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.

NOTE: 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, MogDB sequences cannot be used to obtain sequence without gaps.

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

Return type: bigint

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

Example 1:
```
mogdb=# select nextval('seqDemo');
 nextval
---------
       2
(1 row)
```
Example 2:
```
mogdb=# 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.

Return type: bigint

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

Example 1:
```
mogdb=# select currval('seq1');
 currval
---------
       2
(1 row)
```
Example 2:
```
mogdb=# select seq1.currval;
 currval
---------
       2
(1 row)
```
lastval()

Description: 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, invoking lastval will report an error.

Return type: bigint

Example:
```
mogdb=# select lastval();
 lastval
---------
       2
(1 row)
```
setval(regclass, bigint)

Description: Sets the current value of a sequence.

Return type: bigint

Example:
```
mogdb=# select setval('seqDemo',1);
 setval
--------
      1
(1 row)
```
setval(regclass, bigint, Boolean)

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

Return type: bigint

Example:
```
mogdb=# select setval('seqDemo',1,true);
 setval
--------
      1
(1 row)
```
NOTE: 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.

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

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