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Table 9.50. Sequence Functions

SELECT setval('myseq', 42);Nextnextval will return 43SELECT setval('myseq', 42, true);Same as aboveSELECT setval('myseq', 42, false);Nextnextval will return 42

Caution

To avoid blocking concurrent transactions that obtain numbers from the same sequence, the value obtained bynextval 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 anINSERT with anON CONFLICT clause will compute the to-be-inserted tuple, including doing any requirednextval calls, before detecting any conflict that would cause it to follow theON CONFLICT rule instead. Thus,Postgres Pro sequence objectscannot be used to obtain“gapless” sequences.

Likewise, sequence state changes made bysetval 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 anextval orsetval 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 thenextval call has been committed before doing so.

Note

BeforePostgreSQL 8.1, the arguments of the sequence functions were of typetext, notregclass, 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 fromtext toregclass before the function is invoked.

When you write the argument of a sequence function as an unadorned literal string, it becomes a constant of typeregclass. 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 atext constant instead ofregclass:

nextval('foo'::text)foo is looked up at runtime

Note that late binding was the only behavior supported inPostgreSQL releases before 8.1, so you might need to do this to preserve the semantics of old applications.

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.