This section describes functions for operating on sequence objects, also called sequence generators or just sequences. Sequence objects are special single-row tables created with CREATE SEQUENCE. Sequence objects are commonly used to generate unique identifiers for rows of a table. The sequence functions, listed in Table 9.51, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Table 9.51. Sequence Functions
Function Description |
---|
Advances the sequence object to its next value and returns that value.
This is done atomically: even if multiple sessions
execute
This function requires |
Sets the sequence object's current value, and optionally
its SELECT setval('myseq', 42); Next
The result returned by
This function requires |
Returns the value most recently obtained
by
This function requires |
Returns the value most recently returned by
This function requires |
To avoid blocking concurrent transactions that obtain numbers from
the same sequence, the value obtained by nextval
is not reclaimed for re-use if the calling transaction later aborts.
This means that transaction aborts or database crashes can result in
gaps in the sequence of assigned values. That can happen without a
transaction abort, too. For example an INSERT
with
an ON CONFLICT
clause will compute the to-be-inserted
tuple, including doing any required nextval
calls, before detecting any conflict that would cause it to follow
the ON CONFLICT
rule instead.
Thus, LightDB sequence
objects cannot be used to obtain “gapless”
sequences.
Likewise, sequence state changes made by setval
are immediately visible to other transactions, and are not undone if
the calling transaction rolls back.
If the database cluster crashes before committing a transaction
containing a nextval
or setval
call, the sequence state change might
not have made its way to persistent storage, so that it is uncertain
whether the sequence will have its original or updated state after the
cluster restarts. This is harmless for usage of the sequence within
the database, since other effects of uncommitted transactions will not
be visible either. However, if you wish to use a sequence value for
persistent outside-the-database purposes, make sure that the
nextval
call has been committed before doing so.
The sequence to be operated on by a sequence function is specified by
a regclass
argument, which is simply the OID of the sequence in the
pg_class
system catalog. You do not have to look up the
OID by hand, however, since the regclass
data type's input
converter will do the work for you. Just write the sequence name enclosed
in single quotes so that it looks like a literal constant. For
compatibility with the handling of ordinary
SQL names, the string will be converted to lower case
unless it contains double quotes around the sequence name. Thus:
nextval('foo') operates on sequencefoo
nextval('FOO') operates on sequencefoo
nextval('"Foo"') operates on sequenceFoo
The sequence name can be schema-qualified if necessary:
nextval('myschema.foo') operates onmyschema.foo
nextval('"myschema".foo') same as above nextval('foo') searches search path forfoo
See Section 8.19 for more information about
regclass
.
Before LightDB 8.1, the arguments of the
sequence functions were of type text
, not regclass
, and
the above-described conversion from a text string to an OID value would
happen at run time during each call. For backward compatibility, this
facility still exists, but internally it is now handled as an implicit
coercion from text
to regclass
before the function is
invoked.
When you write the argument of a sequence function as an unadorned
literal string, it becomes a constant of type regclass
.
Since this is really just an OID, it will track the originally
identified sequence despite later renaming, schema reassignment,
etc. This “early binding” behavior is usually desirable for
sequence references in column defaults and views. But sometimes you might
want “late binding” where the sequence reference is resolved
at run time. To get late-binding behavior, force the constant to be
stored as a text
constant instead of regclass
:
nextval('foo'::text) foo
is looked up at runtime
Of course, the argument of a sequence function can be an expression as well as a constant. If it is a text expression then the implicit coercion will result in a run-time lookup.