Re: Unknown-type resolution rules, redux - Mailing list pgsql-hackers

Added to TODO.detail.

> parse_coerce.c contains the following conversation --- I believe the
> first XXX comment is from me and the second from you:
> 
>     /*
>      * Still too many candidates? Try assigning types for the unknown
>      * columns.
>      *
>      * We do this by examining each unknown argument position to see if all
>      * the candidates agree on the type category of that slot.  If so, and
>      * if some candidates accept the preferred type in that category,
>      * eliminate the candidates with other input types.  If we are down to
>      * one candidate at the end, we win.
>      *
>      * XXX It's kinda bogus to do this left-to-right, isn't it?  If we
>      * eliminate some candidates because they are non-preferred at the
>      * first slot, we won't notice that they didn't have the same type
>      * category for a later slot.
>      * XXX Hmm. How else would you do this? These candidates are here because
>      * they all have the same number of matches on arguments with explicit
>      * types, so from here on left-to-right resolution is as good as any.
>      * Need a counterexample to see otherwise...
>      */
> 
> The comment is out of date anyway because it fails to mention the new
> rule about preferring STRING category.  But to answer your request for
> a counterexample: consider
> 
>     SELECT foo('bar', 'baz')
> 
> First, suppose the available candidates are
> 
>     foo(float8, int4)
>     foo(float8, point)
> 
> In this case, we examine the first argument position, see that all the
> candidates agree on NUMERIC category, so we consider resolving the first
> unknown input to float8.  That eliminates neither candidate so we move
> on to the second argument position.  Here there is a conflict of
> categories so we can't eliminate anything, and we decide the call is
> ambiguous.  That's correct (or at least Operating As Designed ;-)).
> 
> But now suppose we have
> 
>     foo(float8, int4)
>     foo(float4, point)
> 
> Here, at the first position we will still see that all candidates agree
> on NUMERIC category, and then we will eliminate candidate 2 because it
> isn't the preferred type in that category.  Now when we come to the
> second argument position, there's only one candidate left so there's
> no category conflict.  Result: this call is considered non-ambiguous.
> 
> This means there is a left-to-right bias in the algorithm.  For example,
> the exact same call *would* be considered ambiguous if the candidates'
> argument orders were reversed:
> 
>     foo(int4, float8)
>     foo(point, float4)
> 
> I do not like that.  You could maybe argue that earlier arguments are
> more important than later ones for functions, but it's harder to make
> that case for binary operators --- and in any case this behavior is
> extremely difficult to explain in prose.
> 
> To fix this, I think we need to split the loop into two passes.
> The first pass does *not* remove any candidates.  What it does is to
> look separately at each UNKNOWN-argument position and attempt to deduce
> a probable category for it, using the following rules:
> 
> * If any candidate has an input type of STRING category, use STRING
> category; else if all candidates agree on the category, use that
> category; else fail because no resolution can be made.
> 
> * The first pass must also remember whether any candidates are of a
> preferred type within the selected category.
> 
> The probable categories and exists-preferred-type booleans are saved in
> local arrays.  (Note this has to be done this way because
> IsPreferredType currently allows more than one type to be considered
> preferred in a category ... so the first pass cannot try to determine a
> unique type, only a category.)
> 
> If we find a category for every UNKNOWN arg, then we enter a second loop
> in which we discard candidates.  In this pass we discard a candidate if
> (a) it is of the wrong category, or (b) it is of the right category but
> is not of preferred type in that category, *and* we found candidate(s)
> of preferred type at this slot.
> 
> If we end with exactly one candidate then we win.
> 
> It is clear in this algorithm that there is no order dependency: the
> conditions for keeping or discarding a candidate are fixed before we
> start the second pass, and do not vary depending on which other
> candidates were discarded before it.
> 
> Comments?
> 
>             regards, tom lane
> 


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