Improve Performance with Teradata Partition Elimination: The Stored Procedure Approach

Teradata Partition Elimination – the Stored Procedure Approach

Occasionally, we may want to select rows in a table’s character column that begin with a specific prefix, yet we lack prior knowledge of all the values contained in that column. This could include newly introduced codes in the future.

If we are lucky, there might be a convention for these codes, such as: “All code values have to start with an ‘A’ character”.

With SQL, this problem can be resolved using the following approach:

SELECT * FROM The_Table WHERE CODE_COLUMN LIKE ‘A%’;

or

SELECT * FROM The_Table WHERE SUBSTR(CODE_COLUMN,1,1) = ‘A’;

While the SQL statement above resolves our issue, accessing a table with numerous rows could potentially result in performance problems.

Even if the column CODE_COLUMN were to partition the table, we might end up with a full table scan (FTS), as partition elimination cannot be applied if a LIKE or SUBSTR function is defined on the partition column(s):

CREATE TABLE The_Table
(
PK INTEGER NOT NULL,
Code_Column CHAR(100)
) PRIMARY INDEX (PK)
PARTITION BY (Code_Column);

Fortunately, a Teradata Stored Procedure can resolve this issue.

1. All distinct codes are extracted from the table and written into a variable.
2. The list of code values is dynamically inserted into a SQL statement and executed:

REPLACE PROCEDURE MY_SP()
BEGIN
DECLARE CODE_LIST VARCHAR(3200);
SET MySQL = ‘CREATE VOLATILE MULTISET TABLE MY_CODES AS
(
SELECT CODE_COLUMN FROM THE_TABLE WHERE CODE_COLUMN LIKE ”A%” GROUP BY 1
) WITH DATA PRIMARY INDEX (CODE_COLUMN) ON COMMIT PRESERVE ROWS;’;

CALL DBC.SysExecSQL(MySQL);
SET CODE_LIST=”’Ax”’;
FOR TheRow AS CODES_CURSOR CURSOR
FOR
  SELECT
  CODE_COLUMN,RANK(CODE_COLUMN) AS RNK
  FROM MY_CODES
  DO
  IF TheRow. RNK = 1
  THEN
    SET CODE_LIST = ”” || TheRow. CODE_COLUMN || ””;
  ELSE
    SET CODE_LIST = CODE_LIST || ‘,”’ || TheRow. CODE_COLUMN || ””;
  END IF;
END FOR;

SET MySQL =’

INSERT INTO TARGET_TABLE
SELECT PK,COUNT(*)
FROM THE_TABLE
WHERE
CODE_COLUMN IN (”’ || CODE_LIST || ”’)
GROUP BY 1
;’

CALL DBC.SysExecSQL(MySQL);
END;