Enhanced Window Clause functionality in Oracle 21c (20c)
Updated: Changed 20c to Oracle 21c, as Oracle 20c Database never really existed 🙂
The Oracle Database has had advanced analytical functions for some time now and with each release we get to have some new additions or some enhancements to existing functionality.
One new enhancement, available and documented in 21c (not yet released at time of writing this), is changing in the way the Window Clause can be defined for analytic functions. Oracle 21c is available on Oracle Cloud as a pre-release for evaluation purposes (but it won’t be available for much longer!). The examples shown below are based on using this 21c pre-release of the database.
NOTE: At this point, no one really knows when or if 20c will be released. I’m sure all the documented 20c new features will be rolled into 21c, whenever that will be released.
Before giving some examples of the new Window Clause functionality, lets have a quick recap on how we could use it up to now (up to 19c database). Here is a simple example of windowing the data by creating partitions based on the distinct values in DEPTNO column
avg (salary) over (partition by DEPTNO) avg_sal
order by deptno;
Here we get to see the average salary being calculated for each window partition and being reset for the next windwo partition.
The SQL:2011 standard support the defining of the Window clause in the query block, after defining the list tables for the query. This allows us to define the window clause one and then reference this for analytic function that need it. The following example illustrate this. I’ve take the able query and altered it to have the newer syntax. I’ve highlighted the new or changed code in blue. In the analytic function, the w1 refers to the Window clause defined later, and is more in keeping with how a query is logically processed.
sum(sal) over (w1) sum_sal
window w1 as (partition by deptno);
As you would expect we get the same results returned.
This newer syntax is particularly useful when we have many more analytic function in our queries, and some of these are using slightly different windowing. To me it makes it easier to read and to make edits, allowing an edit to be preformed once instead of for each analytic function, and avoids any errors. But making it easier to read and understand is by far the greatest benefit. Here is another example which uses different window clauses using the previous syntax.
AVG(sal) OVER (PARTITION BY deptno ORDER BY sal) AS avg_dept_sal,
AVG(sal) OVER (PARTITION BY deptno ) AS avg_dept_sal2,
SUM(sal) OVER (PARTITION BY deptno ORDER BY sal desc) AS sum_dept_sal
Using the newer syntax this gets transformed into the following.
AVG(sal) OVER (w1) AS avg_dept_sal,
AVG(sal) OVER (w2) AS avg_dept_sal2,
SUM(sal) OVER (w2) AS avg_dept_sal
window w1 as (PARTITION BY deptno ORDER BY sal),
w2 as (PARTITION BY deptno),
w3 as (PARTITION BY deptno ORDER BY sal desc);
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