How to query range of data in DB2 with highest performance?

问题

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?

回答1:

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

回答2:

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.

回答3:

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

回答4:

It's simple, to make a pagination just follow the syntax.

LIMIT (pageSize) OFFSET ((currentPage) * (pageSize))

来源：https://stackoverflow.com/questions/265850/how-to-query-range-of-data-in-db2-with-highest-performance

标签

sql

database

pagination

db2