Re: Can an SQL common table expression be used for many different selects?

Hi Maria

I am trying to understand what is happening - I don't know how 1 static statement can return a COUNT(*) in one execution and the rows in another execution - it can't be the same, since the column list isn't the same - but maybe an example can help me.

By dynamic filters, do you mean values from the request that are used with host variables? Performance is improved, as you probably know, with host variables, because the optimization does not need to be done more than once under certain conditions, as I understand it - performance guide gives details, I think multiple use in the same RPG module is one of those. And in many cases, the ODP for the processing is not completely closed, so is available for other values of host variables.

I wonder, without knowing more, if any of the OLAP functions can be used. The result set can include various subtotals in addition to the data rows.

As to Common Table Expressions - they apply only to the statement of which they are a part. They are recommended by the team at IBM, since the SQL engine can optimize things very well when there are supporting indexes and appropriate JOINs and WHERE clauses. But they are limited to the statement - they are not available to other statements, while global temporary tables (and temporary result table you also create, maybe in QTEMP) are available in the job.

It does seem that if you have many SELECTs over the same temporary result set, with different WHERE clauses (not just different values for host variables), that a temporary table is needed. But I'm not able to say more without seeing more of what the statements look like.

Hope this helps some!
Vern

On 7/28/2020 2:16 AM, Maria Lucia Stoppa wrote:

Hi all,
Hope you are well.

A REST API I am working on returns data split into pages: many calls are
necessary to get the complete set of data, but in this way, the user
doesn't wait too much for the first page. Data are retrieved within an RPG
ILE service program by a static SQL statement which applies some dynamic
filters that come within the request.

Everything works fine, except the same static SQL statement is run at least
twice to know the total number of rows (a simple count(*)) and the rows
themselves page by page.
Now, on the same data retrieved by this SQL statement, others select
statements must be run to get some totals according to different group by
clauses in order to present the data set distribution to the final user.

I hardly accepted the idea of running the same SQL statement (which, by
the way, is pretty complex) at least twice to get both the total and the
row, but I can't stand the idea of even more runs to get data distribution.

There might be errors in my design of how the procedure should work,
nonetheless, I wonder if a single common table expression might be used
many times to serve many different select statements, as I would avoid the
use of other solutions like global temporary tables.

Any suggestion is really appreciated.

Many thanks