[elephant-devel] Querying Advice

[Pierre THIERRY]

Scribit Daniel Salama dies 12/11/2006 hora 10:28:
> I guess we would have to sequentially navigate thru the  results in  
> order to "manually" select each record based on all the other  
> possible search arguments. I suppose, in a way, this can be done  
> relatively painless by using macros

I don't think custom macros are needed here. The search could be made
with the various searching functions of Lisp and a predicate built from
the constraints. Here is an example of a predicate building function for
an application about real estate:

(defun search-predicate (&key min-price max-price min-surface max-surface)
  (lambda (x)
    (and
      (if min-price (>= (price x) min-price) t)
      (if max-price (<= (price x) max-price) t)
      (if min-surface (>= (surface x) min-surface) t)
      (if max-surface (<= (surface x) max-surface) t))))

Macros could be used to return an optimized closure where some useless
branching is avoided, but some implementation may already do that
optimization by themselves (I think at least SBCL removes unreachable
code when compiling).

It should be pretty easy to write some macros to avoid writing the
predicated building function by hand, and we may provide some more as
some sort of query language for the most common cases (min and max for
the result of a function, and so on). I think we should keep the ability
for the user to provide it's own predicate code (it would be so
unlispy not to...).

> Then we have the issue of the sorting. I suppose it falls into a
> similar situation: once we get the matching resultset from the
> previous step, we would have to perform some "efficient" sort
> algorithm on the data set dynamically based on the user's sorting
> desire. I also suppose we could create a macro for this as well.

I'm sure sorting is an absolutely pure functional thing. It's "just" a
matter of finding an efficient algorithm here. Beware of premature
optimization though. I think we should build a correct solution first,
and check the state of the art of database implementations if profiling
show us a bad performance.

Functionally,
Nowhere man
-- 
nowhere.man at levallois.eu.org
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