Help Center/ GaussDB(DWS)/ SQL Syntax Reference (8.1.3.x)/ Functions and Operators/ SEQUENCE Functions
Updated on 2024-12-19 GMT+08:00
View PDF

SEQUENCE Functions

The sequence functions provide a simple method to ensure security of multiple users for users to obtain sequence values from sequence objects.

The hybrid data warehouse (standalone) does not support SEQUENCE and related functions.

nextval(regclass)

Specifies an increasing sequence and returns a new value.

  • In order to avoid blocking concurrent transactions that retrieve values from the same sequence, the nextval operation does not undo its action. This means that once a value is obtained, it is considered as used and will not be given again. This applies even if the operation is part of a transaction that gets interrupted or if the calling query does not end up using the value. This means that aborted transactions may leave unused "holes" in the sequence of assigned values. Therefore, sequences in GaussDB(DWS) cannot be used to obtain sequence without gaps.
  • If the nextval function is pushed to DNs, each DN will automatically connect to the GTM and requests the next value. For example, in the INSERT INTO t1 SELECTxxx statement, a column in table t1 needs to invoke the nextval function. If maximum number of connections on the GTM is 8,192, this type of pushed statements occupies too many GTM connections. Therefore, the number of concurrent connections for these statements is limited to 7,000 divided by the number of cluster DNs. The other 1,192 connections are reserved for other statements.

Return type: bigint

The nextval function can be invoked in either of the following ways: (In example 2, the Oracle syntax is supported. Currently, the sequence name cannot contain a dot.)

Example 1:

1
2
3
4
5
 
SELECT nextval('seqDemo'); 
 nextval
---------
       2
(1 row)

Example 2:

1
2
3
4
5
 
SELECT seqDemo.nextval; 
 nextval
---------
       2
(1 row)

currval(regclass)

Returns the last value of nextval for a specified sequence in the current session. If nextval has not been invoked for the specified sequence in the current session, an error is reported when currval is invoked. By default, currval is disabled. To enable it, set enable_beta_features to true. After currval is enabled, nextval will not be pushed down.

Return type: bigint

The currval function can be invoked in either of the following ways: (In example 2, the Oracle syntax is supported. Currently, the sequence name cannot contain a dot.)

Example 1:

1
2
3
4
5
 
SELECT currval('seq1'); 
 currval
---------
       2
(1 row)

Example 2:

1
2
3
4
5
 
SELECT seq1.currval seq1; 
 currval
---------
       2
(1 row)

lastval()

Returns the last value of nextval in the current session. This function is equivalent to currval, but lastval does not have a parameter. If nextval has not been invoked in the current session, an error is reported when lastval is invoked.

By default, lastval is disabled. To enable it, set enable_beta_features or lastval_supported to true. After lastval is enabled, nextval will not be pushed down.

Return type: bigint

For example:

1
2
3
4
5
 
SELECT lastval(); 
 lastval
---------
       2
(1 row)

setval(regclass, bigint)

Sets the current value of a sequence.

Return type: bigint

For example:

1
2
3
4
5
 
SELECT setval('seqDemo',1);
 setval
--------
      1
(1 row)

setval(regclass, bigint, boolean)

Sets the current value of a sequence and the is_called sign.

Return type: bigint

For example:

1
2
3
4
5
 
SELECT setval('seqDemo',1,true);
 setval
--------
      1
(1 row)

The current session and GTM will take effect immediately after setval is performed. If other sessions have buffered sequence values, setval will take effect only after the values are used up. Therefore, to prevent sequence value conflicts, you are advised to use setval with caution.

Because the sequence is non-transactional, changes made by setval will not be canceled when a transaction rolled back.

Parent topic: Functions and Operators