Re: COPY WHERE clause generated/system column reference

On 04.11.25 12:43, jian he wrote:
>>> generated column allow tableoid system column reference, COPY WHERE clause also
>>> allow tableoid column reference, should be fine.
>>>
> 
> for virtual generated column, adding
> ``whereClause = expand_generated_columns_in_expr(whereClause, rel, 1);``
> 
> should be able to solve the problem.
> 
> For stored generated columns, we can either
> A. document that the stored generated column is not yet computed, it
> will be NULL
> B. error out if the WHERE clause has a stored generated column.
> C. add a temp slot and the computed stored generated column value
> stored in the temp slot.
> 
> attached v2-0003 using option C to address this problem.

For backpatching, I suggest that we prohibit both stored and virtual 
generated column in the COPY WHERE clause.  They don't work anyway, so 
this doesn't change anything except get a better error message.

We can then consider adding support in future releases, similar to how 
we are expanding their use in other contexts in other patches.

Attached is my proposed patch.  I kept it similar to the recently 
committed fix in commit ba99c9491c4.  Note that we also need to consider 
whole-row references, as that patch did.

>>> please check the attached file:
>>> v1-0001 fix COPY WHERE with system column reference
>>
>> It seems to make sense to disallow users to specify system columns in
>> the WHERE clause of COPY FROM. But why do we need to have an exception
>> for tableoid? In the context of COPY FROM, specifying tableoid doesn't
>> not make sense to me as tuples don't come from any relations. If we
>> accept tableoid, I think it's better to explain why here.
>>
> In function CopyFrom, we have below comment, which indicates
> At that time, tableoid was considered in the WHERE clause.
> 
>          /*
>           * Constraints and where clause might reference the tableoid column,
>           * so (re-)initialize tts_tableOid before evaluating them.
>           */
>          myslot->tts_tableOid =
> RelationGetRelid(target_resultRelInfo->ri_RelationDesc);

I think this doesn't actually work correctly.  I started a separate 
thread about this:

https://www.postgresql.org/message-id/flat/30c39ee8-bb11-4b8f-9697-45f7e018a8d3%40eisentraut.org

Until that is solved, I think we don't need to do anything about system 
columns.  System columns other than tableoid are already rejected.  Once 
we know what, if anything, to do about tableoid, we can implement a more 
complete check.