Creating and Using Sequences
Creating and Using Sequences
A Greenplum Database sequence object is a special single row table that functions as a number generator. You can use a sequence to generate unique integer identifiers for a row that you add to a table. Declaring a column of type SERIAL implicitly creates a sequence counter for use in that table column.
Greenplum Database provides commands to create, alter, and drop a squence. Greenplum Database also provides built-in functions to return the next value in the sequence (nextval()) or to set the sequence to a specific start value (setval()).
Attributes of a sequence object include the name of the sequence, its increment value, and the last, minimum, and maximum values of the sequence counter. Sequences also have a special boolean attribute named is_called that governs the auto-increment behavior of a nextval() operation on the sequence counter. When a sequence's is_called attribute is true, nextval() increments the sequence counter before returning the value. When the is_called attribute value of a sequence is false, nextval() does not increment the counter before returning the value.
Creating a Sequence
The CREATE SEQUENCE command creates and initializes a sequence with the given sequence name and optional start value. The sequence name must be distinct from the name of any other sequence, table, index, or view in the same schema. For example:
CREATE SEQUENCE myserial START 101;
When you create a new sequence, Greenplum Database sets the sequence is_called attribute to false. Invoking nextval() on a newly-created sequence does not increment the sequence counter, but returns the sequence start value and sets is_called to true.
Using a Sequence
After you create a sequence with the CREATE SEQUENCE command, you can examine the sequence and use the sequence built-in functions.
Examining Sequence Attributes
To examine the current attributes of a sequence, query the sequence directly. For example, to examine a sequence named myserial:
SELECT * FROM myserial;
Returning the Next Sequence Counter Value
You can invoke the nextval() built-in function to return and use the next value in a sequence. The following command inserts the next value of the sequence named myserial into the first column of a table named vendors:
INSERT INTO vendors VALUES (nextval('myserial'), 'acme');
nextval() uses the sequence's is_called attribute value to determine whether or not to increment the sequence counter before returning the value. nextval() advances the counter when is_called is true. nextval() sets the sequence is_called attribute to true before returning.
A nextval() operation is never rolled back. A fetched value is considered used, even if the transaction that performed the nextval() fails. This means that failed transactions can leave unused holes in the sequence of assigned values.
Setting the Sequence Counter Value
You can use the Greenplum Database setval() built-in function to set the counter value for a sequence. For example, the following command sets the counter value of the sequence named myserial to 201:
SELECT setval('myserial', 201);
setval() has two function signatures: setval(sequence, start_val) and setval(sequence, start_val, is_called). The default behaviour of setval( sequence, start_val) sets the sequence is_called attribute value to true.
If you do not want the sequence counter advanced on the next nextval() call, use the setval( sequence, start_val, is_called) function signature, passing a false argument:
SELECT setval('myserial', 201, false);
setval() operations are never rolled back.
Altering a Sequence
The ALTER SEQUENCE command changes the attributes of an existing sequence. You can alter the sequence minimum, maximum, and increment values. You can also restart the sequence at a specific value.
Any parameters not set in the ALTER SEQUENCE command retain their prior settings.
The following command restarts the sequence named myserial at value 105:
ALTER SEQUENCE myserial RESTART WITH 105;
When you alter a sequence start value with the ALTER SEQUENCE command, Greenplum Database sets the sequence's is_called attribute to false. The first nextval() invoked after restarting a sequence does not advance the sequence counter, but returns the sequence restart value and sets is_called to true.
Dropping a Sequence
The DROP SEQUENCE command removes a sequence. For example, the following command removes the sequence named myserial:
DROP SEQUENCE myserial;
Specifying a Sequence as the Default Value for a Column
You can reference a sequence directly in the CREATE TABLE command in addition to using the SERIAL or BIGSERIAL types. For example:
CREATE TABLE tablename ( id INT4 DEFAULT nextval('myserial'), name text );
You can also alter a table column to set its default value to a sequence counter:
ALTER TABLE tablename ALTER COLUMN id SET DEFAULT nextval('myserial');
By default, a sequence does not wrap around. That is, when a sequence reaches the max value (+32767 for SMALLSERIAL, +2147483647 for SERIAL, +9223372036854775807 for BIGSERIAL), every subsequent nextval() call produces an error. You can alter a sequence to make it cycle around and start at 1 again:
ALTER SEQUENCE myserial CYCLE;
You can also specify the wraparound behaviour when you create the sequence:
CREATE SEQUENCE myserial CYCLE;