Teradata Lock，

There are four types of locks:

Exclusive

prevents any other type of concurrent access

–  Applies only to databases or tables, not rows.
–  Very restrictive

Write

prevents other Read, Write, Exclusive locks.

Read

prevents Write and Exclusive locks

Access

prevents Exclusive locks only.

Locks may be applied at three database levels:

Database

applies to all tables/views in the database.

Table/View

applies to all rows in the table/views.

Row Hash

applies to all rows with same row hash.

Lock types are automatically applied based on the SQL command:

SELECT—applies a Read lock.
UPDATE—applies a Write lock.
CREATE TABLE—applies an Exclusive lock.

Example : A new ACCESS lock request is granted immediately.

Advantages of Access locks:
•Permit quicker access to table in multi-user environment.
•Have minimal blocking effect on other queries.
•Very useful for aggregating large numbers of rows.
Disadvantages of Access locks:
•May produce erroneous results if performed during table
maintenance.