Inserting Rows Into Queue Tables - Teradata Database

Inserting Rows Into Queue Tables

The first column of a queue table is defined as a Queue Insertion TimeStamp (QITS) column. The values in the column determine the order of the rows in the queue, resulting in approximate first-in-first-out (FIFO) ordering.

If you want the QITS value of a row to indicate the time that the row was inserted into the queue table, then you can use the default value, the result of CURRENT_TIMESTAMP, instead of supplying a value. If you want to control the placement of a row in the FIFO order, you can supply a TIMESTAMP value for the QITS column.

For a multistatement request containing multiple INSERT requests that do not supply values for the QITS column, the QITS values are the same for every row inserted.

If you want unique QITS values for every row in a queue table, you can do any of the following things:

Supply a TIMESTAMP value for the QITS column in every INSERT request.

Avoid multistatement requests containing multiple INSERT statements that do not supply values for the QITS column.

Add incremental offsets to the current timestamp for the QITS column value in each INSERT request.

For example:

     INSERT shopping_cart(CURRENT_TIMESTAMP + INTERVAL '0.001', 100)

    ;INSERT shopping_cart(CURRENT_TIMESTAMP + INTERVAL '0.002', 200)

    ;INSERT shopping_cart(CURRENT_TIMESTAMP + INTERVAL '0.003', 300);

Regarding performance, an INSERT operation into a queue table has the following effects:

Does not affect response time when the system is not CPU-bound.

Is more expensive than an INSERT into a base table because it requires the update of an internal in-memory queue.

For details on queue tables and the queue table cache, see “CREATE TABLE” in SQL Data Definition Language.