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Summary

nextval advances a PostgreSQL sequence atomically, so concurrent callers receive distinct allocated values. Allocation is not rolled back merely because the calling transaction aborts. [S1]

Details

currval reports the last value obtained for that sequence in the current session, not the latest value across all sessions. CREATE SEQUENCE makes the generator used by these functions. Since failed transactions and conflicts can consume values, a sequence is not a gapless counter. setval also changes sequence state outside ordinary transaction rollback behavior. [S1, S2]

Claims

• id: C1 claim: "nextval allocates distinct values atomically across concurrent callers." source: S1 evidence: "each will safely receive a distinct sequence value"
• id: C2 claim: "Aborted transactions do not reclaim a nextval allocation." source: S1 evidence: "not reclaimed for re-use if the calling transaction later aborts"
• id: C3 claim: "CREATE SEQUENCE creates a sequence generator." source: S2 evidence: "CREATE SEQUENCE creates a new sequence number generator"

Sources

• [S1] Sequence manipulation functions. PostgreSQL Global Development Group. https://www.postgresql.org/docs/current/functions-sequence.html. PostgreSQL; license: https://www.postgresql.org/about/licence/. Accessed 2026-11-05.
• [S2] CREATE SEQUENCE. PostgreSQL Global Development Group. https://www.postgresql.org/docs/current/sql-createsequence.html. PostgreSQL; license: https://www.postgresql.org/about/licence/. Accessed 2026-11-05.

Attribution and notices

This note is CC BY 4.0. Source material has been condensed and restated; short evidence excerpts retain their source license.

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@misc{iil_note_gfnwrhq9,
  author = {{Librarian}},
  title = {{PostgreSQL sequences allocate values without promising gapless numbering}},
  howpublished = {iinventedlight Library},
  url = {https://iinventedlight.com/library/notes/postgres-sequences/},
  note = {Permanent ID iil:note:gfnwrhq9. License: CC-BY-4.0.},
}

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