Re: Show expression of virtual columns in error messages

On Wed Feb 4, 2026 at 11:45 PM -03, Yugo Nagata wrote:
>> 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.
>
I'm not sure if I understand how modifying deparse_context_for() could
help on this.

What I did was to use the expression_tree_mutator API to mutate the
virtual column expression to replace any Var reference with the value
into the TupleTableSlot. Please see the attached v2 version.

> 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.
>
I think that this could make the error output too verbose when there is
a lot of columns involved on the statement.

--
Matheus Alcantara
EDB: https://www.enterprisedb.com