Postgresql 中文操作指南

CREATE SEQUENCE

CREATE SEQUENCE — 定义新的序列生成器

CREATE SEQUENCE — define a new sequence generator

Synopsis

CREATE [ { TEMPORARY | TEMP } | UNLOGGED ] SEQUENCE [ IF NOT EXISTS ] name
    [ AS data_type ]
    [ INCREMENT [ BY ] increment ]
    [ MINVALUE minvalue | NO MINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE ]
    [ START [ WITH ] start ] [ CACHE cache ] [ [ NO ] CYCLE ]
    [ OWNED BY { table_name.column_name | NONE } ]

Description

CREATE SEQUENCE 创建一个新的序列号生成器。这涉及创建和初始化一个新的特殊单行表，名称为 name 。该生成器将归 issuing 该命令的用户所有。

CREATE SEQUENCE creates a new sequence number generator. This involves creating and initializing a new special single-row table with the name name. The generator will be owned by the user issuing the command.

如果给出了模式名称，则在指定的模式中创建序列。否则将在当前模式中创建序列。临时序列存在于特殊模式中，因此在创建临时序列时不能给出模式名称。序列名称必须不同于同一模式中任何其他关系（表、序列、索引、视图、物化视图或外部表）的名称。

If a schema name is given then the sequence is created in the specified schema. Otherwise it is created in the current schema. Temporary sequences exist in a special schema, so a schema name cannot be given when creating a temporary sequence. The sequence name must be distinct from the name of any other relation (table, sequence, index, view, materialized view, or foreign table) in the same schema.

创建序列后，使用函数 nextval 、 currval 和 setval 对序列执行操作。这些函数在 Section 9.17 中有记录。

After a sequence is created, you use the functions nextval, currval, and setval to operate on the sequence. These functions are documented in Section 9.17.

虽然不能直接更新序列，但可以使用如下查询：

Although you cannot update a sequence directly, you can use a query like:

SELECT * FROM name;

来检查序列的参数和当前状态。具体而言，序列的 last_value 字段显示由任何会话分配的最后一个值。（当然，如果其他会话主动执行 nextval 调用，此值在打印时可能已过期。）

to examine the parameters and current state of a sequence. In particular, the last_value field of the sequence shows the last value allocated by any session. (Of course, this value might be obsolete by the time it’s printed, if other sessions are actively doing nextval calls.)

Parameters

  • TEMPORARY or TEMP

    • If specified, the sequence object is created only for this session, and is automatically dropped on session exit. Existing permanent sequences with the same name are not visible (in this session) while the temporary sequence exists, unless they are referenced with schema-qualified names.

  • UNLOGGED

    • If specified, the sequence is created as an unlogged sequence. Changes to unlogged sequences are not written to the write-ahead log. They are not crash-safe: an unlogged sequence is automatically reset to its initial state after a crash or unclean shutdown. Unlogged sequences are also not replicated to standby servers.

    • Unlike unlogged tables, unlogged sequences do not offer a significant performance advantage. This option is mainly intended for sequences associated with unlogged tables via identity columns or serial columns. In those cases, it usually wouldn’t make sense to have the sequence WAL-logged and replicated but not its associated table.

  • IF NOT EXISTS

    • Do not throw an error if a relation with the same name already exists. A notice is issued in this case. Note that there is no guarantee that the existing relation is anything like the sequence that would have been created — it might not even be a sequence.

  • name

    • The name (optionally schema-qualified) of the sequence to be created.

  • data_type

    • The optional clause AS _data_type_ specifies the data type of the sequence. Valid types are smallint, integer, and bigint. bigint is the default. The data type determines the default minimum and maximum values of the sequence.

  • increment

    • The optional clause INCREMENT BY _increment_ specifies which value is added to the current sequence value to create a new value. A positive value will make an ascending sequence, a negative one a descending sequence. The default value is 1.

  • minvalue__NO MINVALUE

    • The optional clause MINVALUE _minvalue_ determines the minimum value a sequence can generate. If this clause is not supplied or NO MINVALUE is specified, then defaults will be used. The default for an ascending sequence is 1. The default for a descending sequence is the minimum value of the data type.

  • maxvalue__NO MAXVALUE

    • The optional clause MAXVALUE _maxvalue_ determines the maximum value for the sequence. If this clause is not supplied or NO MAXVALUE is specified, then default values will be used. The default for an ascending sequence is the maximum value of the data type. The default for a descending sequence is -1.

  • start

    • The optional clause START WITH _start_ allows the sequence to begin anywhere. The default starting value is minvalue for ascending sequences and maxvalue for descending ones.

  • cache

    • The optional clause CACHE _cache_ specifies how many sequence numbers are to be preallocated and stored in memory for faster access. The minimum value is 1 (only one value can be generated at a time, i.e., no cache), and this is also the default.

  • CYCLE__NO CYCLE

    • The CYCLE option allows the sequence to wrap around when the maxvalue or minvalue has been reached by an ascending or descending sequence respectively. If the limit is reached, the next number generated will be the minvalue or maxvalue, respectively.

    • If NO CYCLE is specified, any calls to nextval after the sequence has reached its maximum value will return an error. If neither CYCLE or NO CYCLE are specified, NO CYCLE is the default.

  • OWNED BY table_name.column_name__OWNED BY NONE

    • The OWNED BY option causes the sequence to be associated with a specific table column, such that if that column (or its whole table) is dropped, the sequence will be automatically dropped as well. The specified table must have the same owner and be in the same schema as the sequence. OWNED BY NONE, the default, specifies that there is no such association.

Notes

使用 DROP SEQUENCE 删除序列。

Use DROP SEQUENCE to remove a sequence.

序列基于 bigint 算法，因此该范围不能超出一字节整数的范围（-9223372036854775808 至 9223372036854775807）。

Sequences are based on bigint arithmetic, so the range cannot exceed the range of an eight-byte integer (-9223372036854775808 to 9223372036854775807).

因为永远不会回滚 nextval 和 setval 调用，所以如果需要序列号的“无间隔”分配，则不能使用序列对象。通过对包含计数器的表使用独占锁定可以构建无间隔分配；但是与序列对象相比，这种解决方案要昂贵得多，特别是如果许多事务需要同时使用序列号。

Because nextval and setval calls are never rolled back, sequence objects cannot be used if “gapless” assignment of sequence numbers is needed. It is possible to build gapless assignment by using exclusive locking of a table containing a counter; but this solution is much more expensive than sequence objects, especially if many transactions need sequence numbers concurrently.

对于多个会话同时使用的序列对象，如果使用大于 1 的 cache 设置，可能会获得意外的结果。每个会话会在对序列对象的一次访问期间分配并缓存连续的序列值，并相应地增加序列对象的 last_value 。然后，该会话中接下来的 cache -1 次 nextval 使用只会返回预分配的值，而不会接触该序列对象。因此，会话中分配但未使用的任何数字都将在该会话结束时丢失，从而导致序列中出现“空洞”。

Unexpected results might be obtained if a cache setting greater than one is used for a sequence object that will be used concurrently by multiple sessions. Each session will allocate and cache successive sequence values during one access to the sequence object and increase the sequence object’s last_value accordingly. Then, the next cache-1 uses of nextval within that session simply return the preallocated values without touching the sequence object. So, any numbers allocated but not used within a session will be lost when that session ends, resulting in “holes” in the sequence.

此外，虽然可以确保多个会话分配不同的序列值，但从所有会话考虑时这些值可能会乱序生成。例如，对于一个 cache 设置为 10，会话 A 可能会预留值 1..10 并返回 nextval =1，然后会话 B 可能会预留值 11..20 并返回 nextval =11，在会话 A 生成了 nextval =2 之前。因此，对于 cache 设置为 1，可以放心假设 nextval 值是按顺序生成的；对于 cache 设置大于 1，你应该只假设 nextval 值都是不同的，而不是完全按顺序生成的。另外， last_value 会反映任何会话预留的最新值，无论它是否已由 nextval 返回。

Furthermore, although multiple sessions are guaranteed to allocate distinct sequence values, the values might be generated out of sequence when all the sessions are considered. For example, with a cache setting of 10, session A might reserve values 1..10 and return nextval=1, then session B might reserve values 11..20 and return nextval=11 before session A has generated nextval=2. Thus, with a cache setting of one it is safe to assume that nextval values are generated sequentially; with a cache setting greater than one you should only assume that the nextval values are all distinct, not that they are generated purely sequentially. Also, last_value will reflect the latest value reserved by any session, whether or not it has yet been returned by nextval.

另一点需要注意的是，对于此类序列执行的 setval 会在其他会话用完他们已缓存的任何预分配值后，才能为其他会话注意到。

Another consideration is that a setval executed on such a sequence will not be noticed by other sessions until they have used up any preallocated values they have cached.

Examples

创建一个从 101 开始的升序序列，称为 serial ：

Create an ascending sequence called serial, starting at 101:

CREATE SEQUENCE serial START 101;

从此序列选择下一个数字：

Select the next number from this sequence:

SELECT nextval('serial');

 nextval
---------
     101

从此序列选择下一个数字：

Select the next number from this sequence:

SELECT nextval('serial');

 nextval
---------
     102

在一个 INSERT 命令中使用此序列：

Use this sequence in an INSERT command:

INSERT INTO distributors VALUES (nextval('serial'), 'nothing');

在一个 COPY FROM 之后更新序列值：

Update the sequence value after a COPY FROM:

BEGIN;
COPY distributors FROM 'input_file';
SELECT setval('serial', max(id)) FROM distributors;
END;

Compatibility

除了以下例外， CREATE SEQUENCE 符合 SQL 标准：

CREATE SEQUENCE conforms to the SQL standard, with the following exceptions:

See Also

ALTER SEQUENCE 、 DROP SEQUENCE

ALTER SEQUENCE, DROP SEQUENCE