15.10 - Example of Two Serial Transactions - Teradata Database

This example shows 2 transactions. The first transaction begins its operations before the second transaction.

Consider the following table definition.

     CREATE TABLE table_1 (

       column_1 INTEGER, 

       column_2 INTEGER, 

       column_3 INTEGER, 

       column_4 INTEGER) 

     PRIMARY INDEX (column_1);

The following two transactions are running concurrently, with the first having started prior to the second.

LOCKING table_1 FOR READ 

CREATE INDEX (column_3, column_4) ON table_1;

SELECT * 

FROM table_1 

WHERE column_3 = 124 

AND column_4 = 93;

Each transaction places a table-level READ lock on table_1. The transactions obtain access to table_1 and run concurrently.

Note: The SELECT request does not recognize the index being created by the CREATE INDEX request.

To eliminate concurrency, change the coding of transaction 1 to make its explicit lock EXCLUSIVE.

LOCKING table_1 FOR EXCLUSIVE 

CREATE INDEX (column_3, column_4) ON table_1;

SELECT * 

FROM table_1 

WHERE column_3 = 124

AND column_4 = 93;

The upgraded LOCKING FOR EXCLUSIVE modifier in transaction 1 blocks the table‑level READ lock request on table_1 in transaction 2.