转自 http://blog.csdn.net/tianlesoftware/article/details/5995051

之前整理的一篇文章：

http://blog.csdn.net/tianlesoftware/archive/2009/10/30/4745039.aspx

之前整理的一篇文章。那是还是写blog初期的作品。 2009年10月份的。转眼一年，写Blog 也比以前成熟了很多。

一. 理论知识

先看一个创建Sequence的语句：

SQL> create sequence seq_tmp

2 increment by 1

3 start with 1

4 nomaxvalue

5 nocycle

6 ;

序列已创建。

二. 实验

一个网友RAC 系统上的测试时结果：

nocache：　　 2100s

cache ＝1000：　　 55s

差别很明显。

测试一：

SQL> create sequence seq_1 nocache;

序列已创建。

SQL> set timing on;

SQL> declare

2 x number;

3 begin

4 for i in 1 .. 10000 loop

5 select seq_1.nextval into x from dual;

6 end loop;

7 end;

8 /

PL/SQL 过程已成功完成。

已用时间: 00: 00: 02.26

测试二：

SQL> create sequence seq_2 cache 20;

序列已创建。

已用时间: 00: 00: 00.01

SQL> declare

2 x number;

3 begin

4 for i in 1 .. 10000 loop

5 select seq_2.nextval into x from dual;

6 end loop;

7 end;

8 /

PL/SQL 过程已成功完成。

已用时间: 00: 00: 00.46

测试三：

SQL> create sequence seq_3 cache 100;

序列已创建。

已用时间: 00: 00: 00.05

SQL> declare

2 x number;

3 begin

4 for i in 1 .. 10000 loop

5 select seq_3.nextval into x from dual;

6 end loop;

7 end;

8 /

PL/SQL 过程已成功完成。

已用时间: 00: 00: 00.37

测试四：

SQL> create sequence seq_4 cache 1000;

序列已创建。

已用时间: 00: 00: 00.04

SQL> declare

2 x number;

3 begin

4 for i in 1 .. 40000 loop

5 select seq_4.nextval into x from dual;

6 end loop;

7 end;

8 /

PL/SQL 过程已成功完成。

已用时间: 00: 00: 01.31

SQL> declare

2 x number;

3 begin

4 for i in 1 .. 40000 loop

5 select seq_1.nextval into x from dual;

6 end loop;

7 end;

8 /

PL/SQL 过程已成功完成。

已用时间: 00: 00: 09.33

SQL>

小结：

在自己的本本上测试的，Oracle 11gR2. 单Instance数据库单会话循环不间断取1-4万个值。

nocache： 2.26s 10000　　　

cache:20 0.46s 10000

cache:100 0.37s 10000

cache:1000 1.31s 40000

nocache： 9.33s 40000

基本上cache 大于20的时候性能基本可以接受，nocache的时候性能确实很差．

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以下是本人添加的官网文档说明

Sequence Pseudocolumns

A sequence is a schema object that can generate unique sequential values. These values are often used for primary and unique keys. You can refer to sequence values in SQL statements with these pseudocolumns:

CURRVAL: Returns the current value of a sequence
NEXTVAL: Increments the sequence and returns the next value

You must qualify CURRVAL and NEXTVAL with the name of the sequence:

sequence.CURRVAL

sequence.NEXTVAL

To refer to the current or next value of a sequence in the schema of another user, you must have been granted either SELECT object privilege on the sequence or SELECT ANY SEQUENCE system privilege, and you must qualify the sequence with the schema containing it:

schema.sequence.CURRVAL

schema.sequence.NEXTVAL

To refer to the value of a sequence on a remote database, you must qualify the sequence with a complete or partial name of a database link:

schema.sequence.CURRVAL@dblink

schema.sequence.NEXTVAL@dblink

A sequence can be accessed by many users concurrently with no waiting or locking.

Where to Use Sequence Values

You can use CURRVAL and NEXTVAL in the following locations:

The select list of a SELECT statement that is not contained in a subquery, materialized view, or view
The select list of a subquery in an INSERT statement
The VALUES clause of an INSERT statement
The SET clause of an UPDATE statement

Restrictions on Sequence Values You cannot use CURRVAL and NEXTVAL in the following constructs:

A subquery in a DELETE, SELECT, or UPDATE statement
A query of a view or of a materialized view
A SELECT statement with the DISTINCT operator
A SELECT statement with a GROUP BY clause or ORDER BY clause
A SELECT statement that is combined with another SELECT statement with the UNION, INTERSECT, or MINUS set operator
The WHERE clause of a SELECT statement
The DEFAULT value of a column in a CREATE TABLE or ALTER TABLE statement
The condition of a CHECK constraint

Within a single SQL statement that uses CURRVAL or NEXTVAL, all referenced LONG columns, updated tables, and locked tables must be located on the same database.

How to Use Sequence Values

When you create a sequence, you can define its initial value and the increment between its values. The first reference to NEXTVAL returns the initial value of the sequence. Subsequent references to NEXTVAL increment the sequence value by the defined increment and return the new value. Any reference to CURRVALalways returns the current value of the sequence, which is the value returned by the last reference to NEXTVAL.

Before you use CURRVAL for a sequence in your session, you must first initialize the sequence with NEXTVAL. Refer to CREATE SEQUENCE for information on sequences.

Within a single SQL statement containing a reference to NEXTVAL, Oracle increments the sequence once:

For each row returned by the outer query block of a SELECT statement. Such a query block can appear in the following places:
- A top-level SELECT statement
- An INSERT ... SELECT statement (either single-table or multitable). For a multitable insert, the reference to NEXTVAL must appear in the VALUESclause, and the sequence is updated once for each row returned by the subquery, even though NEXTVAL may be referenced in multiple branches of the multitable insert.
- A CREATE TABLE ... AS SELECT statement
- A CREATE MATERIALIZED VIEW ... AS SELECT statement
For each row updated in an UPDATE statement
For each INSERT statement containing a VALUES clause
For each INSERT ... [ALL | FIRST] statement (multitable insert). A multitable insert is considered a single SQL statement. Therefore, a reference to theNEXTVAL of a sequence will increase the sequence only once for each input record coming from the SELECT portion of the statement. If NEXTVAL is specified more than once in any part of the INSERT ... [ALL | FIRST ] statement, then the value will be the same for all insert branches, regardless of how often a given record might be inserted.
For each row merged by a MERGE statement. The reference to NEXTVAL can appear in the merge_insert_clause or the merge_update_clause or both. TheNEXTVALUE value is incremented for each row updated and for each row inserted, even if the sequence number is not actually used in the update or insert operation. If NEXTVAL is specified more than once in any of these locations, then the sequence is incremented once for each row and returns the same value for all occurrences of NEXTVAL for that row.
For each input row in a multitable INSERT ALL statement. NEXTVAL is incremented once for each row returned by the subquery, regardless of how many occurrences of the insert_into_clause map to each row.

If any of these locations contains more than one reference to NEXTVAL, then Oracle increments the sequence once and returns the same value for all occurrences of NEXTVAL.

If any of these locations contains references to both CURRVAL and NEXTVAL, then Oracle increments the sequence and returns the same value for both CURRVALand NEXTVAL.

Finding the next value of a sequence: Example This example selects the next value of the employee sequence in the sample schema hr:

SELECT employees_seq.nextval

  FROM DUAL;

Inserting sequence values into a table: Example This example increments the employee sequence and uses its value for a new employee inserted into the sample table hr.employees:

INSERT INTO employees

  VALUES (employees_seq.nextval, 'John', 'Doe', 'jdoe', '555-1212',

          TO_DATE(SYSDATE), 'PU_CLERK', 2500, null, null, 30);

Reusing the current value of a sequence: Example This example adds a new order with the next order number to the master order table. It then adds suborders with this number to the detail order table:

INSERT INTO orders (order_id, order_date, customer_id)

  VALUES (orders_seq.nextval, TO_DATE(SYSDATE), 106);

INSERT INTO order_items (order_id, line_item_id, product_id)

  VALUES (orders_seq.currval, 1, 2359);

INSERT INTO order_items (order_id, line_item_id, product_id)

  VALUES (orders_seq.currval, 2, 3290);

INSERT INTO order_items (order_id, line_item_id, product_id)

  VALUES (orders_seq.currval, 3, 2381);

CREATE SEQUENCE

Purpose

Use the CREATE SEQUENCE statement to create a sequence, which is a database object from which multiple users may generate unique integers. You can use sequences to automatically generate primary key values.

When a sequence number is generated, the sequence is incremented, independent of the transaction committing or rolling back. If two users concurrently increment the same sequence, then the sequence numbers each user acquires may have gaps, because sequence numbers are being generated by the other user. One user can never acquire the sequence number generated by another user. After a sequence value is generated by one user, that user can continue to access that value regardless of whether the sequence is incremented by another user.

Sequence numbers are generated independently of tables, so the same sequence can be used for one or for multiple tables. It is possible that individual sequence numbers will appear to be skipped, because they were generated and used in a transaction that ultimately rolled back. Additionally, a single user may not realize that other users are drawing from the same sequence.

After a sequence is created, you can access its values in SQL statements with the CURRVAL pseudocolumn, which returns the current value of the sequence, or the NEXTVAL pseudocolumn, which increments the sequence and returns the new value.

Note on Using Sequences with Deferred Segments If you attempt to insert a sequence value into a table that uses deferred segment creation, the first value that the sequence returns will be skipped.

See Also:

Chapter 2, "Pseudocolumns" for more information on using the CURRVAL and NEXTVAL
"How to Use Sequence Values" for information on using sequences
ALTER SEQUENCE or DROP SEQUENCE for information on modifying or dropping a sequence

Prerequisites

To create a sequence in your own schema, you must have the CREATE SEQUENCE system privilege.

To create a sequence in another user's schema, you must have the CREATE ANY SEQUENCE system privilege.

Syntax

create_sequence::=

Oracle Sequence Cache 参数说明
Description of the illustration create_sequence.gif

Semantics

schema

Specify the schema to contain the sequence. If you omit schema, then Oracle Database creates the sequence in your own schema.

sequence

Specify the name of the sequence to be created. The name must satisfy the requirements listed in "Database Object Naming Rules".

If you specify none of the following clauses, then you create an ascending sequence that starts with 1 and increases by 1 with no upper limit. Specifying onlyINCREMENT BY -1 creates a descending sequence that starts with -1 and decreases with no lower limit.

To create a sequence that increments without bound, for ascending sequences, omit the MAXVALUE parameter or specify NOMAXVALUE. For descending sequences, omit the MINVALUE parameter or specify the NOMINVALUE.
To create a sequence that stops at a predefined limit, for an ascending sequence, specify a value for the MAXVALUE parameter. For a descending sequence, specify a value for the MINVALUE parameter. Also specify NOCYCLE. Any attempt to generate a sequence number once the sequence has reached its limit results in an error.
To create a sequence that restarts after reaching a predefined limit, specify values for both the MAXVALUE and MINVALUE parameters. Also specify CYCLE.

INCREMENT BY Specify the interval between sequence numbers. This integer value can be any positive or negative integer, but it cannot be 0. This value can have 28 or fewer digits for an ascending sequence and 27 or fewer digits for a descending sequence. The absolute of this value must be less than the difference of MAXVALUE and MINVALUE. If this value is negative, then the sequence descends. If the value is positive, then the sequence ascends. If you omit this clause, then the interval defaults to 1.

START WITH Specify the first sequence number to be generated. Use this clause to start an ascending sequence at a value greater than its minimum or to start a descending sequence at a value less than its maximum. For ascending sequences, the default value is the minimum value of the sequence. For descending sequences, the default value is the maximum value of the sequence. This integer value can have 28 or fewer digits for positive values and 27 or fewer digits for negative values.

Note:

This value is not necessarily the value to which an ascending or descending cycling sequence cycles after reaching its maximum or minimum value, respectively.

MAXVALUE Specify the maximum value the sequence can generate. This integer value can have 28 or fewer digits for positive values and 27 or fewer digits for negative values. MAXVALUE must be equal to or greater than START WITH and must be greater than MINVALUE.

NOMAXVALUE Specify NOMAXVALUE to indicate a maximum value of 10²⁸-1 for an ascending sequence or -1 for a descending sequence. This is the default.

MINVALUE Specify the minimum value of the sequence. This integer value can have 28 or fewer digits for positive values and 27 or fewer digits for negative values. MINVALUE must be less than or equal to START WITH and must be less than MAXVALUE.

NOMINVALUE Specify NOMINVALUE to indicate a minimum value of 1 for an ascending sequence or -(10²⁷ -1) for a descending sequence. This is the default.

CYCLE Specify CYCLE to indicate that the sequence continues to generate values after reaching either its maximum or minimum value. After an ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum value.

NOCYCLE Specify NOCYCLE to indicate that the sequence cannot generate more values after reaching its maximum or minimum value. This is the default.

CACHE Specify how many values of the sequence the database preallocates and keeps in memory for faster access. This integer value can have 28 or fewer digits. The minimum value for this parameter is 2. For sequences that cycle, this value must be less than the number of values in the cycle. You cannot cache more values than will fit in a given cycle of sequence numbers. Therefore, the maximum value allowed for CACHE must be less than the value determined by the following formula:

(CEIL (MAXVALUE - MINVALUE)) / ABS (INCREMENT)

If a system failure occurs, then all cached sequence values that have not been used in committed DML statements are lost. The potential number of lost values is equal to the value of the CACHE parameter.

Note:

Oracle recommends using the CACHE setting to enhance performance if you are using sequences in an Oracle Real Application Clusters environment.

NOCACHE Specify NOCACHE to indicate that values of the sequence are not preallocated. If you omit both CACHE and NOCACHE, then the database caches 20 sequence numbers by default.

ORDER Specify ORDER to guarantee that sequence numbers are generated in order of request. This clause is useful if you are using the sequence numbers as timestamps. Guaranteeing order is usually not important for sequences used to generate primary keys.

ORDER is necessary only to guarantee ordered generation if you are using Oracle Real Application Clusters. If you are using exclusive mode, then sequence numbers are always generated in order.

NOORDER Specify NOORDER if you do not want to guarantee sequence numbers are generated in order of request. This is the default.

Example

Creating a Sequence: Example The following statement creates the sequence customers_seq in the sample schema oe. This sequence could be used to provide customer ID numbers when rows are added to the customers table.

CREATE SEQUENCE customers_seq

 START WITH     1000

 INCREMENT BY   1

 NOCACHE

 NOCYCLE;

The first reference to customers_seq.nextval returns 1000. The second returns 1001. Each subsequent reference will return a value 1 greater than the previous reference.

秒客网

Oracle Sequence Cache 参数说明

一. 理论知识

二. 实验

Sequence Pseudocolumns

Where to Use Sequence Values

How to Use Sequence Values

CREATE SEQUENCE

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