Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 10 ■ DATABASE TABLES
Sorted Hash Clustered Tables
Sorted hash clusters are new in Oracle 10g. They combine the qualities of the hash cluster just
described with those of an IOT. They are most appropriate when you constantly retrieve data
using a query similar to this:
Select *
From t
Where KEY=:x
Order by SORTED_COLUMN
That is, you retrieve the data by some key and need that data ordered by some other column.
Using a sorted hash cluster, Oracle can return the data without performing a sort at all. It
accomplishes this by storing the data upon insert in sorted order physically—by key. Suppose
you have a customer order table:
ops$tkyte@ORA10G> select cust_id, order_dt, order_number
2 from cust_orders
3 order by cust_id, order_dt;
CUST_ID ORDER_DT
ORDER_NUMBER
------- ---------------------------- ------------
1 31-MAR-05 09.13.57.000000 PM 21453
11-APR-05 08.30.45.000000 AM 21454
28-APR-05 06.21.09.000000 AM 21455
2 08-APR-05 03.42.45.000000 AM 21456
19-APR-05 08.59.33.000000 AM 21457
27-APR-05 06.35.34.000000 AM 21458
30-APR-05 01.47.34.000000 AM 21459
7 rows selected.
The table is stored in a sorted hash cluster, whereby the HASH key is CUST_ID and the field
to sort on is ORDER_DT. Graphically, it might look like Figure 10-10, where 1, 2, 3, 4, . . . represent
the records stored sorted on each block.
Figure 10-10. Sorted hash cluster depiction