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Evidence that a large index blocksizes reduces I/O

May 27,  2003

Oracle Tips by Robin Schumacher

(Note: Robin's book excerpt work below omits some important external factors that influence multiple blocksize read performance. 


For the latest consensus on using multiple blocksizes in Oracle, see The 2009 consensus on multiple blocksizes.)


Empirical evidence suggests that you can use the large (16-32K) blocksize and separate data caches to improve response time under certain conditions.  Does such a thing really help performance?  A small but revealing test can answer that question.   This is from Robin Schumacher's book "Oracle Performance Troubleshooting", demonstrating that a larger blocksize can result in a reduction in logical I/O with larger blocksizes. However, be aware that external influences (SAN, NAS, RAID level, Stripe Size, &c) will influence your response time.  YMMV . . . . 



For the test, the following query will be used against a database that has a database block size of 8K, but also has the 16K cache enabled along with a 16K tablespace:






      patient_id between 1 and 40000;



The ERADMIN.ADMISSION table has 150,000 rows in it and has an index build on the PATIENT_ID column.  An EXPLAIN of the query reveals that it uses an index range scan to produce the desired end result:


Execution Plan


0                   SELECT STATEMENT Optimizer=CHOOSE

1                   (Cost=41 Card=1 Bytes=4)

   1    0   SORT (AGGREGATE)


              (NON-UNIQUE) (Cost=41 Card=120002 Bytes=480008)


Executing the query (twice to eliminate parse activity and to cache any data) with the index residing in a standard 8K tablespace produces these runtime statistics:




          0  recursive calls

          0  db block gets

        421  consistent gets

          0  physical reads

          0  redo size

        371  bytes sent via SQL*Net to client

        430  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed


To test the effectiveness of the new 16K cache and 16K tablespace, the index used by the query will be rebuilt into the 16K tablespace that has the exact same characteristics as the original 8K tablespace, except for the larger blocksize:


alter index
      rebuild nologging noreverse tablespace indx_16k;


Once the index is nestled firmly into the 16K tablespace, the query is re-executed (again twice) with the following runtime statistics being produced:




          0  recursive calls

          0  db block gets

        211  consistent gets

          0  physical reads

          0  redo size

        371  bytes sent via SQL*Net to client

        430  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed


As you can see, the amount of logical reads has been reduced in half simply by using the new 16K tablespace and accompanying 16K data cache.  Clearly, the benefits of properly using the new data caches and multi-block tablespace feature of Oracleand above are worth your investigation and trials in your own database. 


These and many other revelations are now available in Robin's phenomenal book "Oracle Performance Troubleshooting".  IMHO, this is one of the best Oracle performance books in many years.  Robin's book is available at this link:

If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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