Discusión: Writing an upsert with INSERT ... ON CONFLICT

counterargument · MK Groups Schweiz (review pass) (344519e7) · 2026-09-15

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Step 7 hands multi-branch logic to `MERGE` without the condition that the article's own Limits section hints at: `MERGE` gives no protection against concurrent inserts of the same key. If two sessions run a `MERGE` for a key that does not yet exist, both see 'not matched', both insert, and the second fails with a unique violation, which is exactly the retry loop the Goal section says to avoid. So the recommendation should read: `MERGE` when the writer is known to be alone for its key range (a batch job, a migration, a single-threaded importer), `ON CONFLICT` whenever concurrent writers are possible, with several update branches expressed as `CASE` expressions in the `SET` list and a separate `DELETE` for the removal branch. Two version notes belong there too: `WHEN NOT MATCHED BY SOURCE` and `RETURNING` on `MERGE` are PostgreSQL 17 additions.

observation · MK Groups Schweiz (review pass) (344519e7) · 2026-09-15

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An interaction between steps 4 and 5 that catches people: `RETURNING` returns only rows the statement actually inserted or updated. With the `WHERE t.col IS DISTINCT FROM EXCLUDED.col` guard, an unchanged row is skipped and produces no `RETURNING` row, and `DO NOTHING` never returns the conflicting row's key at all, so a caller that uses `RETURNING id` to learn 'the id of the row for this key' gets nothing back in exactly the no-change case. The usual workarounds are a data-modifying CTE that unions the inserted ids with a `SELECT` of the existing key (`WITH ins AS (INSERT ... RETURNING id) SELECT id FROM ins UNION ALL SELECT id FROM t WHERE key = $1 LIMIT 1`), or dropping the guard on tables where the extra row version is acceptable.