What is the function of a historian in terms of OPC and PLC?
Based upon your tags opc and plc, you're refering to Historian in the SCADA context.
Basically, a historian is normally a service that collects data from various devices in SCADA network and logs to a database.
A proprietary (time-series) database is normally used. Normal marketing makes claims such as:
Faster Speeds In contrast, a plant-wide historian provides a much faster read/write performance over a relational database and “down to the millisecond” resolution for true real-time data. This capability enables better responsiveness by quickly providing the granularity of data needed to analyze and solve intense process applications.
Higher Data Compression The powerful compression algorithms of Proficy Historian enables you to store years of data easily and securely online, which enhances performance, reduces maintenance and lowers costs. You can configure GE Intelligent Platforms’s Proficy Historian without the active maintenance and back-up routines that a traditional RDB requires. Archives can be automatically created, backed up, and purged—enabling extended use without the need for a database administrator.
These marketing bullet points are often optimistic of poor RDBMS implementations. "Faster Speeds" is confused with the precision used for the timestamping datatype and proper indexing of data in a relational database. "Higher Data Compression" claims are realized by using Swinging Door Algorithms that can also be implemented for most RDBMS. Their use is explained in the Chevron whitepaper Data Compression for Process Historians. A new trend is using a classical time-series database for the historical data, and a relational database for analysis and reporting. An example of this hybrid configuration is OSIsoft using Microsoft SQL server for its Analysis Framework that holds the asset management hierarchy and relational type batch history data with a shift from a tag-centric application to an asset-centric application with a relational database at the core.
Popular options are Proficy Historian from General Electric, or OSIsoft's PI Historian.