CREATE SEQUENCSQL)- Language Statements (2002-1CREATE SEQUENCE(l)
NAME
CREATE SEQUENCE - define a new sequence generator
SYNOPSIS
CREATE [ TEMPORARY | TEMP ] SEQUENCE seqname [ INCREMENT increment ]
[ MINVALUE minvalue ] [ MAXVALUE maxvalue ]
[ START start ] [ CACHE cache ] [ CYCLE ]
INPUTS
TEMPORARY or TEMP
If specified, the sequence object is created only for
this session, and is automatically dropped on session
exit. Existing permanent sequences with the same name
are not visible (in this session) while the temporary
sequence exists, unless they are referenced with
schema-qualified names.
seqname
The name (optionally schema-qualified) of a sequence to
be created.
increment
The INCREMENT increment clause is optional. A positive
value will make an ascending sequence, a negative one a
descending sequence. The default value is one (1).
minvalue
The optional clause MINVALUE minvalue determines the
minimum value a sequence can generate. The defaults are
1 and -2^63-1 for ascending and descending sequences,
respectively.
maxvalue
The optional clause MAXVALUE maxvalue determines the
maximum value for the sequence. The defaults are 2^63-1
and -1 for ascending and descending sequences,
respectively.
start
The optional START start clause enables the sequence to
begin anywhere. The default starting value is minvalue
for ascending sequences and maxvalue for descending
ones.
cache
The CACHE cache option enables sequence numbers to be
preallocated and stored in memory for faster access.
The minimum value is 1 (only one value can be generated
at a time, i.e., no cache) and this is also the
default.
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CREATE SEQUENCSQL)- Language Statements (2002-1CREATE SEQUENCE(l)
CYCLE
The optional CYCLE keyword may be used to enable the
sequence to wrap around when the maxvalue or minvalue
has been reached by an ascending or descending sequence
respectively. If the limit is reached, the next number
generated will be the minvalue or maxvalue,
respectively. Without CYCLE, after the limit is
reached nextval calls will return an error.
OUTPUTS
CREATE SEQUENCE
Message returned if the command is successful.
ERROR: Relation 'seqname' already exists
If the sequence specified already exists.
(max)
ERROR: DefineSequence: MINVALUE (start) can't be >= MAXVALUE
If the specified starting value is out of range.
(min)
ERROR: DefineSequence: START value (start) can't be < MINVALUE
If the specified starting value is out of range.
ERROR: DefineSequence: MINVALUE (min) can't be >= MAXVALUE (max)
If the minimum and maximum values are inconsistent.
DESCRIPTION
CREATE SEQUENCE will enter a new sequence number generator
into the current database. This involves creating and
initializing a new single-row table with the name seqname.
The generator will be owned by the user issuing the command.
If a schema name is given then the sequence is created in
the specified schema. Otherwise it is created in the current
schema (the one at the front of the search path; see
CURRENT_SCHEMA()). TEMP sequences exist in a special
schema, so a schema name may not be given when creating a
TEMP sequence. The sequence name must be distinct from the
name of any other sequence, table, index, or view in the
same schema.
After a sequence is created, you use the functions nextval,
currval and setval to operate on the sequence. These
functions are documented in the User's Guide.
Although you cannot update a sequence directly, you can use
a query like
SELECT * FROM seqname;
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CREATE SEQUENCSQL)- Language Statements (2002-1CREATE SEQUENCE(l)
to examine the parameters and current state of a sequence.
In particular, the last_value field of the sequence shows
the last value allocated by any backend process. (Of course,
this value may be obsolete by the time it's printed, if
other processes are actively doing nextval calls.)
Caution: Unexpected results may be obtained if a cache
setting greater than one is used for a sequence object
that will be used concurrently by multiple backends.
Each backend will allocate and cache successive
sequence values during one access to the sequence
object and increase the sequence object's last_value
accordingly. Then, the next cache-1 uses of nextval
within that backend simply return the preallocated
values without touching the shared object. So, any
numbers allocated but not used within a session will be
lost when that session ends. Furthermore, although
multiple backends are guaranteed to allocate distinct
sequence values, the values may be generated out of
sequence when all the backends are considered. (For
example, with a cache setting of 10, backend A might
reserve values 1..10 and return nextval=1, then backend
B might reserve values 11..20 and return nextval=11
before backend A has generated nextval=2.) Thus, with a
cache setting of one it is safe to assume that nextval
values are generated sequentially; with a cache setting
greater than one you should only assume that the
nextval values are all distinct, not that they are
generated purely sequentially. Also, last_value will
reflect the latest value reserved by any backend,
whether or not it has yet been returned by nextval.
Another consideration is that a setval executed on such
a sequence will not be noticed by other backends until
they have used up any preallocated values they have
cached.
NOTES
Use DROP SEQUENCE to remove a sequence.
Sequences are based on bigint arithmetic, so the range
cannot exceed the range of an eight-byte integer (-
9223372036854775808 to 9223372036854775807). On some older
platforms, there may be no compiler support for eight-byte
integers, in which case sequences use regular integer
arithmetic (range -2147483648 to +2147483647).
When cache is greater than one, each backend uses its own
cache to store preallocated numbers. Numbers that are
cached but not used in the current session will be lost,
resulting in ``holes'' in the sequence.
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CREATE SEQUENCSQL)- Language Statements (2002-1CREATE SEQUENCE(l)
USAGE
Create an ascending sequence called serial, starting at 101:
CREATE SEQUENCE serial START 101;
Select the next number from this sequence:
SELECT nextval('serial');
nextval
-------
114
Use this sequence in an INSERT:
INSERT INTO distributors VALUES (nextval('serial'), 'nothing');
Update the sequence value after a COPY FROM:
BEGIN;
COPY distributors FROM 'input_file';
SELECT setval('serial', max(id)) FROM distributors;
END;
COMPATIBILITY
SQL92
CREATE SEQUENCE is a PostgreSQL language extension. There
is no CREATE SEQUENCE statement in SQL92.
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