---
title: PostgreSQL sequences allocate values without promising gapless numbering
slug: postgres-sequences
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source: https://iinventedlight.com/library/notes/postgres-sequences.md
source_license: PostgreSQL
attribution: Original synthesis by iinventedlight, drafted by Librarian with AI assistance; based on the attributed sources
  below.
author: Librarian
client: Librarian
date: '2026-11-05'
checked_on: '2026-11-05'
as_of: '2026-11-05'
status: published
confidence: medium
category: coding-references
tags:
- postgres
- sequences
untested: Reference note. No code or commands were run to validate behavior.
sources:
- id: S1
  title: Sequence manipulation functions
  publisher: PostgreSQL Global Development Group
  url: https://www.postgresql.org/docs/current/functions-sequence.html
  accessed: '2026-11-05'
  license: PostgreSQL
  license_url: https://www.postgresql.org/about/licence/
  license_verified: '2026-11-05'
  green: true
  attribution: PostgreSQL Global Development Group; Sequence manipulation functions; PostgreSQL
  snapshot: _fetched/pg-seq.txt
- id: S2
  title: CREATE SEQUENCE
  publisher: PostgreSQL Global Development Group
  url: https://www.postgresql.org/docs/current/sql-createsequence.html
  accessed: '2026-11-05'
  license: PostgreSQL
  license_url: https://www.postgresql.org/about/licence/
  license_verified: '2026-11-05'
  green: true
  attribution: PostgreSQL Global Development Group; CREATE SEQUENCE; PostgreSQL
  snapshot: _fetched/pg-create.txt
---
# PostgreSQL sequences allocate values without promising gapless numbering

## 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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