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Don Burleson Blog 







offset fetch first rows only tips

Oracle Database Tips by Donald BurlesonMarch 11, 2015

Prior to Oracle12c, you had to use special techniques to display the first "n" number of rows within a query.

Getting sub-sets of data in sorted order from the database has always been problematic in SQL, and many people use the "where rownum , n" clause, randomly displaying rows as they are found in the data blocks.

Oracle has many ways to display the top n rows from a table, but none off them has straightforward syntax until Oracle introduced the "row limit" functionality with the "fetch:" and "offset" syntax.

.Here is a review of the fetch top-n SQL methods in Oracle:

  • Row Limit plan:  This Oracle 12c new feature offset  x fetch first y rows only makes it easy to display the first n rows from a table.  That is the method that we discuss below.

Prior to Oracle 12c, we were constrained by these methods:

  • Top-n SQL using subselect with ROWNUM. You can use an inline view with ROWNUM to get the top-10 rows for any SQL query, and the rownum can be used to materialize an in-line view. Beware, this is not always a good thing, as adding the where rownum WITH clause.
  • Top-n SQL with dense_rank and SQL ranking functions. Oracle SQL includes ranking functions that provide support for common OLAP rankings, such as the top 10, bottom 10, top 10 percent, and bottom 10 percent.
  • Top-n SQL using the row_number function: You can query the top 100 rows using the Oracle row_number() and "over" syntax.

Let's consider a way to display the top n rows from a table:

   (select empno, sal row_number()
   (order by
            sal desc) rnk from emp)
where rnk <= 10;

This works to display the first 10 rows from the table, but the syntax is cryptic and in Oracle 12c we get a SQL extension that makes it easy and straightforward when display the first n rows from a table.

  • select . . . . order by x fetch first 10 rows only:  This will display the first rows of the table, in the order that was specified in the order by clause.

  • select . . . order by x offset 20 fetch first 10 rows only:  This will offset into the table (in sorted order) and then fetch the next 10 rows of the table.

Consider these examples of :

-- ************************
-- fetch first 5 rows,
-- ordered by ename

-- ************************


order by ename
fetch first 5 rows only;


-- ************************
-- fetch next 5 rows,
-- start a 5th sorted row
-- ordered by ename
-- ************************


order by ename
offset 5 rows
fetch next 5 rows only;


Here we see that Oracle 12c has again extended their dialect of SQL to make it easier to paginate a SQL result set and reduce the amount of cryptic ANSI 99 syntax that was previously required to display the "next n" and "first n" rows from a pre-sorted result set.

Performance of offset and fetch statements

In PL/SQL, a programmer could declare a cursor and fetch a page of data using the "fetch" syntax, and this SQL "fetch" appears to have similar functionality.  While Oracle does not publish the internals of these offset and fetch operands, that manifest themselves as a "row limit" in execution plans.

This suggests that there will be some performance gain to using this syntax, especially with the "result set" syntax, that will prevent the need for the SQL to fetch the data multiple times.

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