What is the function of a historian in terms of OPC and PLC?
Based upon your tags
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:
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.