Re: Embedded SQL with Coalesce

Then, Chuck, maybe the issue is using scalar functions on a column
you are using to test - like

where substring(somecolumn, 5, 4) = '1234'

That, IIRC, would prevent use of an index. I think!

On 5/30/2012 1:26 PM, CRPence wrote:

The host variable has a value that is constant for the scope of
the statement. Thus there would be no reason to calculate the
expression more than once. The expression (2011 concat :RPTTXYR)
can be evaluated [as cast to the data type\attributes of the column
being compared; implicit vs explicit CAST should be moot] and
implemented as the effective literal it is, before the rest of the
query evaluation\optimization begins. That expression, the
effective constant, just as a literal would is constant, is a good
potential candidate for implementation on an index defined on
column ARTXYR when coded in the WHERE clause as:
and A.artxyr = 2011 concat :RPTTXYR

While it is entirely possible the optimizer does not have the
capability to do so [I believe it does], the query implementation
_should_ be capable of effecting the equivalent of the following
for its optimization _because_ the given expression is as good as
a literal:
and A.artxyr = :Evaluated_Expression_2011_concat_hvRPTTXYR

I would expect VE to confirm that potential.

On 30 May 2012 09:50, Vern Hamberg wrote:

Tom - I'll jump in here - the calculation has to be done for
every record you are testing - this prevents the use of any index
on A.artxyr.

This is generally true, and may depend on where the calculation
is.

Also, I'm inclined to think implicit casts may have some bearing
on performance. Not sure, would have to run through Visual
Explain to see.

In this case, it'd be easy enough to run 2 SELECTs in Navigator,
one with this concatenation of 2011 and a constant that is a
valid value for RPTTXYR. The other would use directly the value
that resulted from the concatenation.

On 5/30/2012 10:06 AM, Tom E Stieger wrote:

On a side note, why does this matter for performance?

Birgitta Hauser on Tuesday, May 29, 2012 10:38 PM wrote:
Scott Klement on Tuesday, 29.5 2012 23:58 wrote:

This line looks suspicious to me:
and A.artxyr = 2011 || :RPTTXYR

To me too!
For performance issues you should avoid concatenating those
things in the SQL statement, calculate the variable value in
your RPG and use it as follows:

MyVar = '2011' + RPTTXYR;

Select ...
Where a.ARTXYR = :MyVar