Marten Mickos may no longer run MySQL, but his ghost still haunts the database market.

Years ago, Mickos declared, "The relational database market is a $9 billion a year market. I want to shrink it to $3 billion and take a third of the market." MySQL never got to that point, generating roughly $100 million in sales before being acquired by Sun for $1 billion, but that commoditization urge has hit the database market hard.

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How hard? Today's open source databases comprise a fraction of the paid database market, yet now constitute as much as a quarter of all database usage. The database market, in short, is ripe for open source cannibalization.

A big market ... and target

The database market is a huge and growing market. According to IDC, the overall database market tops $40 billion today and should reach $50 billion by 2017.

Most of that market is composed of proprietary databases like Oracle and Microsoft's SQL Server.

Yet despite what appears to be reasonable growth, all is not well in database land -- at least, not if you're a legacy vendor.

Growing slower all the time

As Gartner highlights in a recent research report, open source databases now consume 25 percent of relational database usage.

Why should we care? Because, according to Gartner, "The potential impact of [open source databases] capturing workloads that would otherwise go to commercial products will manifest in declining growth rates for the latter."

In fact, it's already happening. Though the proprietary RDBMS market grew at a sluggish 5.4 percent in 2014, the open source database market grew 31 percent to hit $562 million.

Think about that: $562 million in revenue but 25 percent overall database usage in a market worth more than $40 billion. What will happen to that $40 billion when open source databases claim 50 percent market share in terms of database use?

Physician, eat thyself

The database vendors need an answer -- soon. A quick look at DB-Engines, which ranks database popularity, suggests that this trend toward open source will only continue:

With very few exceptions, open source databases -- both relational and NoSQL -- are chewing into proprietary database popularity.

This isn't a development a vendor can combat through ever-tightening account control, not given the rising importance of developers. As Gartner highlights, "Developer choice, an increasingly important determinant of product usage, often occurs without reference to corporate standard preferences, as usage increasingly falls outside of IT organizations."

While it used to be the case that picking open source databases was a trade-off of robustness and performance for developer convenience, that's no longer the case according to Gartner for open source RDBMSes: "Open source ... RDBMSes have matured and today can be considered by information leaders, DBAs and application development management as a standard infrastructure choice for a large majority of new enterprise applications."

Assuming the same happens in the NoSQL market -- a very good assumption-- the hitherto indomitable proprietary database market looks ripe for open source cannibalization. Mickos had the vision, but today's open source database leaders will really get to live it.