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From Jeremy Boynes <jboy...@apache.org>
Subject Re: About improvement of DERBY-134
Date Sun, 27 Feb 2005 20:29:16 GMT
Jack Klebanoff wrote:
> 
> 1. sqlgrammar.jj. I think that creating a new method,
> isNonReservedKeyword() to determine whether a token is a non-reserved
> keyword or not, is a maintenance problem. Whenever we add a new
> non-reserved keyword we must add it to the list of tokens, to
> nonReservedKeyword(), and now to isNonReservedKeyword(). Having to add
> it in two places is difficult enough to discover or remember. If we need
> isNonReservedKeyword then we should find a way of combining
> nonReservedKeyword and isNonReservedKeyword so that only one of them
> keeps the list of non-reserved key words.
> 
> It is not necessary for the parser to recognize 3 cases of ORDER BY sort
> key type. A column name is just one kind of <expression>. If the parser
> treats it as an expression we should still get the right ordering. I
> think that it would better if the parser did so. The OrderByColumn class
> can special case a simple column reference expression, as an
> optimization. This considerably simplifies parsing sort keys.
> 
> The only sort key type that has to be handled specially is that of an
> integer constant. That specifies one of the select list columns as the
> sort key. This case can be recognized in the parser, as is done in the
> patch, or it can be recognized in OrderByColumn. In this alternative the
> parser always creates OrderByColumn nodes with the sort key given by an
> expression (a ValueNode). At bind time OrderByColumn can determine
> whether the sort key expression is an integer constant, and if so treat
> it as a column position.
> 
> The two alternatives differ in the way that they treat constant integer
> expressions like "ORDER BY 2-1". The patch orders the rows by the
> constant 1, which is not usefull. With the patch "ORDER BY 2-1 ASC" and
> "ORDER BY 2-1 DESC" produce the same ordering. If OrderByColumn treated
> an integer constant sort key expression as a result column position then
> "ORDER BY 2-1" would cause the rows to be ordered on the first result
> column, which I think is more usefull.
> 

 From the SQL spec, the <sort-key> is simply a value expression 
evaluated for each row. Hence "ORDER BY 2-1" means that all rows are 
peers and the ordering is implementation-dependent.

I think allowing the value-expressions to evaluate to column position is 
potentially confusing. For example, suppose you have "ORDER BY ?" - do 
we order by the supplied value (which would be the same for all rows 
making the actual ordering implementation-dependent) or do we order by 
the column position (with the potential need to re-select indexes used 
to assist in ordering).

I believe it is simple and SQL-compliant to have very simple rules here:
1) if all <sort-key>s are integer literals, then sort by the column
    position they imply. This is SQL-compliant as the comparison criteria
    for each row will be the same, making all rows peers and the actual
    order implementation dependent (we just define ours as using the
    appropriate column value).

2) if any <sort-key> is not an integer literal then we treat all of them
    as <value expression> and compare rows accordingly.

--
Jeremy

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