|Lock Mode||Blocks/Blocked By Modes|
|Exclusive||Access, Write, Read|
|Write||Read, Exclusive, other Writes|
Unity holds locks for the duration of a transaction. Session mode and TCL statements determine transaction duration. Only one transaction can hold a Write or Exclusive lock on an object at one time, but multiple transactions can be granted Read locks and Access locks simultaneously.
For Read queries, the session isolation level determines whether an Access or Read lock is used. It also determines if any locking modifiers exist in the query and relevant database configuration parameter values. For Write queries, any Read of any object always results in a Read lock. This ensures consistent results across all Teradata systems.