Hi Ian,
Thanks for your comments. They do make some points a bit clearer and
bring others to the table. I'd like to see others comment as well
before we continue moving forward.
In summary, I agree to follow your suggestion. However, one thing that
still remains unclear to me is the type of result expected from the
query system.
I was under the assumption that we want to continue handling objects
of the persistent classes in Elephant. Thus, the results from the
query should be a set (list, etc) of objects matching the criteria.
However, reading your comments makes me think that what you're
suggesting is that the result set be something closer to what a SQL
query would return. So, going back to my example, instead of receiving
a set of Books, you would get back something like:
((book_oid book_title book_author publisher_oid publisher_name
publisher_year) (value11 value12 value 13, value14, value15 value16)
(value21 value22 value23 value24 value25 value26) ... )
In other words, maybe something like a list that contains a list of
the slot names returned and then the list of matching values, where
the list of slots is either the concatenation of all the slots in the
persistent classes (in the case of something like SELECT Books.*,
Publishers.* ...) or the specific slots requested (e.g. SELECT
Books.author, Publishers.name ...).
And therefore, the result set would be treated as simply a set of
values instead of a set of objects. Within the result set, you could
include the OIDs so that you could eventually instantiate the
particular object and work in via the object model, or you could
simply just use the values returned, which is what you originally
asked for in the SELECT statement.
If my assumption is correct from what you're saying, that certainly
clarifies a lot or my concerns and doubts. I'm sure more details will
arise along the way, but that could be a starting point to sketch the
system.
Comments?
Thanks,
Daniel
On May 9, 2008, at 9:54 PM, Ian Eslick wrote:
Welcome back Daniel, we all know the work drill!
Here are a few thoughts to throw into the mix...
One advantage of the relational model is that you have implicit data
structures (tables) that can be assembled from existing tables via
the SQL query. This is nice because it means we don't have to
explicitly create and maintain the structure for all these derived
data structures. In a pure lisp model, you actually have to do all
this maintenance yourself, especially the optimizations necessary
for efficiency that add to complexity. I feel that Elephant should
probably fall somewhere in-between. You maintain the data
structures that you want to work with in your program logic, but the
system can maintain pointers and indices and other relationships
that make it easy and efficient to generate and work with subsets of
objects efficiently (a user's inbox, for example).
Some of the limitations/frustrations with the current system may be
caused by people trying to do familiar relational tasks in the OODB
framework.
I also think that Robert's lisp-as-query-language works well for the
prevalence model when all objects are in memory, but I think it's
less practical in, say, BDB where you are going to disk alot.
However, it's a good discipline to consider - when does it makes
sense to add new syntax/apis and when does it make sense to use lisp
directly.
You mentioned associations. The best way to think about
associations is that it is an easy way to maintain back pointers.
For example, if a message object has a slot that contains a
reference to a user, we may also want the user object to have an
accessor that provides quick and efficient access to the collection
of messages that point to it. That's what associations are for.
You could do this by declaring after methods on (setf (user message)
value) that add the message to a pset sitting in a user instance
slot, but that gets tedious. As Leslie says, we're trying to make
common cases simple and reasonably efficient.
So the approach I'd like to see taken to designing the query
framework is to capture the use cases and metaphors that people are
really interested in and are encountering in real-world use and pick
the largest subset that fits nicely into a clean, theoretical
conceptual model. There are already a good number (Leslie, Alex,
etc) on the list that we could start with.
For example, I often find myself wanting to filter a set of objects
by more than one parameter (messages from user U that are high
priority between 4/1/08 and 5/1/08). What is the complexity of
different approaches afforded by the existing Elephant implementation?
In order of computational efficiency (I surmise):
1. scan all messages and collect/operate on only those matching all
criteria
2. scan an index on messages instead of all messages; pick the one
likely to yield the smallest subset
3. intersection: scan two or more indexes for subsets represented as
sequences of oids, instantiate, filter and operate on the objects
represented by the intersection.
4. create an index that orders objects by all three parameters and
just walk the matching set. Trade off space for time.
Any others?
The other consideration is the conceptual framework we want to use
to approach the problem. Procedural? Constraint satisfaction?
Logical form? Graph matching? There are some good examples of
existing OODB systems in lisp out there (PLOB, AllegroStore/
AllegroCache, Statice, etc). If you search the list archives, I
think I've forwarded references in the past.
I tend to lean towards a constraint satisfaction approach, as my
sketch demonstrations. "Operate on the set of objects that satisfy
these constraints." There are a bunch of practical issues. Do we
map query sets? Do we cache them? Do we represent them as lists?
Are they lazily evaluated? If we don't have a DSL, but allow
arbitrary lisp expressions, then there isn't enough information to
automatically select indexes, perform intersections, etc.
My other strong suggestion, besides starting by capturing the major
use cases, is that we begin by implementing a procedural approach by
implementing the building blocks for filter, sort, intersect, etc.
If we take the list of four filtering approaches above, we can start
writing code that do these things and use them to implement some of
the use cases. The common building blocks and problems that we
discover will inform the additions we'll want to the MOP, new
implicit data structures like associations, the most convenient
query syntax, etc. Plus it will be useful in the meantime. This
fits into the classic lisp bottom-up DSL development model (well
proselytized by Paul. Graham).
Ian
On May 9, 2008, at 6:02 PM, [EMAIL PROTECTED] wrote:
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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