Bookshelf of Bookmarks

• Slashdot is Going out of Style in 2006 "2006 will the year in which the once great Slashdot dies." More up-to-date and more stories over comments?
• Comparing LINQ and Its Contemporaries Comparison of Ruby on Rails, Hibernate and other languages/APIs and LINQ.
• Fire & Motion: The Trouble with Competing with Google How Google picks its fights in order to cause the most cost to its competitors.
• RDF For Rapid Development Of A Data Model How to use RDF to eventually get to a relational or other integration system (mentions Metis).
• RuleML "The aim is to build an expert system shell which implements native support for RuleML and incorporates the latest concepts in web-based and distributed processing." Via Sumatra Posted to Sourceforge.
• From the same site: Grand Challenge, Neural nets and rule engines How to distribute the RETE algorithm by processing locally and distributing indexes.
• A couple of links on the Ruby VM, Rite: Rite and YARV (Yet Another Ruby VM)
• Luke 6:41 "Our Leader hasn't caught Osama bin Laden, but he's doing a bang up job rounding up brown people."
• Modularity Rules "IBM’s investment changed the nature of the industry so completely that modularization is the name of the game."
• JAHAH Home AJAX is so 2005.

Bug or Feature

ImplicitInterfaceImplementation "What would it mean to implement an implicit interface? Essentially it would tell the type system that the ValuedCustomer class implements all the methods declared in the public interface of Customer but does not take any of its implementation, that is its public method bodies, and non public methods or data. In other words we have interface-inheritance but not implementation-inheritance."

InterfaceImplementationPair "Using interfaces when you aren't going to have multiple implementations is extra effort to keep everything in sync (although good IDEs help). Furthermore it hides the cases where you actually do provide multiple implementations."

Interfaces "Somewhat related to this is one of Robert Martin's Principles of Object-Oriented Design: "The Interface Segregation Principle", which says "Clients should not be forced to depend on methods that they do not use. Interfaces belong to client, not to hierarchies."

In a dynamic system like Smalltalk, Ruby, or Python, we don't need explicit interfaces, so "The Interface Segregation Principle" doesn't seem to apply. In static systems like Java and C++, one could declare interfaces specific to each client, but it seems that few people do.

The principle seems to more about dependency-breaking than anything else. I'd be interested in hearing from people who have actually done this extensively."

Implicit Interfaces "Of course Martin's suggestion would make it easier to use third party code without wrapping it up in a shim of your own. Personally I think it's a good thing to get in the habit of segregating third party code and putting it behind a thin, object based firewall of your own devising. The shim/wrapper code allows you to blend the alien API into your standard ways of doing things and gives you a chance to refine the abstraction that the third party API provides. Chances are their abstractions are in terms of the problem that they solve whereas yours should be in terms of the problem that you are solving and might only use a small part of the API... What's more, if you have slipped in an interface you can test without the third party code and you can, potentially, replace the third party code with other code if required..."

This last point is very much in line with my own experience and the point in A RATIONAL DESIGN PROCESS: HOW AND WHY TO FAKE IT "Often we are encouraged, for economic reasons, to use software that was developed for some other
project...The resulting software may not be the ideal software for either project, i.e., not the software that we would develop based on its requirements alone, but it is good enough and will save effort."

Mac XP

Sort of like the "Pooh and the Philosophers" only for XP and the Mac development team.

Project Metaphor: Creative Think "Find a central metaphor that's so good that everything aligns to it. Design meetings are no longer necessary, it designs itself. The metaphor should be crisp and fun."

Refactoring: -2000 Lines Of Code "He recently was working on optimizing Quickdraw's region calculation machinery, and had completely rewritten the region engine using a simpler, more general algorithm which, after some tweaking, made region operations almost six times faster. As a by-product, the rewrite also saved around 2,000 lines of code."

Simple Design: MacPaint Evolution "I was surprised a few days later when Bill told me that he decided to remove the character recognition feature from MacPaint. He was afraid that if he left it in, people would actually use it a lot, and MacPaint would be regarded as an inadequate word processor instead of a great drawing program. It was probably the right decision, although I didn't think so at the time. I was amazed that he was able to detach himself from all the effort that he put into creating the discarded feature; I know that I probably wouldn't have been able to do the same."

Constant Integration: Real Artists Ship "We starting doing release cycles that were only a few hours apart, re-releasing every time we fixed a significant problem."

Pair Programming: Real Artists Ship "At one point, around 2am on Sunday night, I stumbled across a bug in the clipboard code. I thought I knew what it might be, but I was so tired that I didn't want to deal with it. I tried to pretend that I didn't see the problem, but Steve Capps was watching my expression and knew there was something wrong. I also was too tired to sustain a pretense; he grilled me about the problem and then helped me craft a fix, since I was too tired to do it on my own."

Automated Testing: Monkey Lives "The Monkey was a small desk accessory that used the journaling hooks to feed random events to the current application, so the Macintosh seemed to be operated by an incredibly fast, somewhat angry monkey, banging away at the mouse and keyboard, generating clicks and drags at random positions with wild abandon. It had great potential as a testing tool, so Capps refined it to generate more semantically rich events, with a certain percentage of the events as menu commands, a certain percentage as window drags, etc."

Coding Standard: Hungarian "Bud decided that it would be too error prone to try to translate the Hungarian memory manager directly into assembly language. First, he made a pass through it to strip the type prefixes and restore the vowels to all the identifier names, so you could read the code without getting a headache, before adding lots of block comments to explain the purpose of various sub-components.

A few weeks later, when Bud came back to attend one of our first retreats, he brought with him a nicely coded, efficient assembly language version of the memory manager, complete with easy to read variable names, which immediately became a cornerstone of our rapidly evolving Macintosh operating system."

The importance of on-site customers: Round Rects Are Everywhere! "Steve suddenly got more intense. "Rectangles with rounded corners are everywhere! Just look around this room!". And sure enough, there were lots of them, like the whiteboard and some of the desks and tables. Then he pointed out the window. "And look outside, there's even more, practically everywhere you look!". He even persuaded Bill to take a quick walk around the block with him, pointing out every rectangle with rounded corners that he could find. ...Over the next few months, roundrects worked their way into various parts of the user interface, and soon became indispensable."

New Newsy News

I should've done this a little while ago, a link to The News before The News "Tech opinion from James Webster". Perhaps one of the few people to constantly surprise me with stuff that I hadn't read about before.

Includes links to my blog including: "All Yuor Google Base Are Belong To Us!" and "DabbleDb - Naked Objects for Web 2.0".

Rocketman

First jet powered Birdman flight "After checking the altimeter several times, it was apparent that there was no appreciable loss in altitude for this period of time. Visa next changed his angle of attack by redirected the thrust and changing his body position to attain vertical climb...The jump has proven empirically that level human flight is possible and sustainable using the combination of jet engines and a bird-man suit. The strength required to control level flight was relatively easy, and controlling the direction of flight feels surprisingly natural. The duration of flight is simply a factor of the consumption of fuel of the engine(s) powering the flight."

Video here.

Smalltalk meets the Semantic Web

Smalltalk:::OWL-Project "OWL has emerged from the AI/semantic community and tends to be in the open-source community which appears to be a direction for Smalltalk (e.g. Smalltalk Solutions at Linux World) Much of the work to date has been implemented in Python and Ruby which, from a language perspective, is very close to Smalltalk. However, those languages become less appealing if you have ever worked in the IDE's supporting those languages. OWL can provide the Smalltalk community with a "market" that is a good fit for the features of the ST language and supporting IDE's."

"Agilense provides a product named EA WebModeler...is an implementation of the Adaptive Object Model pattern..."

A good summary of AOM: "We call these systems "Adaptive Object-Models", because the users' object model is interpreted at runtime and can be changed with immediate (but controlled) effects on the system interpreting it. The real power in Adaptive Object-Models is that the definition of a domain model and rules for its integrity can be configured by domain experts external to the execution of the program."...If you got just one reason why EJB is flawed, this has got to be the one, you can't build systems for Enterprises at the same time take away control form the business stakeholders."

RETE Rebuked

A recent blog I started reading after criticizing SPARQL. This time its criticizing an entry Paul made about the scalability of the RETE algorithm: "I am a bit confused by the statement that RETE does not scale. This is contrary to a mountain of papers by researchers and developers around the world. From the paragraph, a couple of things come to mind. "(loading indexes) does not need to be done often" tells me the author doesn't understand the purpose and goal of RETE. RETE was designed to solve machine learning problems where data changes rapidly and reasoning is a continuous process. What the author wants is something closer to BitMap indexes used in OLAP products."

A lot of the points raised, like RETE being for changing data, are mentioned in a previous post under Meeting. Drools was chosen as a starting to point to see what kind of system needed to be developed in Kowari.

Also mentioned, bitmap indexing: "Given Tucana is indexing everything, they might as well adapt Bitmap indexing and get better than linear performance. The problem described by the blog is a well understood problem in the OLAP world."

An previous entry, "Relational theory, RETE and Derby" points to some interesting articles about bitmap indexes (available in Oracle 9) and high scalability requirements: "In a large financial institution like a mutual fund company, they may have 1-20 million customers. If each customer has an average of 20-30 positions (aka specific holding of an equity) that means the potential dataset for firm wide compliance rule could involve 20million+ rows. Doing this within 2-5 seconds is rather hard, so it requires using lots of different techniques. In the extreme cases, a company might have 20 million accounts, which means the potential dataset is 600 million rows."

From the OTN article: "B-tree indexes are usually used when columns are unique or near unique; bitmap indexes should be used, or at least considered, in all other cases. While you would not generally use a b-tree index when retrieving 40 percent of the rows of a table, a bitmap index is often still faster than doing a full table scan. This is seemingly in violation of the 80/20 rule, which is to generally use an index when retrieving 20 percent or less of the rows and do a full table scan when retrieving more. Bitmap indexes are smaller and work differently from the 80/20 rule. You can effectively use bitmap indexes even when retrieving large percentages (20 to 80 percent) of a table. Bitmaps can also be used to retrieve conditions based on nulls (since nulls are also indexed) and for "not equal" conditions."

It would appear that this would be suitable for predicate indexation but not generally as both subjects and objects are near unique.

Good API Design

Java API Design Guidelines "If your API is worth anything, it will evolve over time...decide what sort of compatibility you will guarantee between revisions...What should the design goals of your API be?...absolutely correct...easy to use...easy to learn...fast enough...small enough...it's much easier to put things in than to take them out."

This seems all well and good.

"Interfaces can be implemented by anybody. Suppose String were an interface. Then you could never be sure that a String you got from somewhere obeyed the semantics you expect: it is immutable; its hashCode() is computed in a certain way; its length is never negative; and so on."

Adding interfaces on top of String like CharSequence was a good thing. It meant that you could process a String or a StringBuffer the same as well as consistently treat something that may have been in memory or on-disk (including being memory mapped via NIO). The way to ensure that String implementations do follow the correct semantics is test driving the interfaces and this requires being able to create a Mock object of that interface - pretty much sealing the deal as far as interfaces are concerned.

Actually, the whole section on interfaces is pretty much a wash as this can be solved by test driving 3 of the 4 points raised. As far as using an abstract class to help with the evolution of the API, you can have both - an interface and a default abstract class for some base implementation.

Exceptions get a going over too, "Use a checked exception "if the exceptional condition cannot be prevented by proper use of the API and the programmer using the API can take some useful action once confronted with the exception." In practice this usually means that a checked exception reflects a problem in interaction with the outside world, such as the network, filesystem, or windowing system."

Related: Evolution Not Creation, Reminder About Incremental and Test Driven Development and 10 Minute Commits for Better Code.

5 Minutes with Monad

I've recently spent a bit of time trying to solve some problems using Microsoft's new Monad shell. It's interesting that the creature for the O'Reilly book is the common toad. It was a different experience, although the pain is eased a little as the default installation comes with all the new commands (cmdlets) mapped to Unix ones (like ls and ps).

The default security setting prevents remotely signed objects from being executed and there seems to be no way to turn it on. The documentation is missing. To make it usable it's:

set-property
 `HKLM:\SOFTWARE\Microsoft\Msh\Microsoft.Management.Automation.msh` 
-property ExecutionPolicy -value RemoteSigned

Going through the tutorials it did show itself to be kind of cool. For example, being able to select the top 10 processes based on VirtualMemorySize:

get-process | sort-object VirtualMemorySize | select-object -last 10

You can whack on a "convert-HTML" or an "export-csv" to produce the result in a format you want or connect to Excel or SQL Server to retrieve data. A lot has been made of its native XML support and how it passes around strongly typed objects rather than just Unix's streams.

One of the problems was trying to do line by line processing. There was promises of pipelining via XML streams but according to "Replace lines in a text file?" (the first hit on Google) Monad doesn't support it. The lack of streaming appears to be a crucial omission in a toolset designed for system administrators - although it might not be fatal as log files and the like don't usually come close to the available memory of modern systems.

It does support accessing the .NET APIs which provides a loophole. For example, to read a file line by line and replace "xxx" with "yyy":

$f = [System.IO.File]::OpenText("c:\file.txt")
while($line = $f.ReadLine()) 
{ 
  $line -replace "xxx","yyy"
}

It was all for nothing, as I later found out that it didn't support Windows 2000 and it needed to be deployed on that - it is supported by Windows XP, 2003 and Vista. Back to Windows Script Host (maybe using Ruby) I guess.

Merry Bag Of Links

Political:

• Support Creative Commons "We are down to the last $100,000, and really need your support — both for the very cool projects we’re launching (see, e.g., the license interoperability project, discussed recently in Technology Review, and the two new projects announced this week), and for the very uncool pressure we’re under from IRS regulations to demonstrate “public support” as a condition for keeping our (absolutely essential as in we can’t live with out it) tax exempt status." Via We've got 10 days, and we need $100,000. Please help


• Passion of the Spaghetti Monster and Intelligent Design


• Top 12 media myths and falsehoods on the Bush administration's spying scandal "...the Bush administration and its conservative allies in the media have defended the secret spying operation with false and misleading claims that have subsequently been reported without challenge across the media."


• The Curious Section 126 of the Patriot Act "Congress is seeking assurances that "the privacy and due process rights of individuals" is protected in the course of the government using massive databases of non-publicly available data; both proprietary databases and its own compiled intelligence and law enforcement databases to "search" for terrorists and terrorist connections."

Agile:

• Client vs. Developer Wars "This, to me, is another indictment of dysfunctional specifications. I learned long ago that clients won't listen to what you say, and they certainly won't read what you write. You're much better off putting that wasted effort into a working model and setting it in front of the client. Let them play with it for a while. Refine the working model based on that feedback, then keep turning the crank on this cycle until you run out of resources."


• Continuous Testing - in spirals "I want tests to run ‘inside out’ Imagine a spiral, with the unit test for the bit of code you’re currently editing to be the focal point. Ideally, I’d want a test to run first for the method I changed last, then for the whole class, then for the suite the class is in, then further out to other dependencies. Tests run outward only when green bars are encountered. If there is a red bar somewhere, the spiraling stops, so we can examine the failure, fix it, and see again from the inside which tests run."


• Essential Advice for Agile Coaches

Programming:

• Ruby Off the Rails "Ruby's syntax is quite different from that of the Java language, but it's amazingly easy to pick up. Moreover, some things are just plain easier to do in Ruby than they are in the Java language."


• Seven Habits of Highly Effective Programmers "Sorry, there's no shortcut - you have to learn and practice and make some mistakes."


• Networking Libraries using NIO: EJOE, Coconut AIO and MINA. MINA's features include: "unit testability using mock objects".


• ONJava: 2005 Year in Review "Java is still by far the most widely used programming language, if book sales are any indication, about 2x C#, 2.5x PHP, 4x Perl, and 9x Ruby/Python."
  


• Automate acceptance tests with Selenium.
  

General Technical:

• What is Songbird? "Songbird is built atop the Mozilla Foundation's XULRunner platform also used by the Firefox browser, the Thunderbird email client and other desktop applications." The user preview has been delayed


• Mac Mini - Big Ideas


• Lack of focus and death march at Google


• Mac IE's Death: A Case for Microsoft Disbanding or Transfering the Windows IE Team ""Then why on earth did we pursue IE in the first place? Just so that the DOJ would sue us?""


• The Bubble Cycle is Replacing the Business Cycle Housing, bond, etc bubbles.

Know When to Hold Them, Know When to Fold Them

So what is an RDF merge and when should you apply it in SPARQL?

Very succinctly, in "The Semantics of SPARQL" it says: "The RDF merge <G₁...G_n> of a sequence of graphs <G₁...G_n> (i.e., a dataset) is the ordered merge union of the graphs, where repeated bnodes are substituted with fresh ones, by keeping the names of the bnodes coming first in the sequence order."

In "SPARQL Query Language for RDF" it gives a simple example:

Graph 1:
_:a foaf:name "Bob" .
_:a foaf:mbox .

Graph 2:
_:a foaf:name "Alice" .
_:a foaf:mbox .

The result of the merge, upon which queries are made:
_:x foaf:name "Bob" .
_:x foaf:mbox .

_:y foaf:name "Alice" .
_:y foaf:mbox .

Section 9 details querying multiple graphs in SPARQL, including a new dataset where the default graph is a merge of the graphs in the FROM clause.

In summary, when SPARQL operations are performed across graphs you get new blank nodes which prevents, for example, being able to JOIN across graphs using them.

What is generally required by RDF applications is something like smushing. For example, an "...RDF spider (often known as a "scutter") can gather up FOAF files and "smush" them together into a single model that unifies the individual pieces of information into a network." (from "A Semantic Web Shoebox - Annotating Photos with RSS and RDF").

To actually achieve smushing, Leo has an example algorithm or it might be appropriate to adapt RDF graph isomorphism algorithms.

More on Disjunction

I think this is just going to happen time and time again, "RDF non-sense": "The W3C Working Group members argue SPARQL is a mixed mode language that does support OR, though they are calling it an "optional union". Frankly, I see no point in renaming something people understand to mean one thing. It gives me the impression the W3C want to be thought police and enforce a certain way of thinking.

On the practical side, many analysts happen to like OR disjunctions and would complain loudly. Of course, there are plenty of cases where users abuse the power and write deeply nested disjunctions. That is not a valid reason in my mind to avoid disjunction. It saves the user time and allows them to write simpler rules using disjunction. The W3C seems love RDF and wants the world to love it. Unfortunately, the current specification is a complete piece of junk. I hope RDF dies a quick and public death."

I declare...backward chaining suits me fine! "My two favorite declarative tools right now are Pellet and Prova, both of which are open source java SemWeb tools that are highly compatible with Jena, which recently got a bump to 2.3 with fairly complete SPARQL support. Pellet is an implementation of OWL-DL and some related description logic facilities by the Mindswap guys in Maryland, who have absorbed some of the l33t Kowari/Tucana guys, too (Tucana was recently picked up by Northrop, BTW)."

"Prova is a prolog-variant built on top of Mandarax. It is a very effective and fun medium for scripting of high-level relationships and operations. The integration of prolog unification, java types, java methods, and java exceptions is done very nicely, and yields fine code economy. There are some rough edges in the docs, but we are helping to get these worked out in the pretty soon."

I guess that means David W is l33t! :-)

I Hope Not

Ruby is to Perl what C++ was to C. He qualified this by saying, "Ruby improves and simplifies the Perl language" which isn't really what I saw C++ doing at all.

Quote: "These are the folks that assert that Java's verbosity is "just finger typing that Eclipse/IntelliJ will do for me," and it doesn't matter if the resulting code has 20 times the visual bulk of a simpler approach. One of the basic tenets of the Python language has been that code should be simple and clear to express and to read, and Ruby has followed this idea, although not as far as Python has because of the inherited Perlisms. But for someone who has invested Herculean effort to use EJBs just to baby-sit a database, Rails must seem like the essence of simplicity. The understandable reaction for such a person is that everything they did in Java was a waste of time, and that Ruby is the one true path."

Related: Rocking With Ruby and One way Java is better than Ruby

A Better PageRank

This paper gives an example of some of the flaws with Google's PageRank algorithm and they suggest they have an algorithm that fixes it. Something is Wrong with Google’s Mathematical Model "In their original paper : ”The PageRank Citation Ranking: Bringing Order to the Web” [1], Page et al. suggest a new ranking algorithm named - PageRank. It is shown there that implementing the new algorithm boils down to solving a huge eigenvalues problem Ax = x (1) where A is a matrix which represents a graph related to the web. It is claimed that in order for the model to work properly, the graph should be strongly connected. In general, the graph is not strongly connected and we have ’sink’ set of pages."

"We have developed a new algorithm which can be considered as a modification of the original PageRank algorithm. The modified algorithm is stable and gives a correct ranking vector. The mathematical complexity of the suggested algorithm is the same as the complexity of the original one."

In Google's Librarian Center they've published as similar explanation of PageRank called "How does Google collect and rank results?".

LiveConnect, Lives as LAJAX

I ignored this when I first heard about it, as noted here: ""I was disappointed when [Bray] said that he was going to make a product announcement, and I was unenthusiastic when the announcement turned out to be about a Sun version of Derby," Leung wrote.".

Derby Demo hits a nerve "I think we hit a nerve with this demo. I think many of us within the Derby community recognized the potential for Derby within a web browser environment, but it's wonderful, great, fantastic to see how the community is "getting" it and running with it."

Derby ApacheCon demo and ApacheCon 2005: Ok, Tim, I'm not jaded anymore.

Minimum Union

The key operation to provide outer joins that are associative and communitive, as noted in Outer Joins Aren't Primitive, is called "minimum union". Minimum union (I've also seen "outer union") pads with nulls the tuples of two schemas and then unions them (without duplicate removal).

The original thesis "Algebraic Optimization of Outerjoin Queries" gives a variant of relational algebra that allows tuples defined by different sets of attributes (schemes) rather than padding with nulls. This actually removes the requirement for nulls. It also includes presenting the data in a nested relational form, where instead of having one tuple in a parent-child relationship, children are a set to a parent. This is just like the way Kowari presents its results (Figure 3.2 in the paper).

A Link in Time

• Getting more than what you pay for with open source databases. The tale of using a free database, using a commercial one and then putting the free one back. A quote: "There are a lot of companies out there paying dearly for commercial databases (and operating systems for that matter). As far as I'm concerned they might as well be flushing that money down the toilet. Actually, they might be better off. We certainly would have been."


• For the metrically minded, "Two Motivational Metrics for Agile Teams". So as long as you convince people that "Time to Obstacle Removal" and "Obstacles Removed per Iteration" are good metrics.


• The, "I'm sick of the comparison between building and software" rant.


• Martin Fowler has recently updated his "The New Methodology" with half a dozen or so flavours of agile, which one are you?


• Don't Click It for those people with zero button mice. The first example of a gesture to replace a click just seems ridiculously hard.


• Examples of using Chickenfoot for Firefox: Rewrite the Web
  The table sort, how to get a book from the library, simple examples and timely.


• Another REST framework - Restlet.


• Getting stuff done - the quick version


• How not to get a PhD

Event Horizon

• Semantic Web, Here We Come "The “Structured Blogging Initiative” is an attempt to jump-start the “semantic web,” the idea of giving deeper meaning to the Internet advocated by World Wide Web creator Tim Berners-Lee. By incorporating descriptive information into the code of web pages, laypeople will be able to designate their content as a movie review, an event posting, or an item available for sale." StructureBlogging initiative and other entries: "Structured blogging initiative taking off", "More StructuredBlogging feedback" and Structured Blogging is a thing you do -- not a format.


• Bill de hÓra discusses RDF and database schemas: "Using RDF storage provides flexibility at the domain level. Altering tables isn't needed because RDF, being a graph based, is naturally additive...My (somewhat anecdotal) experience with RDF is that datasets in the order of 10⁶ and greater aren't uncommon and that you should budget for an order of magnitude increase in terms of the number of rows required for the domain storage compared to an entity relational approach...It's an interesting question whether using RDBMSes to store RDF counts as some form of abuse, or bad engineering."

Ion Inside

First Mass Producible Quantum Computer Chip "Using the same semiconductor fabrication technology that is used in everyday computer chips, researchers were able to trap a single atom within an integrated semiconductor chip and control it using electrical signals, said Christopher Monroe, U-M physics professor and the principal investigator and co-author of the paper, "Ion Trap in a Semiconductor Chip." The paper appeared in the Dec. 11 issue of Nature Physics."

Marks

The Man Who Wasn't There: problems of missing or partially missing data in geoscience databases "In the literature discussions between Codd and Date on the propriety or otherwise of NULLs in relational databases, there seems to have been some confusion on both sides, on one very important question. That is the distinction between database representation and function evaluation. NULLs are one approach to the problem of handling missing data within the database."

So in this respect RDF is great - you don't have to come up with a value or values to represent missing data. You only have to worry about function evaluation.

"In fact the Codd 'mark' solution does not in itself require, as unfortunately implied by Codd himself, and vigorously attacked by Date, the use of 3- or 4-valued logic, and therefore cannot be dismissed so easily. Relational database theory is based on first-order predicate logic, which uses two truth values TRUE and FALSE. If there is no value for a data item, then the logical statement corresponding to the tuple containing that item can simply omit any mention of that particular column. If the value of this data item is required in an operation, then there is only one truth value which can be returned: FALSE. This applies to database set operations such as JOINs and also to numerical operations such as totals and averages where the absence of any required data value prevents the computation from being carried out. If a total or average is required in such a situation, then the problem can be circumvented only by first selecting non-absent data. This is the correct treatment, to ensure that statistics are computed on a valid data set."

Another example of marks, tuple marks.

SH writes in about incomplete data in observational science databases, the open world assumption, 3VL and NULL, McGoveran responds saying: "In a scientific database such as the type to which you allude, a reasonable interpretation of True and False under CWA is "valid by experiment and consistent with hypotheses" and "not validated by experiment or inconsistent with hypotheses". If you give this differentiation up with CWA and nulls, you've given up scientific reasoning and the scientific method."

In Kowari, if you have the following triples: _b1, <urn:sno>, "S1"; _b2, <urn:sno>, "S2"; _b3, <urn:pno> "P1".

And performed the following query:

select $s1
...
where $s1 <urn:sno> $o1 or $s2 <urn:pno> $o2;

It returns: _b1, _b2, null (really unconstrained).

However, if you select $s2 instead it returns: null, b3. Using the above idea, it would return _b1, _b2 for the first query and _b3 for the second.

I'm not sure I really like this solution, preserving unknown seems to make more sense as demonstrated in How FirstSQL Solves the EXISTS and Other Problems.