics
———————————————————-
15 recursive calls
0 db block gets
592 consistent gets
588 physical reads
0 redo size
516 bytes sent via SQL*Net to client
468 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)

SQL> SELECT name,height,lf_rows,lf_blks,del_lf_rows,distinct_keys,used_space FROM INDEX_STATS;

NAME HEIGHT LF_ROWS LF_BLKS DEL_LF_ROWS
—————————— ———- ———- ———- ———–
TEST_INDX 4 865410 15434 0

This clearly indicates rebuild helped my query performance. The height of index is reduced to 4 and DEL_LF_ROWS is 0

Now coming to second of part of Harvinder’s comment.

Possible ways of Rebuilding.

– Online/Offline.

ONLINE Rebuild (8i onwards)

SQL> Alter index rebuild online;

This allows parallel DML to go on while Index is getting rebuild. Remember, online index requires more space, as it creates a new index before dropping the old one.

Index Rebuild is primarily a 3 step process

Prepare phase: Oracle locks the table for a fraction of second (actually not felt) to build index structure and populate data-dictionary.

Build phase: Most of the work is done in this phase. Oracle engine populates the index using the table and allows parallel DML’s, parallel DML’s uses a temporary journal table (b tree index like structure) to host the entries while the new index is getting populated.

Merge phase: Now the final stage, Oracle merges the new index with the journal table and drops the old index. Even during the merge phase, any changes to the table are recorded in the journal table and they get merged towards end of this phase.

9i onwards, online index rebuild feature includes Reverse Key, Function Based and Key Compressed indexes.

Offline Index rebuild.

SQL> Alter index rebuild;

This is conventional rebuild which was used(still available) before 8i. In this rebuild process, oracle drops the old index and creates a new one. In this process, no extra space is required, but parallel dml’s are not supported.

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