Single hash clustered table: 1

1：When creating a hash cluster, if the hashkeys is not a prime number, then oracle will use the next nearest prime number as the hashkeys.

 

2: When creating a table in the hash cluster, as the cluster is already pre-allocated, so we could see the table will take space even if there is no data inside.

 

SQL> create cluster hash_cluster ( hash_key number(10) )

  2  hashkeys 1000

  3  size 1200

  4  single table

  5  hash is HASH_KEY;

 

Cluster created

SQL> select a.cluster_name,a.hashkeys from user_clusters a where a.cluster_name='HASH_CLUSTER';

 

CLUSTER_NAME                     HASHKEYS

------------------------------ ----------

HASH_CLUSTER                         1009     ===> 1009 is used instead of 1000

 

SQL> create table t_hashed

  2  (a number(10) primary key, b char(1000))

  3  cluster hash_cluster(a);

 

Table created

 

SQL> set serveroutput on

 

SQL> exec show_space('T_HASHED');

 

Free Blocks.............................0

Total Blocks............................256

Total Bytes.............................2097152

Unused Blocks...........................86

Unused Bytes............................704512

Last Used Ext FileId....................6

Last Used Ext BlockId...................28169

Last Used Block.........................42

 

PL/SQL procedure successfully completed

 

 

SQL> select count(*) from T_HASHED;

 

  COUNT(*)

----------

         0                         ===> no data, but table already takes space