batch update a column using sql - sql

Question

I have a database table (db2) with a very large number of rows (a couple million). I need to change the datatype of one of the columns.
In DB2 LUW there does not seem to be a way to directly change the datatype of a column (ALTER TABLE ALTER COLUMN SET DATA TYPE does not work). So I am creating a new column, copying data to it and dropping the old column.
Since its not a good idea to do a direct update on the table, I'm creating a procedure which will update and commit 10000 rows at a time.
Given this, I have the following questions:
What is the best way to carry out the update here? - as far as I can tell, the cursor allows iteration over 1 row at a time. Is updating 10000 rows one at a time, then committing, and repeating until the table is updated the correct way to do it?
Is there any better way to handle the original issue of changing the data type of a column in a simpler way?

Answer 1

DB2 has a feature called LOAD FROM CURSOR that allows for fast migration of data.
The following is an example of this using LOAD FROM CURSOR:
-- this is the original table
CREATE TABLE TEST (
ID INT NOT NULL GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
TEXT VARCHAR(50)
)#
CREATE TABLE TEST_NEW (
ID INT NOT NULL GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
TEXT CLOB(5000000)
)#
DECLARE C1 CURSOR FOR SELECT ID, TEXT FROM TEST#
LOAD FROM C1 OF CURSOR INSERT INTO TEST_NEW (ID,TEXT)#
DROP TABLE TEST#
RENAME TABLE TEST_NEW TO TEST#
In addition, the following procedure can also be used (commit after every 10000 records):
-- this is the original table
CREATE TABLE TEST (
ID INT NOT NULL GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
TEXT VARCHAR(50)
)#
CREATE OR REPLACE PROCEDURE MIGRATE_TEST()
LANGUAGE SQL
BEGIN
DECLARE EOF INT DEFAULT 0;
DECLARE CUR_COUNT INT DEFAULT 0;
DECLARE CUR_ID INT DEFAULT 0;
DECLARE C CURSOR WITH HOLD FOR
SELECT ID FROM TEST WHERE TEXT_NEW IS NULL;
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET EOF = 1;
OPEN C;
FETCH_LOOP: LOOP
FETCH FROM C INTO CUR_ID;
IF EOF <> 0 THEN
LEAVE FETCH_LOOP;
END IF;
UPDATE TEST
SET TEXT_NEW = TEXT
WHERE ID = CUR_ID ;
SET CUR_COUNT = CUR_COUNT + 1;
IF CUR_COUNT >= 10000 THEN
CALL DBMS_OUTPUT.PUT_LINE('COMMITTING');
COMMIT WORK;
SET CUR_COUNT = 0;
END IF;
END LOOP FETCH_LOOP;
COMMIT WORK;
CLOSE C;
END#