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Don Burleson Blog 







Oracle sorted hash cluster tips

Oracle Database Tips by Donald Burleson

If your database has a high volume of transactions that join two tables with a subordinate index range scan, grouping the rows together with sorted hash clusters can have a profound impact on reducing I/O. 

For example, clustering all orders for a customer together on the same data block allows the query "show me all orders for this customer" to be retrieved in a single fetch.  See here for more details on using hashing to group data rows onto related data blocks.  The clustering_factor metric shows how in-sync the rows are.

With row re-sequencing, adjacent rows are together, greatly reducing I/O:

Figure 15.8: This column has a low clustering factor, small rows and large blocks.

For queries that access common rows with a table (e.g. get all items in order 123), unordered tables can experience huge I/O as the index retrieves a separate data block for each row requested.

If we group like rows together (as measured by the clustering_factor in dba_indexes) we can get all of the row with a single block read because the rows are together.  You can use 10g hash cluster tables, single table clusters, or manual row re-sequencing (CTAS with ORDER BY) to achieve this goal:

Oracle has had several methods for re-sequencing table rows together, but the best is the Oracle sorted hash cluster:

  • Single table cluster tables - This uses an overflow area

  • CTAS with order by clause - The DBA manually reorgs the table, reordering the table rows together on adjacent data blocks.

  • Sorted hash clusters - The new 10g way of assigning rows to adjacent data blocks.

The latest method, the Oracle sorted hash cluster, is a great way to ensure row adjacency.  Let's take a closer look at sorted hash clusters.

Creating a sorted hash cluster

The Oracle 10g documentation notes "In a sorted hash cluster, the rows corresponding to each value of the hash function are sorted on a specified set of columns in ascending order, which can improve response time during subsequent operations on the clustered data.".  In Oracle10g and beyond, a sorted hash cluster allows you to define a hash sort key and have Oracle place the row on the target

create cluster
    ordor_nbr      number,
    ordor_date     date,
    customer_nbr   number sort
hashkeys 10000
hash is ora_hash(customer_nbr)
size 256;

create table ordor
    ordor_nbr      number,
    ordor_date     date,
    customer_nbr   number sort
   customer_nbr, transdate

As we see, the Oracle sorted hash clusters are a great way to reduce I/O stress on Oracle databases where related rows are grouped together physically on the data block.

For a complete treatment of using Oracle sorted hash clusters, see my book.  "Oracle Tuning: The Definitive Reference". 

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

Sorted hash clusters references:

Oracle has some great advice for tuning-down "db file sequential read" waits by taking read-only tables and reorganization the table in index row-order: 

- If Index Range scans are involved, more blocks than necessary could be being visited if the index is unselective: by forcing or enabling the  use of a more selective index, we can access the same table data by  visiting fewer index blocks (and doing fewer physical I/Os).   

- If the index being used has a large Clustering Factor (in dba_indexes), then more table data blocks have to be visited in order to get the rows in each Index block:

My related pages on sorted hash clusters:



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