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Date: Mon, 28 Apr 2025 14:55:58 +1000
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To: pgsql-general@lists.postgresql.org
From: Tim Starling <tstarling@wikimedia.org>
Subject: Upsert error "column reference is ambiguous"
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Regarding upsert syntax.

psql (16.8 (Ubuntu 16.8-0ubuntu0.24.04.1), server 14.13 (Ubuntu 
14.13-0ubuntu0.22.04.1))
=> CREATE TABLE t (k INTEGER, v INTEGER);
=> CREATE UNIQUE INDEX t_k ON t (k);
=> INSERT INTO t VALUES (1,1);
INSERT 0 1
=> INSERT INTO t VALUES (1,1) ON CONFLICT (k) DO UPDATE SET v=v+1;
ERROR:  column reference "v" is ambiguous

Please convince me that this is not a bug.

If I understand correctly, in the expression "v+1", both EXCLUDED.v 
and t.v are present as the unqualified name "v". This is always the 
case and it is never possible to reference an unqualified field name 
in the expression of a conflict action.

Thus, any query with an unqualified name is statically known to be 
invalid. It is not a b/c break to remove EXCLUDED.v from the list of 
unqualified fields in a new major release of PG, thus allowing it to DWIM.

I'm a maintainer of MediaWiki. Some kind person contributed PostgreSQL 
support many years ago so now I am required to maintain it in 
perpetuity. The work seems out of proportion to the benefit, but 
that's the industry I guess. A handful of users benefit, such as 
wiki.postgresql.org.

Our application has an upsert method which takes the assignment 
"v=v+1" as a string. It is feasible to split it on the equals sign 
into the destination field and expression components, but it is not 
feasible to parse the expression or to require callers to supply an 
AST tree for the expressions they give us. It is not feasible to 
require callers to prefix all field names with the table name.

We currently emulate upsert on PostgreSQL using several awkward and 
inefficient queries. It would be nice to be able to use PostgreSQL's 
native upsert feature. But unless someone here has an idea for a 
workaround, I think this field name resolution policy is a total 
blocker. We can implement upsert on MySQL and SQLite but on PostgreSQL 
it will remain emulated.

-- Tim Starling