Unity routes session Read requests to a single Teradata Database system. If no routing rules are defined for a client logon, Unity provides a default routing rule (DefaultRouting).
When the requested data resides in tables on a subset of Teradata Database systems, the Sequencer determines which Teradata Database systems have all of the tables requested in the Read operation.
The algorithm selected for routing depends on both the routing rule specific to the session client logon and current system and table states. Only tables in Active or Read-Only state are available for reading. If there are no tables in Active or Read-Only state, but one or more tables are in Standby state, then Unity uses one of the tables in Standby state for the Read operation.