Databases - Practical PostgreSQL

Databases - Practical PostgreSQL
Previous Page	Home	Next Page

Sequences

A sequence in PostgreSQL is a database object that is essentially an automatically incrementing numeric value. For this reason, sequences are commonly known in other database products as auto-increment values. Sequences can be extremely useful in assigning non-random, unique identification numbers to tables that require such values. A sequence consists of a current numeric value, and a set of characteristics that determine how to automatically increment (or alternatively, decrement) that value upon use.

Along with its current value, a sequence also includes a minimum value, a maximum value, a starting value, and the amount to increment the sequence by. This increment is usually 1, but may be any whole integer.

In practice, sequences are not meant to be accessed directly. Instead, they are used through a set of functions built into PostgreSQL which either set, increment, or return the current value of the sequence.

Creating a sequence

Sequences are created with the CREATE SEQUENCE SQL command. The sequence can be specified to increment or decrement. The syntax for CREATE SEQUENCE is:

  CREATE SEQUENCE 
sequencename

         [ INCREMENT 
increment
 ]
         [ MINVALUE 
minvalue
 ]
         [ MAXVALUE 
maxvalue
 ]
         [ START 
start
 ]
         [ CACHE 
cache
 ]
         [ CYCLE ]

In this syntax, sequencename is the name of the sequence to be created. This is the only required parameter. A sequence uses the integer data type, and it therefore shares its maximum and minimum limitations of 2147483647 and –2147483647, respectively.

The optional CREATE SEQUENCE clauses are as follows:

INCREMENT increment_val

Sets the numeric quantity with which to modify the sequence's value to increment_val. This is used when the nextval() function is called on the sequence. Setting increment_val to a negative number results in a descending sequence. The default value is 1.

MINVALUE minvalue

Sets the fixed minimum value for the sequence to minvalue. Any attempt to lower a sequence below this value will result in an error, or in the value cycling to its maximum value (if the CYCLE keyword was used when the sequence was created).

The default value is 1 for ascending sequences, and –2147483647 for descending sequences.

MAXVALUE maxvalue

Sets the fixed maximum value for the sequence to maxvalue. Any attempt to raise a sequence above this value will result in an error, or in the value cycling to its minimum value.

The default value is 2147483647 for ascending sequences, and –1 for descending sequences.

START start_val

Sets the value that the sequence begins at. It may be any integer between the minimum and maximum values. The sequence defaults to start at its minimum value for ascending sequences, and its maximum value for descending sequences.

CACHE cache

Provides the ability for sequence values to be pre-allocated and stored in memory. This can result in faster access times to highly used sequences. The minimum and default value is 1; a higher value of cache results in more values being cached.

CYCLE

Enables the sequence to continue generating new values after it has reached its maximum or minimum value. When the limit is reached, the sequence starts over at the minimum value (for ascending sequences), or at the maximum value (descending sequences).

Example 7-28 creates a simple ascending sequence named shipments_ship_id_seq that starts at a value of 0, and will be incremented by the default increment of 1 until it reaches the default maximum limit of 2147483647. By not using the CYCLE keyword, the sequence is guaranteed to always return a unique value.

Example 7-28. Creating a sequence

booktown=# 
CREATE SEQUENCE shipments_ship_id_seq

booktown-# 
                MINVALUE 0;

CREATE

Viewing a sequence

The output from the \d command within psql shows whether or not a database object is a sequence, table, view or index. More specifically, the \ds command can be used to view all sequences in the currently connected database. For example:

booktown=# 
\ds

             List of relations
         Name          |   Type   |  Owner
-----------------------+----------+---------
 book_ids              | sequence | manager
 shipments_ship_id_seq | sequence | manager
 subject_ids           | sequence | manager
(3 rows)

While not often necessary, sequences can be directly queried with SELECT statements, as if they were a table or view. When you query a sentence, you use the attributes of that sequence as columns in your select list. The attributes of a sequence are shown in Table 7-1.

Table 7-1. Sequence attributes

Attribute	Type
`sequence_name`	`name`
`last_value`	`integer`
`increment_by`	`integer`
`max_value`	`integer`
`min_value`	`integer`
`cache_value`	`integer`
`log_cnt`	`integer`
`is_cycled`	`"char"`
`is_called`	`"char"`

Example 7-29 illustrates a query to the shipments_ship_id_seq sequence. This query selects the last_value attribute, which is the most currently selected value from the sequence, and the increment_by attribute, which is the amount the sequence is to be incremented each time the nextval() function is called.

Example 7-29. Viewing a sequence

booktown=# 
SELECT last_value, increment_by

booktown-# 
       FROM shipments_ship_id_seq;

 last_value | increment_by
------------+--------------
          0 |            1
(1 row)

Since the sequence in question has just been created, its last_value is still set to 0.

Using a sequence

Sequences are typically not queried directly, but are instead used through functions. There are three functions in PostgreSQL which apply exclusively to sequences:

nextval(' sequence_name '): Increments the value of the specified sequence named sequence_name, and returns the new value, which is of type integer.
currval(' sequence_name '): Returns the most recently returned value from nextval(' sequence_name '). This value is associated with a PostgreSQL session, and if the nextval() function has not yet been called in the connected session on the specified sequence sequence_name, there will be no "current" value returned.
setval(' sequence_name ', n ): Sets the current value of the specified sequence to the numeric value n. The value returned by the next call to nextval() will return n + increment, where increment is the amount that the sequence increments by each iteration.
setval(' sequence_name ', n , b ): Also sets the current value of the specified sequence to the numeric value n. However, if b (a value of type boolean) is false , the value returned by the next call to nextval() will be just n . If b is true , the next call to nextval() will return n + increment , as it would without specifying the Boolean argument at all.

The most commonly used sequence function is nextval(). This is the function that actually pushes the increment of the value. It requires the name of the sequence as the argument (bound by single quotes), and returns a value of type integer.

Example 7-30 selects a couple of incremented values from the sequence named shipments_ship_id_seq.

Example 7-30. Incrementing a sequence

booktown=# 
SELECT nextval('shipments_ship_id_seq');

 nextval
---------
       1
(1 row)

booktown=# 
SELECT nextval('shipments_ship_id_seq');

 nextval
---------
       2
(1 row)

Note: The first call to nextval() will return the sequence's initial value (set by the START keyword), since it has not yet been called to increment the starting value. All subsequent calls increment the last_value column.

Sequences are commonly used as default values for tables which require unique integer identifiers. The shipments table within the booktown database, shown in Table 7-2, exemplifies this.

Table 7-2. The shipments table

Column	Type	Modifier
`id`	`integer`	`NOT NULL DEFAULT nextval('shipments_ship_id_seq')`
`customer_id`	`integer`
`isbn`	`text`
`ship_date`	`timestamp with time zone`

The syntax to create the table in Table 7-2, with the auto-incrementing DEFAULT and PRIMARY KEY constraint, is:

  CREATE TABLE shipments
         (id integer DEFAULT nextval('"shipments_ship_id_seq"'::text)
                     PRIMARY KEY,
          customer_id integer,
          isbn text,
          ship_date timestamp)

The default value for the id column in Table 7-2 is set to the nextval()'s result on the shipments_ship_id_seq sequence. Insertion of row data that does not specify a value for id will therefore choose its value from the result of this function call.

Warning

Merely placing a DEFAULT constraint on the id column does not enforce the use of that default. A user could still manually insert a value, potentially causing a conflict with future sequence values. This can be disallowed with the use of a trigger. See the Section called Triggers " later in this chapter for more information.

After the nextval() function has been called on a sequence in a given session (a connection to PostgreSQL), the currval() function may be used on that same sequence to return the most recently returned value from the sequence. Note that this function may only be called on a sequence that has been called through nextval() in the active session.

Note: Sequences' "current" values are associated with sessions in order to prevent multiple users from running into mistakes by accessing the same sequence at the same time. Two users may access the same sequence from separate sessions, but the currval() function will return only the most recently incremented value of the sequence from within the same session that calls currval().

Example 7-31 inserts a new row into the shipments column, without specifying the value for the id column. This causes the default value to be used, which (as noted in Table 7-2) is the result of the shipments_ship_id_seq being incremented by the nextval() function. The currval() function is then used to access the row that was just inserted.

Example 7-31. Using currval( )

booktown=# 
INSERT INTO shipments (customer_id, isbn, ship_date)

booktown-# 
            VALUES (221, '0394800753', 'now');

INSERT 3628625 1
booktown=# 
SELECT * FROM shipments

booktown-# 
         WHERE id = currval('shipments_ship_id_seq');

  id  | customer_id |    isbn    |       ship_date
------+-------------+------------+------------------------
 1002 |         107 | 0394800753 | 2001-09-22 11:23:28-07
(1 row)

Finally, a sequence may also have its last_value attribute reset to an arbitrary numeric value (within its maximum and minimum value range) by using the setval() function. This requires the name of the sequence as a single-quote bound character string for the first argument and an integer constant representing the new value for last_value for the second argument.

There are two ways to go about this. By default, setval() assumes that the new setting is for an initialized sequence; this means that the next value returned by nextval() will actually be incremented once past the value set by setval().

Alternatively, an optional false value of type boolean may be added as the last argument to setval(), de-initializing the sequence. This modifies the sequence so that the next value returned by nextval() will be the same numeric value passed to setval() (though the sequence will of course be incremented on the next call to nextval()).

Example 7-32 sets the shipments_ship_id_seq's last_value to 1010 through each method, and selects the nextval() on the same sequence to illustrate the effective result.

Example 7-32. Setting a sequence value

booktown=# 
SELECT setval('shipments_ship_id_seq', 1010);

 setval
--------
   1010
(1 row)

booktown=# 
SELECT nextval('shipments_ship_id_seq');

 nextval
---------
    1011
(1 row)

booktown=# 
SELECT setval('shipments_ship_id_seq', 1010, false);

 setval
--------
   1010
(1 row)

booktown=# 
SELECT nextval('shipments_ship_id_seq');

 nextval
---------
    1010
(1 row)

Warning

Sequences are commonly used to ensure unique values in a column. Be sure that you understand the application of a sequence before you reset its last_value attribute.

Destroying a sequence

To destroy a sequence, or several sequences simultaneously, use the DROP SEQUENCE SQL command. Here is the syntax for DROP SEQUENCE:

  DROP SEQUENCE 
sequencename
 [, ...]

In this syntax, sequencename is the name of the sequence that you wish to remove. Multiple sequence names may be specified, separated by commas.

Example 7-33 removes the shipments_ship_id_seq sequence.

Example 7-33. Removing a sequence

booktown=# 
DROP SEQUENCE shipments_ship_id_seq;

DROP

Before destroying a sequence, make sure that the sequence is not used by another table, function, or any other object in the database. If this check is not performed, then other operations that rely on the sequence will fail. The following query will return the name of any relation which relies on a default sequence value, where sequence_name is the name of the sequence you are interesting in finding dependencies for:

  SELECT p.relname, a.adsrc FROM pg_class p 
         JOIN pg_attrdef a ON (p.relfilenode = a.adrelid)
         WHERE a.adsrc ~ '"
sequence_name
 "';

Example 7-34 uses this query to look up the name of any table with a default value involving the shipments_ship_id_seq sequence.

Example 7-34. Checking sequence dependencies

booktown=# 
SELECT p.relname, a.adsrc FROM pg_class p JOIN pg_attrdef a

booktown-# 
                          ON (p.relfilenode = a.adrelid)

booktown-# 
       WHERE a.adsrc ~ '"shipments_ship_id_seq"';

  relname  |                  adsrc
-----------+------------------------------------------
 shipments | nextval('"shipments_ship_id_seq"'::text)
(1 row)

Databases - Practical PostgreSQL
Previous Page	Home	Next Page