Several major players built alternatives to conventional database systems: Google created BigTable, Amazon built Dynamo and Facebook initiated Cassandra. There are many other comparable open source initiatives such as CouchDB and MongoDB. These systems are part of a trend called NoSQL because it is not centered around the SQL language. While there has always been non SQL-based database systems, the rising popularity of these alternatives in industry is drawing attention.

In The “NoSQL” Discussion has Nothing to Do With SQL, Stonebraker opposes the NoSQL trend in those terms:

(…) blinding performance depends on removing overhead. Such overhead has nothing to do with SQL, but instead revolves around traditional implementations of ACID transactions, multi-threading, and disk management.

In effect, Stonebraker says that all of the benefits of the NoSQL systems have nothing to do with ditching the SQL language. Of course, because the current breed of SQL is Turing complete, it is difficult to argue against SQL at the formal level. In theory, all Turing complete languages are interchangeable. You can do everything (bad and good) in SQL.

However, in practice, SQL is based on joins and related low-level issues like foreign keys. SQL entices people to normalize their data. Normalization fragments databases into smaller tables which is great for data integrity and beneficial for some transactional systems. However, joins are expensive. Moreover, joins require strong consistency and fixed schemas.

In turn, avoiding join operations makes it possible to maintain flexible or informal schemas, and to scale horizontally. Thus, the NoSQL solutions should really be called NoJoin because they are mostly defined by avoidance of the join operation.

How do we compute joins? There are two main techniques :

• When dealing with large tables, you may prefer the sort merge algorithm. Because it requires sorting tables, it runs in O(n log n). (If your tables are already sorted in the correct order, sort merge is automatically the best choice.)
• For in-memory tables, hash joins are preferable because they run in linear time O(n). However, the characteristics of modern hardware are increasing detrimental to the hash join alternative (see C. Kim, et al. Sort vs. Hash revisited. 2009).

(It is also possible to use bitmap indexes to precompute joins.) In any case, short of precomputing the joins, joining large tables is expensive and requires source tables to be consistent.

Conclusion: SQL is a fine language, but it has some biases that may trap developers. What works well in a business transaction system, may fail you in other instances.

I often come across the following type of arguments in research papers:

• You could save 3 bits of storage for every value in your database. Surely that’s irrelevant. Nobody cares about saving 3 bits!
• You can sort arrays in 10 ms. Surely, that cannot be improved upon? You are already down to 10 ms and nobody cares about such small delays.

I hope you can see what is wrong with these statements?

I call it the fallacy of absolute numbers: you express a measure or a gain in absolute value, and then conclude to optimality or near optimality because the number appears small (or large).

Remember: Saving 3 bits of storage out of 6 bits is a 2:1 compression ratio. Sorting in 5 ms instead of 10 ms doubles the speed.

Disclaimer: I am sure that someone else has documented this fallacy, but I could not find any reference to it.

I’d like to know a lot more about XML indexing—if only because I really ought to be teaching this topic. So I decided to write a blog post to expose what I know, hoping that some knowledgeable readers will fill me in on what I am missing.

Mostly, I expect we are interested in indexing XPath queries. Not only is XPath useful on its own, but it is also the basis for the FLWOR expressions in XQuery.

A typical XPath expression will select only a small fraction of any XML document (such as the value of a particular attribute). Thus, a sensible strategy is to represent the XML documents as tables. There are several possible maps from XML documents to tables. One of the most common is ORDPATH.

In the ORDPATH model, the root node receives the identifier 1, the first node contained in the root node receives the identifier 1.1, the second one receives the identifier 1.2, and so on. Given the ORDPATH identifiers, we can easily determine whether two nodes are neighbors, or whether they have a child-parent relationship.

As an example, here’s an XML document and its (simplified) ORDPATH representation:


<liste temps="janvier" >
<bateau />
<bateau >
<canard />
</bateau>
</liste>


Given a table, we can easily index it using standard indexes such as B trees or hash tables. For example, if we index the value column, we can quickly process the XPath expression @temps=”janvier”.

Effectively, we can map XPath and XQuery queries into SQL. This leaves relatively little room for XML-specific indexes. I am certain that XML database designers have even smarter strategies, but do they work significantly better?

Reference: P. O’Neil, et al.. ORDPATHs: insert-friendly XML node labels. 2004.

Further reading: Native XML databases: have they taken the world over yet?

A common advice given out to young researchers is to find a niche. (See Michael’s Branding Your Research). That is certainly good advice. Instead of being another young researcher, you can be the new guy working on topic X. But it always seems to happen no matter what: most Ph.D. thesis address a narrow topic. I believe that the real advice people would like to give is: find yourself a nice topic, and make sure this topic becomes fashionable. Of course, this implies that you can somehow predict the future, or have a thesis supervisor with enough clout that he can either initiate new trends, or have inside knowledge regarding the upcoming trends.

A more interesting question is what you should do with the rest of your career, assuming you landed a research job, somehow. Should you find yourself one or two niche topics and stay there for the rest of your life? That is a common strategy. You save precious time: instead of having to skim 100 research articles a year, you may get by with 20 or 30 research articles, or even less. Moreover, because you are the leading authority on one or two topics, you can never be caught unaware. You never have to worry about finding new topics: you just keep on iteratively improving whatever you are doing right now. With some luck, you can reuse your funding proposals year after year. Finally, you can quickly get to know everyone that matters regarding these narrow topics. And that is a perfectly good strategy.

The problems begin when we associate the lack of a steady trajectory with failure. Encouraging static research topics leads to conservatism. Meanwhile, some of the most innovative researchers have cultivated varied interests. Von Neumann was a set theorist, but he wrote 20 papers in Physics, and even in Mathematics, he covered a wide range of topics (set theory, logic, topological groups, measure theory, ergodic theory, operator theory, and continuous geometry). Would we have been better off had von Neumann remained a pure set theorist?

And I tend to have more trust in researchers who have their eggs in different baskets. They can afford to be a bit more critical.

Warning: I am not urging Ph.D. students to change topic repeatedly while writing up their thesis. Finish whatever you start. And be aware that approaching a new research topic can be costly.

Technological progress tends to increase the available information. Thus, our capacity to manage this information becomes overloaded (hence the term information overload). As Clay Shirky explained: it is not so much an information overload, as a filter failure. The abundance of information is never a problem. The real problem is the lack of efficient strategies to index, summarize, filter, cross-reference and archive information.

But information overload is nothing new. In Reading Strategies for Coping With Information Overload ca. 1550-1700, Blair surveys the techniques our ancestors invented to cope with the abundance of books :

• the alphabetical index;
• the reference book,
• copy and paste (with actual scissors) to save time in note-taking.

What I find fascinating is the historical perspective: while still useful, the alphabetical index is hardly exciting anymore. It has been supplanted by full text search (in e-books). There are still reference books (such as dictionaries), but they are being replaced with online tools. Information overload continues to generate many inventions: the search engine (such as Google), the recommender system (as on Amazon.com), and the social networks (such as Twitter). Literally, these tools expand our minds. We become smarter.

Yet, every time I finish writing a research article, I am amazed at how old fashioned the format is.

• Research journals still ask for silly metadata such as keywords, even though most researchers rely on full text search.
• The format is clearly meant for paper, even though most of my collaborators browse research articles on their computers.
• We have silly things like page limitations.
• It is excessively difficult to correct or improve a “published” article.

There is hope. The PLoS One journal presents research articles in an innovative format. The article is interactive: anyone can rate and comment it. Many journals allow the authors to upload supplementary material. Yet, I predict that in 20 years, we will look back and think that academic publishing in 2010 was archaic. (I admit that it is not a daring prediction.) There is much room for innovation.

Source: Erik Duval.

The greatest challenge for a researcher is to choose projects that have a good chance of delivering impact. Alain Désilets from NRC—co-author of VoiceGrip, Webitext and the Cross Lingual Wiki Engine—shared his strategies with me:

• Look at how many workdays per week you can dedicate to research and make that be the number of projects you can work on in parallel. In other words, if you are one of the lucky few who can dedicate 5 days per week doing research, then you have room for 5 projects.
• Invest your energy proportionally to the amount of positive feedback you receive for each project. This includes collaboration offers, grants, potential users, and so on.
• Never work alone on a project for too long. It’s OK to start exploring a compelling idea on your own for a couple of months, but if you can’t convince someone else to work with you on it, maybe it’s not such a great idea after all. Maybe it’s technically infeasible, maybe there is no need or market for it, or maybe it’s just too much ahead of its time. Don’t completely give up on the idea yet. Put it on the ice for now and keep sharing that idea with people until you meet the right people to make it happen with you.
• Instead of looking for partnership money which will require you to spend months drafting and revising agreements (who wants to deal with lawyers anyway), look for talented people who have control over their own time, and are willing to invest some of that precious resource working with you on an idea. Don’t worry about who will own the baby before it’s actually born (that usually ensures that the work relationship will never get off the ground). Just make sure everyone keeps a lab book documenting who did what so that you will have a basis to argue in a friendly and civilised manner about who owns what share of the baby, if you ever have that nice problem.
• Talk to lots of different people from different walks of life about your idea. You never know who will give you the insight or contact you need to advance to the next level on a given project. Of course if you do this, you pretty much give up on the idea of patenting your idea.
• Make sure you collocate in time and space as much as you can with your collaborators. There was a time when I had 5 projects (those were the happy days of 5 days of research per week), and I had scheduled things so that on Mondays I would work with Joe on project X, Tuesdays were dedicated to working on project Y with Jane, and so on.
• Find and organisation or a type of end users with an interesting problem that you think you could solve using some bleeding edge technology. Become very intimate with the problem, maybe even pretending to do these people’s job for a day. Once you understand their problem well, don’t jump right away to the hi-tech solution. Instead, start with the Simplest Thing That Could Possibly Work, and only add complex technology where and when it is needed. This may not get you a publication in a first tier journal, but it greatly increases your odds of developing a system that will actually be used. Plus, when you DO find that you need sophisticated technology, you know exactly why, and what the actual value added is.
• Use Agile Development practices which allow you to advance your projects in short, highly focused bursts of a few days (1-day burst are even possible). Write lots of short “stories” that describe things you can accomplish in a day or less, and keep re-prioritizing them so that the ones that currently add the most value to your target users are always at the top. Use Test Driven Development to ensure that your system is always stable and that you can put it aside for a few days or months, yet pick up right from where you left. These kinds of techniques are essential if you want to be able to quickly reallocate your effort depending on how hot your different projects are.

Disclaimer: it does not necessarily  reflect the views of his employer.

The publishing house Elsevier invited me to fill out a survey regarding their journals. As a reward, they gave me a glimpse at their statistics.

The three most important considerations when choosing a research journals are (in order) :

1. Speed of review process
2. Standard of reviews
3. Overall reputation of the journal

And the activity researchers complained to most about? Peer reviewing manuscripts.

In any case, if you want to build a good journal and attract great papers, make sure you have fast and competent peer review. (Duh!) Meanwhile, having a good printer or a good editorial board are much less important.