Relational Database or Real-Time Historian for Logging Process Data?

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Know the right tool for logging data from a process control application

Quite a few years ago, while living in a part of the world where personal computers were a relatively new phenomenon, I happened to be in an office watching a secretary busily typing away at her new PC. She was thrilled to have such powerful tool to use. "Look!" she said excitedly. "Now I can write and correct my work so easily!" I looked at the screen, and had to smile. She was composing an entire business letter within a single cell of an Excel spreadsheet.

What determines the right tool for the job? For that secretary, the right tool was the one that was available and that she knew how to use. What's the right tool for logging data from a process control application? Sometimes a CSV file is all that is needed. Sometimes Excel will do. More often, though, engineers and system integrators will use either a relational database or a real-time historian to store permanent records of process data.

Relational databases have the advantage of availability and familiarity. SQL Server, MySQL, Oracle, or any other relational database, including Microsoft Access, are usually already installed at the company. They offer a common interface, ODBC, and the IT department is familiar with them. It's no surprise that relational databases are being used for logging process data, particularly when the requests for data come from management personnel familiar with this kind of database.

But a relational database may not be the ideal choice for all process control applications. Designed for the needs of corporations, businesses, and banks to store transactional data, relational databases are optimized for analyzing complex relationships between data. These databases can afford to focus processing power on these relationships because the data itself gets updated relatively infrequently, usually in terms of seconds, minutes, or hours. While analytical power is good for business applications, process control applications typically don't need it. What they do need is speed.