# 42P09 — Ambiguous table alias

> PostgreSQL SQLSTATE 42P09: Ambiguous table alias (ambiguous_alias), source-backed diagnosis and recovery guidance.
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# 42P09 — Ambiguous table alias

## At a glance {#at-a-glance}

`42P09` (**ambiguous_alias**) is a parser `ERROR` when one visible table reference resolves to more than one relation namespace item. It occurs before execution and has separate named-alias and relation-OID paths.

<!-- BEGIN SQLSTATE FACTS: generated by scripts/generate.py; do not edit -->

| Field | Value |
| --- | --- |
| SQLSTATE | `42P09` |
| Condition | `ambiguous_alias` |
| Status | `active` |
| Known present by | `7.4` |
| Locked snapshots | `9.0.23, 9.1.24, 9.2.24, 9.3.25, 9.4.26, 9.5.25, 9.6.24, 10.23, 11.22, 12.22, 13.23, 14.24, 15.19, 16.15, 17.11, 18.6, 19beta3` |
| Macros | `ERRCODE_AMBIGUOUS_ALIAS` |
| Aliases | `—` |

<!-- source facts: data/errcodes/42P09.json -->
<!-- END SQLSTATE FACTS -->

## Meaning {#meaning}

For a textual reference, `scanNameSpaceForRefname` compares visible namespace aliases and raises `table reference "%s" is ambiguous` when a second match is visible. Column-only items and lateral-only items outside an active LATERAL scope are excluded. A separate `scanNameSpaceForRelid` path matches unaliased relation RTEs by OID and raises `table reference %u is ambiguous` when the same relation appears more than once. Both include parser position and are distinct from column ambiguity.

## Diagnosis {#diagnosis}

Inspect the FROM/JOIN tree, alias names, subquery/CTE visibility, and LATERAL scope. If the primary includes a quoted name, find repeated visible aliases; if it includes an OID, look for repeated unaliased RTEs of that relation. Check whether a join alias hides an inner namespace or whether a LATERAL subquery makes both aliases visible. `search_path` can affect earlier relation lookup, but changing it is not a generic repair for a duplicate item already present in the parse namespace.

## Response {#response}

Give each visible relation a deliberate alias, qualify references through that alias, or remove the redundant relation from the query builder. Preserve intentional LATERAL scope and verify join cardinality after the change. Do not treat `42P09` as `42P01` (missing relation) or `42702` (ambiguous column), and do not repeatedly submit identical text. If the parser `ERROR` occurred inside an explicit transaction, roll back to a savepoint or roll back the transaction before continuing.

## Messages {#messages}

The selected variants are explicit `ERROR`s:

| Parser path | Primary |
| --- | --- |
| duplicate visible alias/name | `table reference "%s" is ambiguous` |
| duplicate unaliased relation OID | `table reference %u is ambiguous` |

Both attach parser position; `%s` and `%u` are dynamic source fields.

## Versions {#versions}

From 7.4 through 18.6 and 19beta3; that catalogue range is not a precise introduction claim. The selected named-alias and internal-OID branches are fixed 18.6 source paths, and runtime was not run.

## Related {#related}

- [`42702`](../42702/)
- [`42P08`](../42p08/)
- [`42P01`](../42p01/)

## Sources {#sources}
- `src.errcodes.42P09.18.6` — `src/backend/utils/errcodes.txt` line 389, fixed at `724edf9bde9d356724ad384a2e196edc3c9f80f7`; SHA-256 `6e8de346643ba84aa3c9c6a73360acfc7b2dfb89162c06c08ce9bf5bcd5bbcba` ([source](https://github.com/postgres/postgres/blob/724edf9bde9d356724ad384a2e196edc3c9f80f7/src/backend/utils/errcodes.txt#L389)).

- `src.call.42P09.ed0c1b7f7e4bd90cb2068e17` — `src/backend/parser/parse_relation.c` lines 224-228, fixed at `724edf9bde9d356724ad384a2e196edc3c9f80f7`; SHA-256 `0d9c88f4a8def4c2d982c33f13208590e21ecf5e6f8cd9d7faa223dc771c9a9a` ([source](https://github.com/postgres/postgres/blob/724edf9bde9d356724ad384a2e196edc3c9f80f7/src/backend/parser/parse_relation.c#L224-L228)).
- `src.call.42P09.171afa1503a4e198e620e1b5` — `src/backend/parser/parse_relation.c` lines 271-275, fixed at `724edf9bde9d356724ad384a2e196edc3c9f80f7`; SHA-256 `0d9c88f4a8def4c2d982c33f13208590e21ecf5e6f8cd9d7faa223dc771c9a9a` ([source](https://github.com/postgres/postgres/blob/724edf9bde9d356724ad384a2e196edc3c9f80f7/src/backend/parser/parse_relation.c#L271-L275)).
- `src.calls.REL_18_6.42P09` — resolved core call groups; SHA-256 `9ee8a0e81d8f0825c5c1ae45583439859a26e602bdd4ce2f2a62aa278867ccbf`.
- [`authored evidence`](../data/evidence/42p09.json) — source claims, message roles, and runtime boundary.