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On 13-Sep-2011 14:13 , Robert Munday wrote:
I am looking for efficiencies and identifying where some processes
can be streamlined.
I have a number of RPG programs using SQL statements in purge and
delete programs which are being called five times, once for each
company. Many of the SQL DELETEs are being performed on non-company
specific fields such as dates. Deleting by dates would cross all
company records.
Are there any performance hits for attempting to do SQL DELETEs
multiple times where the first call of the program has already
deleted the records and subsequent calls are redundant? I would
think not but every nanosecond counts.
Imagine a janitor pushing a 50-gallon trash barrel down a hallway,
stopping, then entering an office. The janitor grabs the waste-paper
basket and carries the basket out into the hallway. At this point the
janitor lifts, tips, and shakes the basket to empty the contents into
the barrel, then places the basket just inside the office.
Now imagine the janitor repeating four more times each of lifting
[invocation and initializations], tipping [allocations, query
evaluation, and query start], shaking [data selection and retrieval],
and returning of the basket [return from invocation, freeing storage and
allocations]. If while tipping there is sufficient light exposing that
the basket is already empty [an index with no matching entries], then
the actual shaking of the basket to empty the contents can easily be
skipped, but the actions of lifting, tipping, and replacing of the
basket are still going to take time.
Unless something had been placed in the basket since the most recent
emptying [new rows added that match the selection], each repeated
attempt to empty the basket is a waste of time. So if truly "every
nanosecond counts", then the time for all of that unnecessary work can
easily add-up, especially if no index exists to easily identify that no
actual data selection is required.
As I recall, the database monitor shows the time those extra DELETE
requests spent; a job trace would as well.
Regards, Chuck
midrange.com
