Hello everyone,
I apologize for being disconnected for so long. I had volunteered to
help in the query system and should have done more progress by now.
Unfortunately, the same as some (most or all) of you, putting food on
the table for my family has a higher priority and my current job has
demanded 110% of my time lately.
Enough excuses! I have been passively reading several of your email
threads. I am convinced that a query system will bring a lot of value
to Elephant. The question that still arises is whether or not people
want a SQL-like syntax or a Lisp-like syntax.
As Ian has suggested, publicly and/or privately, we should start
designing the query system in a very basic form. The most critical
part would be query optimization, which I'd rather work on after we
have the basic query system in place. But there are a lot of decisions
to make before we get there and coming to a consensus of how it should
look and how it should work is of critical importance.
From a simplistic point of view, a SQL-like syntax should allow for
the execution of the basic relational algebraic operations (union,
difference, cartesian product, projection, and selection). For the
most part, these would not be difficult to implement. However, IMHO,
there is an intrinsic "contradiction" in applying a SQL-like syntax on
top of Elephant.
Assume you have the following Tables (relations) in a SQL world:
Books (
book_id,
title,
author
)
Publishers (
publisher_id,
name
)
BooksPublishers (
book_id,
publisher_id,
year
)
Suppose you wanted to get the cartesian product of all the books
published in 2008, you could run a SQL query like:
SELECT Books.*, Publishers.* FROM Books, Publishers, BooksPublishers
WHERE Books.book_id = BooksPublishers.book_id AND
Publishers.publisher_id = BooksPublishers.publisher_id AND
BooksPublishers.year = 2008
The result will be a concatenation of all the columns from the Books
and Publishers tables. In a SQL-world, you would access these results
in a key-value pair type mode (e.g. Books.book_id = 1, Books.title =
"1984", etc). However, when you think in terms of Elephant (at least
my understanding of it), you're dealing with objects and not key-value
pairs from multiple tables. So, instead of getting a concatenation of
all the columns, you "should" be getting just a list of Book objects
(or Publisher objects) that met your query criteria, such that when
you iterate thru them, you could "query" their Publishers (or the
Books). So, if we had something like (please keep in mind this is no
suggestion to syntax or correctness but just for illustrative purposes):
(defpclass book ()
((title :accessor book-title :index t)
(author :accessor book-author :index t)
(published_copies :accessor book-copies :initform (make-pset))))
(defpclass publisher ()
((name :accessor publisher-name :index t)))
(defmethod add-published-copy ((bk book) (pb publisher) year)
(insert-item '(pb year) (book-copies bk)))
(defmethod map-published-copies (fn (bk book))
(map-pset fn (book-copies bk)))
(setq objs (select book :where ((map-published-copies (lambda (item
year) (= (second item) year)) $bk 2008)))))
From then on, you could just iterate through the book objects in the
result set for their respective published copies. The problem with
this is that, ok, you get all the books that met your criteria but if
you then wanted to get a list of all the published copies, you would
need to apply the filter criteria again. The reason I think it "should
behave" this way is because Elephant deals with sets of objects, and
you use Lisp to navigate through the object space, whereas in a SQL-
world you are not dealing with objects but with a result set that
contains all the columns you asked for. If we were to emulate the same
behavior in the query system, that would sort of defeat the purpose of
Elephant. For that matter, you might as well use some of the other
libraries (e.g. CL-SQL, cl-perec, cl-rdbms, etc).
The above example is a very simple example. We haven't looked at
SORTING, LIMIT, OFFSET, etc. Things which will simply make this whole
dilemma more difficult.
I haven't looked into Ian's association mechanism yet. Maybe the query
system could/should be an extension to that with some specialized
features to apply filter criteria instead (and possibly evolve into
something similar to Ruby's ActiveRecord). I know the association
mechanism is still being developed and I haven't really seen anyone
comment much on it other than what Ian has mentioned. In one of Ian's
comments, he said:
"A more general query language is probably the right solution
for this interface. The query language would know about associations,
derived indices, etc and perform query planning via introspection over
the class objects."
At the same time, Robert said on another thread:
"One might philosophically prefer SQL. I personally vaster
prefer to work in a powerful programming language to accomplish these
things. Obviously, whether two classes that refer to each other stand
in a "parent-child" relationship or not depends entirely on the
circumstances. I prefer to write simple functions such as "delete-
order" below, which both utilize and (in a sense) expand the power of
LISP applied to persistent objects."
Leslie said on yet another thread:
"While I'm at it: OFFSET and LIMIT (a real limit which lets you
specify an arbitrary Lisp expression) are things we definitely want to
aim for in 1.0. They are not difficult to implement at all, but they
don't work with GET-INSTANCES-BY-* and, worse, MAP-BTREE. This means
everyone has to write their own version of these functions that take
appropriate arguments and move the cursor around themselves instead of
relying on a simple high-level API.
I'd have implemented these extensions myself, but I thought it better
to wait for the integration of the query language to add it."
And Alex said:
"I think main problem is not how it looks, but that query
language actually makes programming a lot easier."
All those comments make sense. There seems to be a group agreement
that something is needed, but everyone has their own ideas of how it
should work. Both the query language and the associations are still
being developed, so if we get consensus no how these should work, it
may give a better direction to both feature sets. If anyone has any
comments or suggestion as to whether a query system be of real
interest/necessity and if so, which would be the preferred query
syntax and expected behavior, that would really help.
I'm willing to work on this in as much as possible with my limited
knowledge of Lisp and Elephant. However, given a clear direction of
where this should go, I will be able to focus better and learn faster
what I haven't learned so far.
Again, your feedback is much appreciated. I'm hopeful to be able to
work more on this over the weekend, assuming I get some feedback from
you guys.
Thanks
Daniel
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