Help Center/ GaussDB(DWS)/ Standard Data Warehouse (8.1.3.x)/ Defining Database Objects/ Creating and Using Sequences
Updated on 2023-10-31 GMT+08:00
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Creating and Using Sequences

A sequence is a database object that generates unique integers according to a certain rule and is usually used to generate primary key values.

You can create a sequence for a column in either of the following methods:
  • Set the data type of a column to sequence integer. A sequence will be automatically created by the database for this column.
  • Run the CREATE SEQUENCE statement to create a sequence. Set the initial value of the nextval('sequence_name') function to the default value of a column.

Creating a Sequence.

Method 1: Set the data type of a column to a sequence integer. For example:

Method 2: Create a sequence and set the initial value of the nextval('sequence_name') function to the default value of a column. You can cache a specific number of sequence values to reduce the requests to the GTM, improving the performance.

  1. Create a sequence.
  2. Set the initial value of the nextval('sequence_name') function to the default value of a column.

Methods 1 and 2 are similar except that method 2 specifies cache for the sequence. A sequence using cache has holes (non-consecutive values, for example, 1, 4, 5) and cannot keep the order of the values. After a sequence is deleted, its sub-sequences will be deleted automatically. A sequence shared by multiple columns is not forbidden in a database, but you are not advised to do that.

Currently, the preceding two methods cannot be used for existing tables.

Modifying a Sequence

The ALTER SEQUENCE statement changes the attributes of an existing sequence, including the owner, owning column, and maximum value.

  • Associate the sequence with a column.

    The sequence will be deleted when you delete the column or the table where the column resides.

  • Modify the maximum value of serial to 300.

Deleting a Sequence

Run the DROP SEQUENCE command to delete a sequence. For example, to delete the sequence named seq1, run the following command:

Precautions

Sequence values are generated by the GTM. By default, each request for a sequence value is sent to the GTM. The GTM calculates the result of the current value plus the step and then returns the result. The GTM is the only node that can generate sequence values and probably becomes the performance bottleneck. Therefore, you are not advised to use sequences when sequence values need to be generated frequently (for example, using BulkLoad to import data). For example, the INSERT FROM SELECT statement has poor performance in the following scenario:

To improve the performance, run the following statements (assume that data of 10,000 rows will be imported from T1 to newT1):

Rollback is not supported by sequence functions, including nextval() and setval(). The value of the setval function immediately takes effects on nextval in the current session in any cases and takes effects in other sessions only when no cache is specified for them. If cache is specified for a session, it takes effect only after all the cached values have been used. To avoid duplicate values, use setval only when necessary. Do not set it to an existing sequence value or a cached sequence value.

If BulkLoad is used, set sufficient cache for newSeq1 and do not set Maxvalue or Minvalue. To improve the performance, database may push down the invocation of nextval('sequence_name') to DNs. Currently, the concurrent connection requests that can be processed by the GTM are limited. If there are too many DNs, a large number of concurrent connection requests will be sent to the GTM. In this case, you need to limit the concurrent connection of BulkLoad to save the GTM connection resources. If the target table is a replication table (DISTRIBUTE BY REPLICATION), pushdown cannot be performed. If the data volume is large, this will be a disaster for the database. In addition, the database space may be exhausted. After the import is complete, do VACUUM FULL. Therefore, you are not advised to use sequences when BulkLoad is used.

After a sequence is created, a single-row table is maintained on each node to store the sequence definition and value, which is obtained from the last interaction with the GTM rather than updated in real time. The single-row table on a node does not update when other nodes request a new value from the GTM or when the sequence is modified using setval.