以前建序列时从来没注意过这个选项，总是认为序列就一定是有序的，今天被人问道才注意了一下。原来是在Oracle并行服务器上用到的，我理解是不是在并行情况下，如果序列是noorder的，则取序列号时，先发出命令的实例不一定能取到大的序列。在决大多数情况下是用不到的。
CREATE SEQUENCE command
PURPOSE:
To create a sequence. A sequence is a database object from which
multiple users may generate unique integers. You can use sequences
to automatically generate primary key values.
SYNTAX:
CREATE SEQUENCE [schema.]sequence
[INCREMENT BY integer]
[START WITH integer]
[MAXVALUE integer | NOMAXVALUE]
[MINVALUE integer | NOMINVALUE]
[CYCLE | NOCYCLE]
[CACHE integer | NOCACHE]
[ORDER | NOORDER]
where:
schema
is the schema to contain the sequence. If you omit schema, Oracle
creates the sequence in your own schema.
sequence
is the name of the sequence to be created.
INCREMENT BY
specifies the interval between sequence numbers. This value can be
any positive or negative Oracle integer, but it cannot be 0. If
this value is negative, then the sequence descends. If the
increment is positive, then the sequence ascends. If you omit this
clause, the interval defaults to 1.
MINVALUE
specifies the sequence's minimum value.
NOMINVALUE
specifies a minimum value of 1 for an ascending sequence or -10
for a descending sequence.
The default is NOMINVALUE.
MAXVALUE
specifies the maximum value the sequence can generate.
NOMAXVALUE
specifies a maximum value of 10
for a descending sequence.
The default is NOMAXVALUE.
START WITH
specifies the first sequence number to be generated. You can use
this option 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
sequence's minimum value. For descending sequences, the default
value is the sequence's maximum value.
CYCLE
specifies 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.
NOCYCLE
specifies that the sequence cannot generate more values after
reaching its maximum or minimum value.
The default is NOCYCLE.
CACHE
specifies how many values of the sequence Oracle preallocates and
keeps in memory for faster access. 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.
NOCACHE
specifies that values of the sequence are not preallocated.
If you omit both the CACHE parameter and the NOCACHE option, Oracle
caches 20 sequence numbers by default. However, if you are using
Oracle with the Parallel Server option in parallel mode and you
specify the ORDER option, sequence values are never cached,
regardless of whether you specify the CACHE parameter or the NOCACHE
option.
ORDER
guarantees that sequence numbers are generated in order of request.
You may want to use this option if you are using the sequence
numbers as timestamps. Guaranteeing order is usually not important
for sequences used to generate primary keys.
NOORDER
does not guarantee sequence numbers are generated in order of
request.
If you omit both the ORDER and NOORDER options, Oracle chooses
NOORDER by default. Note that the ORDER option is only necessary to
guarantee ordered generation if you are using Oracle with the
Parallel Server option in parallel mode. If you are using exclusive
mode, sequence numbers are always generated in order.
PREREQUISITES:
To create a sequence in your own schema, you must have CREATE
SEQUENCE privilege.
To create a sequence in another user's schema, you must have CREATE
ANY SEQUENCE privilege. If you are using Trusted Oracle in DBMS MAC
mode, your DBMS label must dominate the creation label of the owner
of the schema to contain the sequence.