SEQUENCE Functions

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

• nextval(regclass)

  Description: Specifies an increasing sequence and returns a new value.

  

  To avoid blocking of concurrent transactions that obtain numbers from the same sequence, a nextval operation is never rolled back; that is, once a value has been fetched it is considered used, even if the transaction that did the nextval later aborts. This means that aborted transactions may leave unused "holes" in the sequence of assigned values. Therefore, GaussDB sequences cannot be used to obtain sequence without gaps.

  

  The nextval function can be executed only on the primary node. It is not supported on standby nodes.

  Return type: numeric

  The nextval function can be called in either of the following ways: (In example 2, the sequence name cannot contain a dot.)

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  openGauss=# CREATE SEQUENCE seqDemo; -- Example 1: openGauss=# SELECT nextval('seqDemo'); nextval --------- 1 (1 row) -- Example 2: openGauss=# SELECT seqDemo.nextval; nextval --------- 2 (1 row) openGauss=# DROP SEQUENCE seqDemo;

• currval(regclass)

  Description: Returns the last value returned by nextval in the current session. If nextval has not been called for the specified sequence in the current session, an error is reported when currval is called.

  Return type: numeric

  The currval function can be called in either of the following ways: (In example 2, the sequence name cannot contain a dot.)

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  openGauss=# CREATE SEQUENCE seq1; openGauss=# SELECT nextval('seq1'); -- Example 1: openGauss=# SELECT currval('seq1'); currval --------- 1 (1 row) -- Example 2: openGauss=# SELECT seq1.currval; currval --------- 1 (1 row) openGauss=# DROP SEQUENCE seq1;

• lastval()

  Description: Returns the last value returned by nextval in the current session. This function is equivalent to currval, except that it does not use sequence names as parameters. It fetches the sequence used by nextval last time in the current session. If nextval has not been called in the current session, an error is reported when lastval is called.

  Return type: numeric

  Example:

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  openGauss=# CREATE SEQUENCE seq1; openGauss=# SELECT nextval('seq1'); openGauss=# SELECT lastval(); lastval --------- 1 (1 row) openGauss=# DROP SEQUENCE seq1;

• setval(regclass, numeric)

  Description: Sets the current value of a sequence.

  Return type: numeric

  Example:

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  openGauss=# CREATE SEQUENCE seqDemo; openGauss=# SELECT nextval('seqDemo'); openGauss=# SELECT setval('seqDemo',5); setval -------- 5 (1 row) openGauss=# DROP SEQUENCE seqDemo;

• setval(regclass, numeric, Boolean)

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

  Return type: numeric

  Example:

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  openGauss=# CREATE SEQUENCE seqDemo; openGauss=# SELECT nextval('seqDemo'); openGauss=# SELECT setval('seqDemo',5,true); setval -------- 5 (1 row) openGauss=# DROP SEQUENCE seqDemo;

  

  The current session 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, the change caused by setval will not be undone by transaction rollback.

  

  The nextval function can be executed only on the primary node. It is not supported on standby nodes.

• pg_sequence_last_value(sequence_oid oid, OUT cache_value int16, OUT last_value int16)

  Description: Obtains the parameters of a specified sequence, including the cache value and current value.

  Return type: int16, int16