Re: non-integer constant in ORDER BY: why exactly, and documentation?

On Oct 11, 2012, at 4:48 PM, Ken Tanzer wrote:

> Hi. I recently ran a query that generate the same error as this:
>
> SELECT * FROM generate_series(1,10) ORDER BY 'foo';
> ERROR: non-integer constant in ORDER BY
> LINE 1: SELECT * FROM generate_series(1,10) ORDER BY 'foo';
>
> The query was generated by an app (and the result somewhat inadvertent), so it was easy enough to change and I'm not asking here about a practical problem.
>
> I am curious though about why this "limitation" exists. I get that integer constants are reserved for sorting by column numbers. But if Postgres already knows that it's a non-integer constant, why not let it go through with the (admittedly pointless) ordering?
>
> Also, I couldn't see that this was explictly mentioned in the documentation. The relevant pieces seemed to be:
>
> Each expression can be the name or ordinal number of an output column (SELECT list item), or it can be an arbitrary expression formed from input-column values.
>
> followed closely by:
>
> It is also possible to use arbitrary expressions in the ORDER BY clause, including columns that do not appear in the SELECT output list.
> (http://www.postgresql.org/docs/9.1/static/sql-select.html#SQL-ORDERBY)
>
> And looking at the expressions page (http://www.postgresql.org/docs/8.4/static/sql-expressions.html), the first type of value expression is a "constant or literal expression." So nothing seems to explicitly rule out a literal ORDER BY.
>
> I'm not sure if it would do violence to something I'm missing, but would the following combined statement work for the documentation?
>
> "Each expression can be the name or ordinal number of an output column (SELECT list item), or it can be an arbitrary expression. The expression can include column values--whether they appear in the SELECT output list or not. An expression may not, however, consist solely of a non-integer constant. And an integer constant will be interpreted as the ordinal number of an output column "

Apparently, the parser tries to pull an column index out of any constant appearing in that position. It can be trivially worked around:

select * from generate_series(1,10) order by coalesce('foo');

but that doesn't help if your query is automatically generated.

Cheers,
M