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

rekharaghuram
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05.11.2015 Views

CHAPTER 13 ■ PARTITIONING 589 12 ) 13 / Table created. ops$tkyte@ORA10G> select segment_name, partition_name, segment_type 2 from user_segments; SEGMENT_NAME PARTITION_NAME SEGMENT_TYPE -------------- --------------- ------------------ PARTITIONED PART_1 TABLE PARTITION PARTITIONED PART_2 TABLE PARTITION PARTITIONED_PK INDEX The PARTITIONED_PK index is not even partitioned, let alone locally partitioned, and as we’ll see, it cannot be locally partitioned. Even if we try to trick Oracle by realizing that a primary key can be enforced by a nonunique index as well as a unique index, we’ll find that this approach will not work either: ops$tkyte@ORA10G> CREATE TABLE partitioned 2 ( timestamp date, 3 id int 4 ) 5 PARTITION BY RANGE (timestamp) 6 ( 7 PARTITION part_1 VALUES LESS THAN 8 ( to_date('01-jan-2000','dd-mon-yyyy') ) , 9 PARTITION part_2 VALUES LESS THAN 10 ( to_date('01-jan-2001','dd-mon-yyyy') ) 11 ) 12 / Table created. ops$tkyte@ORA10G> create index partitioned_idx 2 on partitioned(id) local 3 / Index created. ops$tkyte@ORA10G> select segment_name, partition_name, segment_type 2 from user_segments; SEGMENT_NAME PARTITION_NAME SEGMENT_TYPE --------------- --------------- ------------------ PARTITIONED PART_1 TABLE PARTITION PARTITIONED_IDX PART_2 INDEX PARTITION PARTITIONED PART_2 TABLE PARTITION PARTITIONED_IDX PART_1 INDEX PARTITION

590 CHAPTER 13 ■ PARTITIONING ops$tkyte@ORA10G> alter table partitioned 2 add constraint 3 partitioned_pk 4 primary key(id) 5 / alter table partitioned * ERROR at line 1: ORA-01408: such column list already indexed Here, Oracle attempts to create a global index on ID, but finds that it cannot since an index already exists. The preceding statements would work if the index we created was not partitioned, as Oracle would have used that index to enforce the constraint. The reasons why uniqueness cannot be enforced, unless the partition key is part of the constraint, are twofold. First, if Oracle allowed this, it would void most of the advantages of partitions. Availability and scalability would be lost, as each and every partition would always have to be available and scanned to do any inserts and updates. The more partitions you had, the less available the data would be. The more partitions you had, the more index partitions you would have to scan, and the less scalable partitions would become. Instead of providing availability and scalability, doing this would actually decrease both. Additionally, Oracle would have to effectively serialize inserts and updates to this table at the transaction level. This is because if we add ID=1 to PART_1, Oracle would have to somehow prevent anyone else from adding ID=1 to PART_2. The only way to do this would be to prevent others from modifying index partition PART_2, since there isn’t anything to really “lock” in that partition. In an OLTP system, unique constraints must be system enforced (i.e., enforced by Oracle) to ensure the integrity of data. This implies that the logical model of your application will have an impact on the physical design. Uniqueness constraints will either drive the underlying table partitioning scheme, driving the choice of the partition keys, or point you toward the use of global indexes instead. We’ll take a look at global indexes in more depth next. Global Indexes Global indexes are partitioned using a scheme that is different from that used in the underlying table. The table might be partitioned by a TIMESTAMP column into ten partitions, and a global index on that table could be partitioned into five partitions by the REGION column. Unlike local indexes, there is only one class of global index, and that is a prefixed global index. There is no support for a global index whose index key does not begin with the partitioning key for that index. That implies that whatever attribute(s) you use to partition the index will be on the leading edge of the index key itself. Continuing on from our previous example, here is a quick example of the use of a global index. It shows that a global partitioned index can be used to enforce uniqueness for a primary key, so you can have partitioned indexes that enforce uniqueness, but do not include the partition key of TABLE. The following example creates a table partitioned by TIMESTAMP that has an index partitioned by ID:

CHAPTER 13 ■ PARTITIONING 589<br />

12 )<br />

13 /<br />

Table created.<br />

ops$tkyte@ORA10G> select segment_name, partition_name, segment_type<br />

2 from user_segments;<br />

SEGMENT_NAME PARTITION_NAME SEGMENT_TYPE<br />

-------------- --------------- ------------------<br />

PARTITIONED PART_1 TABLE PARTITION<br />

PARTITIONED PART_2 TABLE PARTITION<br />

PARTITIONED_PK<br />

INDEX<br />

The PARTITIONED_PK index is not even partitioned, let alone locally partitioned, <strong>and</strong> as<br />

we’ll see, it cannot be locally partitioned. Even if we try to trick <strong>Oracle</strong> by realizing that a primary<br />

key can be enforced by a nonunique index as well as a unique index, we’ll find that this<br />

approach will not work either:<br />

ops$tkyte@ORA10G> CREATE TABLE partitioned<br />

2 ( timestamp date,<br />

3 id int<br />

4 )<br />

5 PARTITION BY RANGE (timestamp)<br />

6 (<br />

7 PARTITION part_1 VALUES LESS THAN<br />

8 ( to_date('01-jan-2000','dd-mon-yyyy') ) ,<br />

9 PARTITION part_2 VALUES LESS THAN<br />

10 ( to_date('01-jan-2001','dd-mon-yyyy') )<br />

11 )<br />

12 /<br />

Table created.<br />

ops$tkyte@ORA10G> create index partitioned_idx<br />

2 on partitioned(id) local<br />

3 /<br />

Index created.<br />

ops$tkyte@ORA10G> select segment_name, partition_name, segment_type<br />

2 from user_segments;<br />

SEGMENT_NAME PARTITION_NAME SEGMENT_TYPE<br />

--------------- --------------- ------------------<br />

PARTITIONED PART_1 TABLE PARTITION<br />

PARTITIONED_IDX PART_2<br />

INDEX PARTITION<br />

PARTITIONED PART_2 TABLE PARTITION<br />

PARTITIONED_IDX PART_1<br />

INDEX PARTITION

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