After implementing a pipelined top-N query to retrieve the first page efficiently, you will often also need another query to fetch the next pages. The resulting challenge is that it has to skip the rows from the previous pages. There are two different methods to meet this challenge: firstly the offset method, which numbers the rows from the beginning and uses a filter on this row number to discard the rows before the requested page. The second method, which I call the seek method, searches the last entry of the previous page and fetches only the following rows.
The following examples show the more widely used offset method. Its main advantage is that it is very easy to handle—especially with databases that have a dedicated keyword for it (offset
). This keyword was even taken into the SQL standard as part of the fetch first
extension.
- DB2
DB2 supports
offset
since release 11.1. The standard conforming alternative usingROW_NUMBER()
window function (see next section) works in earlier releases. There are two other ways to get offset functionality, none of them recommendable: (1) usingdb2set DB2_COMPATIBILITY_VECTOR=MYS
to enablelimit
andoffset
like MySQL supports it. This does, however, not allow to combinefetch first
withoffset
; (2) usingdb2set DB2_COMPATIBILITY_VECTOR=ORA
to get Oracle’sROWNUM
pseudo column (see Oracle example).- MySQL
MySQL and PostgreSQL offer the
offset
clause for discarding the specified number of rows from the beginning of a top-N query. Thelimit
clause is applied afterwards.SELECT * FROM sales ORDER BY sale_date DESC LIMIT 10 OFFSET 10
- Oracle
The Oracle database supports
offset
since release 12c. Earlier releases provide the pseudo columnROWNUM
that numbers the rows in the result set automatically. It is, however, not possible to apply a greater than or equal to (
) filter on this pseudo-column. To make this work, you need to first “materialize” the row numbers by renaming the column with an alias.>=
SELECT * FROM ( SELECT tmp.*, rownum rn FROM ( SELECT * FROM sales ORDER BY sale_date DESC ) tmp WHERE rownum <= 20 ) WHERE rn > 10
Note the use of the alias
RN
for the lower bound and theROWNUM
pseudo column itself for the upper bound (thanks to Tom Kyte).- PostgreSQL
The
fetch first
extension defines anoffset ... rows
clause as well. PostgreSQL, however, only acceptsoffset
without therows
keyword. The previously usedlimit/offset
syntax still works as shown in the MySQL example.SELECT * FROM sales ORDER BY sale_date DESC OFFSET 10 FETCH NEXT 10 ROWS ONLY
- SQL Server
SQL Server does not have an “offset” extension for its proprietary
top
clause but introduced thefetch first
extension with SQL Server 2012. Theoffset
clause is mandatory although the standard defines it as an optional addendum.SELECT * FROM sales ORDER BY sale_date DESC OFFSET 10 ROWS FETCH NEXT 10 ROWS ONLY
Besides the simplicity, another advantage of this method is that you just need the row offset to fetch an arbitrary page. Nevertheless, the database must count all rows from the beginning until it reaches the requested page. Figure 7.2 shows that the scanned index range becomes greater when fetching more pages.
Figure 7.2 Access Using the Offset Method

This has two disadvantages: (1) the pages drift when inserting new sales because the numbering is always done from scratch; (2) the response time increases when browsing further back.
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The seek method avoids both problems because it uses the values of the previous page as a delimiter. That means it searches for the values that must come behind the last entry from the previous page. This can be expressed with a simple where
clause. To put it the other way around: the seek method simply doesn’t select already shown values.
The next example shows the seek method. For the sake of demonstration, we will start with the assumption that there is only one sale per day. This makes the SALE_DATE
a unique key. To select the sales that must come behind a particular date you must use a less than condition (<
) because of the descending sort order. For an ascending order, you would have to use a greater than (>
) condition. The fetch first
clause is just used to limit the result to ten rows.
SELECT *
FROM sales
WHERE sale_date < ?
ORDER BY sale_date DESC
FETCH FIRST 10 ROWS ONLY
Instead of a row number, you use the last value of the previous page to specify the lower bound. This has a huge benefit in terms of performance because the database can use the SALE_DATE < ?
condition for index access. That means that the database can truly skip the rows from the previous pages. On top of that, you will also get stable results if new rows are inserted.
Nevertheless, this method does not work if there is more than one sale per day—as shown in Figure 7.2—because using the last date from the first page (“yesterday”) skips all results from yesterday—not just the ones already shown on the first page. The problem is that the order by
clause does not establish a deterministic row sequence. That is, however, prerequisite to using a simple range condition for the page breaks.
Without a deterministic order by
clause, the database by definition does not deliver a deterministic row sequence. The only reason you usually get a consistent row sequence is that the database usually executes the query in the same way. Nevertheless, the database could in fact shuffle the rows having the same SALE_DATE
and still fulfill the order by
clause. In recent releases it might indeed happen that you get the result in a different order every time you run the query, not because the database shuffles the result intentionally but because the database might utilize parallel query execution. That means that the same execution plan can result in a different row sequence because the executing threads finish in a non-deterministic order.
Important
Paging requires a deterministic sort order.
Even if the functional specifications only require sorting “by date, latest first”, we as the developers must make sure the order by
clause yields a deterministic row sequence. For this purpose, we might need to extend the order by
clause with arbitrary columns just to make sure we get a deterministic row sequence. If the index that is used for the pipelined order by
has additional columns, it is a good start to add them to the order by
clause so we can continue using this index for the pipelined order by
. If this still does not yield a deterministic sort order, just add any unique column(s) and extend the index accordingly.
In the following example, we extend the order by
clause and the index with the primary key SALE_ID
to get a deterministic row sequence. Furthermore, we must apply the “comes after” logic to both columns together to get the desired result:
CREATE INDEX sl_dtid ON sales (sale_date, sale_id)
SELECT *
FROM sales
WHERE (sale_date, sale_id) < (?, ?)
ORDER BY sale_date DESC, sale_id DESC
FETCH FIRST 10 ROWS ONLY
The where
clause uses the little-known “row values” syntax (see the box entitled “SQL Row Values”). It combines multiple values into a logical unit that is applicable to the regular comparison operators. As with scalar values, the less-than condition corresponds to “comes after” when sorting in descending order. That means the query considers only the sales that come after the given SALE_DATE
, SALE_ID
pair.
Even though the row values syntax is part of the SQL standard, only a few databases support it. SQL Server 2017 does not support row values at all. The Oracle database supports row values in principle, but cannot apply range operators on them (ORA-01796). MySQL evaluates row value expressions correctly but cannot use them as access predicate during an index access. DB2 (only LUW, since 10.1) and PostgreSQL (since 8.4), however, have a proper support of row value predicates and uses them to access the index if there is a corresponding index available.
Nevertheless it is possible to use an approximated variant of the seek method with databases that do not properly support the row values—even though the approximation is not as elegant and efficient as row values in PostgreSQL. For this approximation, we must use “regular” comparisons to express the required logic as shown in this Oracle example:
SELECT *
FROM ( SELECT *
FROM sales
WHERE sale_date <= ?
AND NOT (sale_date = ? AND sale_id >= ?)
ORDER BY sale_date DESC, sale_id DESC
)
WHERE rownum <= 10
The where
clause consists of two parts. The first part considers the SALE_DATE
only and uses a less than or equal to (<=
) condition—it selects more rows as needed. This part of the where
clause is simple enough so that all databases can use it to access the index. The second part of the where
clause removes the excess rows that were already shown on the previous page. The box entitled “Indexing Equivalent Logic” explains why the where
clause is expressed this way.
The execution plan shows that the database uses the first part of the where
clause as access predicate.
---------------------------------------------------------------
|Id | Operation | Name | Rows | Cost |
---------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 4 |
|*1 | COUNT STOPKEY | | | |
| 2 | VIEW | | 10 | 4 |
| 3 | TABLE ACCESS BY INDEX ROWID | SALES | 50218 | 4 |
|*4 | INDEX RANGE SCAN DESCENDING| SL_DTIT | 2 | 3 |
---------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=10)
4 - access("SALE_DATE"<=:SALE_DATE)
filter("SALE_DATE"<>:SALE_DATE
OR "SALE_ID"<TO_NUMBER(:SALE_ID))
The access predicates on SALE_DATE
enables the database to skip over the days that were fully shown on previous pages. The second part of the where
clause is a filter predicate only. That means that the database inspects a few entries from the previous page again, but drops them immediately. Figure 7.3 shows the respective access path.
Figure 7.3 Access Using the Seek Method

Figure 7.4 compares the performance characteristics of the offset and the seek methods. The accuracy of measurement is insufficient to see the difference on the left hand side of the chart, however the difference is clearly visible from about page 20 onwards.
Figure 7.4 Scalability when Fetching the Next Page

where
clause very carefully—you also cannot fetch arbitrary pages. Moreover you need to reverse all comparison and sort operations to change the browsing direction. Precisely these two functions—skipping pages and browsing backwards—are not needed when using an infinite scrolling mechanism for the user interface.Figure 7.5 Database/Feature Matrix
