[elephant-devel] Query System

Sat May 10 01:54:50 UTC 2008

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, lists at infoway.net 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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