Date: Thu, 5 Feb 2026 11:45:21 +0900
From: Yugo Nagata <nagata@sraoss.co.jp>
To: "Matheus Alcantara" <matheusssilv97@gmail.com>
Cc: "Peter Eisentraut" <peter@eisentraut.org>,
 <pgsql-hackers@lists.postgresql.org>
Subject: Re: Show expression of virtual columns in error messages
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On Wed, 04 Feb 2026 14:54:11 -0300
"Matheus Alcantara" <matheusssilv97@gmail.com> wrote:

> On Wed Feb 4, 2026 at 11:57 AM -03, Peter Eisentraut wrote:
> > On 03.02.26 15:06, Matheus Alcantara wrote:
> >> The attached patch changes this behavior to show the virtual column
> >> expression instead:
> >> 
> >>    ERROR:  new row for relation "t" violates check constraint "t_c_check"
> >>    DETAIL:  Failing row contains (5, 10, a * 2).
> >
> > Could be useful, but in this context you don't know which column is "a"?
> 
> I agree but we at least show more information to the user. The user may
> still need to check the table definition to understand the constraint or
> to get the column ordering of the table to understand what the value of
> "a" is being used to compute the expression. 
> 
> If the statement declare the columns that the values are referecing this
> could be easier to know (e.g INSERT INTO t(a, b) VALUES (5, 10)) but the
> user may still need to check the table definition to understand the
> constraint.
> 
> I see this patch as a way to at least give more information to user
> about the error.
> 
> Another possibility would be to get the actual values of "a" for example
> and show it on the error message, e.g:
> 
>     ERROR:  new row for relation "t" violates check constraint "t_c_check"
>     DETAIL:  Failing row contains (5, 10, 5 * 2).

That would indeed be more useful. One way to achieve this might be to
modify deparse_context and get_variable() so that a Var is displayed as its
actual value.

Another possibility would be to include column names in the DETAIL message,
for example:

 ERROR:  new row for relation "t" violates check constraint "t_c_check"
 DETAIL:  Failing row contains (a, b, c)=(5, 10, a * 2).

Although this would change the existing message format, including column
names could generally provide users with more information about the error.

Regards,
Yugo Nagata

-- 
Yugo Nagata <nagata@sraoss.co.jp>