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Date: Mon, 01 Jun 2026 22:36:36 +0200
From: "Joel Jacobson" <joel@compiler.org>
To: "Laurenz Albe" <laurenz.albe@cybertec.at>,
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Cc: "Arne Roland" <arne.roland@malkut.net>,
 "Anders Granlund" <anders.granlund.0@gmail.com>,
 "Andreas Karlsson" <andreas@proxel.se>, "Vik Fearing" <vik@chouppes.com>
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Subject: Re: Key joins
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On Mon, Jun 1, 2026, at 22:06, Joel Jacobson wrote:
> -- Rejected: the view is the PK side and lacks row coverage.
> SELECT *
> FROM orders AS o
> JOIN active_customers AS ac FOR KEY (id) <- o (customer_id);
> ERROR:  key join from referencing relation o to referenced relation ac 
> cannot be proven
> ERROR:  key join from referencing relation o to referenced relation ac 
> cannot be proven
> LINE 3: JOIN active_customers AS ac FOR KEY (id) <- o (customer_id);
>                                     ^
> DETAIL:  Not every o (customer_id) value can be proven to have a 
> matching ac row. Referenced relation ac is filtered before this key 
> join. The relevant operation occurs inside view public.active_customers.

Ops, that extra ERROR: line was a mistake, sorry about that.

The corresponding subsection has now also been fixed in the web version:
https://keyjoin.org/#sec7.4.13

/Joel