Window functions offer yet another way to implement pagination in SQL. This is a flexible, and above all, standards-compliant method. However, only SQL Server and the Oracle database can use them for a pipelined top-N query. MySQL,0 DB2 and PostgreSQL do not abort the index scan after fetching enough rows and therefore execute these queries very inefficiently.
The following example uses the window function ROW_NUMBER for a pagination query:
SELECT *
FROM ( SELECT sales.*
, ROW_NUMBER() OVER (ORDER BY sale_date DESC
, sale_id DESC) rn
FROM sales
) tmp
WHERE rn between 11 and 20
ORDER BY sale_date DESC, sale_id DESCThe ROW_NUMBER function enumerates the rows according
to the sort order defined in the over
clause. The outer where clause uses
this enumeration to limit the result to the second page (rows 11 through
20).
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The Oracle database recognizes the abort condition and uses the
index on SALE_DATE and SALE_ID to produce a
pipelined top-N behavior:
- DB2
Explain Plan ------------------------------------------------------------- ID | Operation | Rows | Cost 1 | RETURN | | 65658 2 | FILTER | 100933 of 1009326 | 65658 3 | FETCH SALES | 1009326 of 1009326 | 65295 4 | IXSCAN (REVERSE) SL_DTID | 1009326 of 1009326 | 5679 Predicate Information 2 - RESID (11 <= Q3.$C8) RESID (Q3.$C8 <= 20)Please note that DB2 LUW 10.5 does not execute this query as top-n query. Although it prevents the sort operation it still reads through the entire index—it does not abort execution after fetching 20 rows.
To get a proper top-n abort after reading 20 rows, you must double-wrap the query to first apply the top-n abort condition and then filtering away the first 10 rows:
SELECT * FROM (SELECT * FROM (SELECT sales.* , ROW_NUMBER() OVER (ORDER BY sale_date DESC , sale_id DESC) rn FROM sales ) tmp WHERE rn <= 20 ) tmp2 WHERE rn > 10 ORDER BY sale_date DESC, sale_id DESC;Explain Plan ----------------------------------------------------------------- ID | Operation | Rows | Cost 1 | RETURN | | 21 2 | FILTER | 7 of 20 | 21 3 | FETCH SALES | 20 of 1009326 | 65352 4 | IXSCAN (REVERSE) SALES_DT_ID | 1009326 of 1009326 | 5736 Predicate Information 2 - RESID (10 < Q3.$C8)Note that the filter
rn <= 20does not appear in the Predicate Information section, yet the row count estimate reflects it.- Oracle
--------------------------------------------------------------- |Id | Operation | Name | Rows | Cost | --------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1004K| 36877 | |*1 | VIEW | | 1004K| 36877 | |*2 | WINDOW NOSORT STOPKEY | | 1004K| 36877 | | 3 | TABLE ACCESS BY INDEX ROWID | SALES | 1004K| 36877 | | 4 | INDEX FULL SCAN DESCENDING | SL_DTID | 1004K| 2955 | --------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("RN">=11 AND "RN"<=20) 2 - filter(ROW_NUMBER() OVER ( ORDER BY "SALE_DATE" DESC, "SALE_ID" DESC )<=20)
The WINDOW NOSORT STOPKEY operation indicates that
there is no sort operation (NOSORT) and that the database aborts the execution when reaching
the upper threshold (STOPKEY). Considering that the aborted operations are executed in a
pipelined manner, it means that this query is as efficient as the offset
method explained in the previous
section.
The strength of window functions is not pagination, however, but analytical calculations. If you have never used window functions before, you should definitely spend a few hours studying the respective documentation.
Links
Oracle: Analytic Functions in 11g
PostgreSQL: Window Functions in 9.1
SQL Server: OVER Clause in 2008R2 and 2012 (enhanced)
