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Posts Tagged ‘Rebuild’

Error Description
Whenever I try to rebuild index online then it fails with message ORA-00604: error occurred at recursive SQL level 1 along with ORA-01450: maximum key length (3215) exceeded. Below is the scenario.
SQL> create table tab1(a varchar2(3000),b varchar2(2000));
Table created.

SQL> create index tab1_I on tab1(a,b);
Index created.

SQL> alter index tab1_I rebuild online;
alter index tab1_I rebuild online
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-01450: maximum key length (3215) exceeded

Let’s now create one table with column length 3000+199=3199 bytes and see what happens.
SQL> create table tab3(a varchar2(3000),b varchar2(199));
Table created.

SQL> create index tab3_I on tab3(a,b);
Index created.

Try to rebuild it online and it works.
SQL> alter index tab3_I rebuild online;
Index altered.

Now just add extra 1 bytes on column b. And whenever we try to rebuild it online it will fail.

SQL> alter table tab3 modify b varchar2(200);
Table altered.

SQL> alter index tab3_I rebuild online;
alter index tab3_I rebuild online
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-01450: maximum key length (3215) exceeded

Cause of the Problem
When creating a index the total length of the index cannot exceed a certain value. Primarily this value depends on DB_BLOCK_SIZE.
If 2K block size then maximum index key length=758
If 4K block size then maximum index key length=1578
If 8K block size then maximum index key length=3218
If 16K block size then maximum index key length=6498
How the maximum index key length is measured by?
Maximum index key length=Total index length (Sum of width of all indexed column+the number of indexed columns)+Length of the key(2 bytes)+ROWID(6 bytes)+the length of the rowid(1 byte)

The index key size is limited by the value of db_block_size, because a key value may not span multiple blocks. So, based on the size of the block size of index depends. In fact, it is required that any index block must contain at least TWO index entries per block.

So we can say that the maximum key length for an index will be less than half of
the DB_BLOCK_SIZE. But we know that in a block there also needed space for PCTFREE, INITRANS and space for block overhead(Block Header,ROW Directory, Table Directory, etc). After considering these bytes the actual space that can be used for the Index key is actually just over 1/3 of the DB_BLOCK_SIZE.

Solution of the Problem
The causes already indicates what might be the solutions. Solution may be,
1)Increase your database block size. Create a tablespace with bigger block size and create index on that tablespace.

2)If you have index on multiple columns then you can split index to single or 2 columns so that size does not extended over it can handle.

3)Rebuild the index without online clause. That is
ALTER INDEX index_name REBUILD;
Because The online rebuild of the index creates a journal table and index. This internal journal IOT table contains more columns in its index. This is a feature of online rebuild. This time the error arises because that current value of the initialization parameter db_block_size is not large enough to create internal journal IOT.

Expert are always welcome for their valuable comment or suggestion for the above post.

 

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There is one more critical aspect which I wanted to discuss is the cost/effort related to rebuilding indexes.

Rebuilding an index is quite a costly operation and you must evaluate the benefit Vs effort before rebuilding an index.

Rebuilding (online) an index requires additional resources like space, cpu usage, time.

Here is one more option, which is less used or probably less popular “coalesce”.

Rebuild Vs Coalesce

Rebuild

  • Can move an index to a different tablespace
  • Resource consuming process
  • Takes more time
  • Creates a new tree
  • Shortens the height of an index if it was increased due to DML activities
  • Rebuilding can affect future DML’s because index becomes compact and for future DML’s index has to be extend dynamically.

Coalesce

  • Cannot move an index to a different tablespace
  • Comparatively less resource consuming
  • Takes relatively less time
  • Only merge the adjacent free space between leaf blocks within a branch
  • Doesn’t shorten height of index
  • Since coalesce doesn’t effect the total size and only frees up the unused space, it doesn’t affect future DML’s

Coalescing the index, frees up space of adjacent leaf blocks within a branch block. This way the number of blocks or extents which an index is using will not change but there will be more number of free blocks which can be used for future inserts or updates.

In an OLTP environment, where data is highly volatile, coalesce is better because it doesn’t shrink the index and the free space remains with the index segment.

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Need is necessary for any change. I hope all agree to this. So why many DBA’s (not all) rebuilds indexes on periodical basis without knowing the impact of it?

Let’s revisit the facts stated by many Oracle experts:

– B Tree indexes can never be unbalanced
– The distance of any leaf block (which contains the index value) from the root block is always same.
– B Tree performance is good for both small and large tables and does not degrade with the growth of table

When will rebuild help?

• When the data in index is sparse (lots of holes in index, due to deletes or updates) and your query is usually range based.

If your SQL’s use “=” predicate, rebuilding of index may not help you at all unless the DML’s (updates/deletes) on the table are cause of increasing the height of index. Due to heavy updates/deletes on a table, the space in the index can be left unused (unless it is reused) due to which block splits and if the splitting goes beyond the level of accomodation, the height of index increases by 1.
In simpler terms, unless you reduce the height (BLEVEL) of index, the time taken to reach the leaf block from root block will not change.

As explained above, the height of index can be decreased by rebuild of an index(only if height was increased to updates/deletes).

Only in case of range scan, rebuild (consider coalesce option also) may help.

Select * from emp where empno between 1000 and 2000;

In above case there could be lot of deleted/updated data which will also be read while reading between 1000 to 2000. Rebuilding will reclaim any unused space and hence the select could be faster.

• Index fast full scan/ Index full scan. (rare case)
If your SQL’s are designed in a way that mostly data is selected from index (it doesn’t goes to table). In this case the whole index is read for data not for redirecting to table.

Ex:
Select count(empno) from emp; — you have an index in empno
Select empno from emp — you have an index in empno

Both the above SELECTs will fetch the data from Index only. Since the whole index is getting read, the space used by deleted rows is also read at the time of SELECT operation

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