In shortA historian is a time-series-oriented data system that records process values at high speed with timestamp and quality information and stores them for a long time.
Historians are designed to store continuously arriving measurements in compressed form and to query them quickly by time range. Most SCADA solutions include a historian or connect to one.
A general-purpose SQL table can also be used for logging; the choice depends on query needs, retention and existing tools. When designing an SQL schema you can use the SQL table designer.
In OPC Router projects
OPC Router can move data to a historian, an SQL database or a time-series store such as InfluxDB.