oracle optimization tutorial - partitioning

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PL/SQL OPTIMIZATION – PARTITIONING

Creating a Partitioned Table

As the number of rows in your tables grows, the management and performance impacts will

increase. Backups will take longer, recoveries will take longer, and queries that span an entire table

will take longer. You can mitigate the administrative and performance issues for large tables by

separating the rows of a single table into multiple parts. Dividing a table’s data in this manner is

called partitioning the table; the table that is partitioned is called a partitioned table, and the parts

are called partitions.

Partitioning is useful for very large tables. By splitting a large table’s rows across multiple smaller

partitions, you accomplish several important goals:

The performance of queries against the tables may improve because Oracle may have to

search only one partition (one part of the table) instead of the entire table to resolve a

query.

Backup and recovery operations may perform better. Because the partitions are smaller than

the partitioned table, you may have more options for backing up and recovering the

partitions than you would have for a single large table.

The table may be easier to manage. Because the partitioned table’s data is stored in multiple

parts, it may be easier to load and delete data in the partitions than in the large table.

To create a partitioned table, you specify how to set up the partitions of the table’s data as part of

the create table command. Typically, tables are partitioned by ranges of values (known as range

partitioning).

CREATE TABLE tb_transactions_part

(

TRANS_ID VARCHAR2(40 BYTE),

COUNTRY VARCHAR2(10 BYTE),

YEAR INT,

SHOP_ID VARCHAR2(20 BYTE),

PRODUCT_ID VARCHAR2(10 BYTE),

SALES_DATA VARCHAR2(26 BYTE),

PRICE NUMBER(28, 2),

PRODUCT_VERSION VARCHAR2(15 BYTE),

CONSTRAINT trans_id_part_pk PRIMARY KEY (trans_id)

)

PARTITION BY RANGE (YEAR)




(PARTITION part1 VALUES LESS THAN (2012)

TABLESPACE SYSTEM,

PARTITION part2 VALUES LESS THAN (MAXVALUE)

TABLESPACE example);

The tb_transactions_part table will be partitioned based on the values in the Year column. For any

Years less than '2012', the records will be stored in the partition named PART1. The PART1 partition

will be stored in the System tablespace. Any other years will be stored in the PART2 partition.

You do not need to specify a maximum value for the last partition; the maxvalue keyword tells Oracle

to use the partition to store any data that could not be stored in the earlier partitions.

Creating a hash partition

In addition to range partitions, Oracle supports hash partitions. A hash partition determines the

physical placement of data by performing a hash function on the values of the partition key. In range

partitioning, consecutive values of the partition key are usually stored in the same partition. In hash

partitioning, consecutive values of the partition key are not generally stored in the same partition.

Hash partitioning distributes a set of records over a greater set of partitions than range partitioning

does, potentially decreasing the likelihood for I/O contention.

To create a hash partition, use the partition by hash clause in place of the partition by range clause.


(

trans_id VARCHAR2(40 BYTE),

country VARCHAR2(10 BYTE),

YEAR INT,

shop_id VARCHAR2(20 BYTE),

product_id VARCHAR2(10 BYTE),

sales_data VARCHAR2(26 BYTE),

price NUMBER(28, 2),

product_version VARCHAR2(15 BYTE),


)

PARTITION BY HASH (YEAR)

PARTITIONS 2

STORE IN (system, example);




The number of tablespaces specified in the store in clause does not have to equal the number of

partitions. If more partitions than tablespaces are specified, the partitions are assigned to the

tablespaces in a round-robin fashion.

You can specify named partitions. In this method, each partition is given a name and a tablespace,

with the option of using an additional lob or varray storage clause. This method gives you more

control over the location of the partitions, with the added benefit of letting you specify meaningful

names for the partitions.

List Partitioning

You can use list partitions in place of range and hash partitioning. In list partitioning, you tell Oracle

all the possible values and designate the partitions into which the corresponding rows should be

inserted.


(



YEAR INT,







)

PARTITION BY LIST (product_version)

(PARTITION part1 VALUES ('standard', 'simple')

TABLESPACE system,

PARTITION part2 VALUES ('full', 'vip')


Creating Subpartitions

You can create subpartitions—that is, partitions of partitions. You can use subpartitions to combine

all types of partitions: range partitions, list partitions, and hash partitions. For example, you can use

hash partitions in combination with range partitions, creating hash partitions of the range partitions.




For very large tables, this composite partitioning may be an effective way of separating the data into

manageable and tunable divisions.

The following example range-partitions the tb_transactions_part table by the year column, and it

hash-partitions the year partitions by product version values.


(



YEAR INT,







)

PARTITION BY RANGE (YEAR)

SUBPARTITION BY HASH (product_version)

SUBPARTITIONS 4

(PARTITION part1 VALUES LESS THAN (2011)

TABLESPACE system,

PARTITION part2 VALUES LESS THAN (MAXVALUE)


The tb_transactions_part table will be range-partitioned into two partitions, using the year value

ranges specified for the named partitions. Each of those partitions will be hash-partitioned on the

product version column.

Indexing Partitions

When you create a partitioned table, you should create an index on the table. The index may be

partitioned according to the same range values used to partition the table.

CREATE INDEX ix_trans_year

ON tb_transactions_part(year)

LOCAL

(PARTITION part1




TABLESPACE system,

PARTITION part2


In this create index command, no ranges are specified. Instead, the local keyword tells Oracle to

create a separate index for each partition of the tb_transactions_part table. Two partitions were

created on tb_transactions_part. This index will create two separate index partitions—one for each

table partition. Because there is one index per partition, the index partitions are “local” to the

partitions.

CREATE INDEX ix_trans_year_2

ON tb_transactions_part(year)

GLOBAL;

The global clause in this create index command allows you to create a nonpartitioned index or to

specify ranges for the index values that are different from the ranges for the table partitions. Local

indexes may be easier to manage than global indexes; however, global indexes may perform

uniqueness checks faster than local (partitioned) indexes perform them.

You cannot create global indexes for hash partitions or subpartitions.

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oracle optimization tutorial - partitioning

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