Thanks for (as always) a very informative and detailed response, Birgitta.
Thanks to the others who've responded as well. Birgitta's response
brings up a few more questions:
Birgitta Hauser wrote on 09/04/2007 12:57:32:
I'll try to explain how SQL works:
1. When executing an SQL statement an access plan must either be built
or at
least validated.
In the access plan the indexes to be used and temporary objects (such
as
hash tables, bitmaps, relative record lists) to be built for to open
the
data path are desrcibed. In this step all access paths (in either DDS
described logical files or SQL indexes) will be checked and the
statistics
will be consulted.
2. After the access plan is built, validated and actualized the
temporary
objects will be created and filled with data, that can be accessed, i.e.
the
access path is opened (ODP).
Both steps are very expensive and should be avoided whenever possible.
Am I correct to assume that these steps are significantly more expensive
than a traditional OPEN?
I generally do an OPEN and CLOSE each time a subprocedure in a service
program is called, mostly to avoid the need for initialization and cleanup
subprocedures (which would respectively open and close any files used by
the SRVPGM). I was trying to apply the same logic to SQL, which it seems
will not work the way I had hoped. Perhaps the solution to my problem is
to rethink my opening/closing approach. On that note - can anyone comment
on the performance of %open()?
Thanks again,
Adam
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