Re: SQL View vs CTE

Greg,

IBM'ers have always told me that a view is performance neutral, is that no
longer the case?

Or is it really the difference between having the SELECT <whaterver> inside
or outside a CTE?

To clarify:
--case 1
with cte1 as (SELECT1)
, cte2 as (select <...> from cte1 join <...>)
<...>

--case 2
create view myview as (SELECT1);

with cte1 as (select from myview)
, cte2 as (select <...> from cte1 join <...>)
<...>

--case 3
create view myview as (SELECT1);

with cte2 as (select <...> from myview join <...>)
<...>

Could there be a difference between case 1 & 2, or is it only if at all in
case 3?

Thanks,
Charles



On Wed, Dec 1, 2021 at 3:36 PM Timothy P Clark <timclark@xxxxxxxxxx> wrote:

Hi Greg,

The answer is a solid "maybe"... Although from an end-user perspective a
CTE and a view can help to solve the same problem (i.e. separating a
complex query into manageable pieces), they have slightly different
semantics from an SQL language perspective. As a result, the optimizer may
end up implementing them somewhat differently. Without knowing more
details about the query and seeing the VE, it's impossible to say for sure
whether a view would improve performance, but it is worth a try.

Thank you,

Tim Clark
DB2 for IBM i / SQL Optimizer

"MIDRANGE-L" <midrange-l-bounces@xxxxxxxxxxxxxxxxxx> wrote on 12/01/2021
03:24:58 PM:

From: "Greg Wilburn" <gwilburn@xxxxxxxxxxxxxxxxxxxxxxx>
To: "Midrange Systems Technical Discussion"

<midrange-l@xxxxxxxxxxxxxxxxxx>,

Date: 12/01/2021 03:25 PM
Subject: [EXTERNAL] SQL View vs CTE
Sent by: "MIDRANGE-L" <midrange-l-bounces@xxxxxxxxxxxxxxxxxx>

I have a complex SQL statement that I'm going to use as a data
source for a browser application. In ACS Run SQL scripts it
consistently takes about 22 seconds to run. The statement contains
a CTE followed by a join to the CTE. The files involved have a
large number of records.

So I'm wondering if someone can tell me whether creating an SQL View
in place of the CTE would yield any performance increase (I haven't
spent much time looking at the Visual Explain in ACS).
I'm thinking it wouldn't make much difference.

TIA
Greg
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Greg Wilburn

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