Tom Lane wrote:

> I do not think we want any client-side sorting in this feature at all,
> because the minute you have any such thing, you are going to have an
> absolutely never-ending stream of demands for more sorting features:
> multi column, numeric vs text, ASC vs DESC, locale-aware, etc etc etc.

It doesn't really do any client-side sorting, the rest of the thread might
refer to it like that by oversimplification, but if the command requests
a header to be sorted,  a "backdoor-style" query of this form
is sent to the server, with PQexecParams():

SELECT n FROM (VALUES ($1,1),($2,2),($3,3)...) ) AS l(x,n) 
where the values to display in the header are bound to 
$1,$2,.. and the type associated with these parameters is
the PQftype() of the field from which these values come.
Then  the <n> values coming back ordered by <x> tell us
where to position the values corresponding to $1,$2... in the
sorted header.

There are some cases when this sort cannot work.
For example if the field is an anonymous type or a ROW().
Or if the field is POINT(x,y), because our "point" type
does not support order by.
I believe these are corner cases for this feature. In these
cases, psql just displays the error message that PQexecParams()

Best regards,
Daniel Vérité
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Twitter: @DanielVerite

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