How to query range of data in DB2 with highest performance?
-
06-07-2019 - |
Question
Usually, I need to retrieve data from a table in some range; for example, a separate page for each search result. In MySQL I use LIMIT keyword but in DB2 I don't know. Now I use this query for retrieve range of data.
SELECT *
FROM(
SELECT
SMALLINT(RANK() OVER(ORDER BY NAME DESC)) AS RUNNING_NO
, DATA_KEY_VALUE
, SHOW_PRIORITY
FROM
EMPLOYEE
WHERE
NAME LIKE 'DEL%'
ORDER BY
NAME DESC
FETCH FIRST 20 ROWS ONLY
) AS TMP
ORDER BY
TMP.RUNNING_NO ASC
FETCH FIRST 10 ROWS ONLY
but I know it's bad style. So, how to query for highest performance?
Solution
My requirement have been added into DB2 9.7.2 already.
DB2 9.7.2 adds new syntax for limit query result as illustrate below:
SELECT * FROM TABLE LIMIT 5 OFFSET 20
the database will retrieve result from row no. 21 - 25
OTHER TIPS
Not sure why you are creating the TMP table. Isn't RUNNING_NO aready in ascending sequence? I would think:
SELECT SMALLINT(RANK() OVER(ORDER BY NAME DESC)) AS RUNNING_NO,
DATA_KEY_VALUE,
SHOW_PRIORITY
FROM EMPLOYEE
WHERE NAME LIKE 'DEL%'
ORDER BY NAME DESC
FETCH FIRST 10 ROWS ONLY
would give the same results.
Having an INDEX over NAME on the EMPLOYEE table will boost performance of this query.
It's very difficult, it is depends which database do you have.
for example:
SELECT * FROM (
SELECT
ROW_NUMBER() OVER (ORDER BY ID_USER ASC) AS ROWNUM,
ID_EMPLOYEE, FIRSTNAME, LASTNAME
FROM EMPLOYEE
WHERE FIRSTNAME LIKE 'DEL%'
) AS A WHERE A.rownum
BETWEEN 1 AND 25