[Tutor] Performance of Python loops vs multiple MySQL queries
kent37 at tds.net
Thu Dec 22 17:20:48 CET 2005
Bernard Lebel wrote:
> On 12/21/05, Kent Johnson <kent37 at tds.net> wrote:
>>- Don't assume there is going to be a problem.
> [Bernard] Okay perhaps by "problem" I have not been very accurate. I
> meant "sync" problems. You see, when the script finds a job, it makes
> updates in the database, that is, it adds an entry into another table,
> and updates a certain field in the main jobs table. Other clients then
> testing if there is something to do rely on information that must be
> totally up-to-date. I just wanted to make sure I would not run into
> the case of multiple clients getting incorrect results because of not
> so up-to-date informations. Perhaps I should investigate table locks?
Yes, you should have a plan to avoid that kind of problem. Does your database support
transactions? If so that is the easy way to ensure this - just put the whole getJob() into
>>- Your getJob() code seems to use some variables before they are assigned, such as
>>tPoolIDs and aJob. Is this working code? Also it would be easier to read if you broke it
>>up into smaller functions that each do a small piece of the problem.
> [Bernard] This is not working code. tPoolIDs is bound after the first
> query of the function, but aJob is an error of mine.
> Indeed I could break down the getJob() function into smaller
> functions. It's just that since the class is already having a fair
> amount of methods and this is becoming some long code, I wanted to
> keep everything into a single function.
hmm, not a good choice. The code will be more readable and maintainable if it is broken
up. If the class gets to big, think about making a helper class for some of the code. For
example you might put the whole getJob() function into a class or module whose job it is
to talk to the database and figure out the next job.
One hallmark of a good design is that each class or module has a single responsibility. In
your class you have several responsibilities that could possibly be broken out into
- maintain the state of a single client - this is the job of the Client class
- low-level database access - the query() function could be in a separate module
- details of a single job might fit well in a Job class, this would greatly simplify
- getJob() might move to a utility module that just accesses the database and returns a
> Also there was a consideration of performance. I have one question on
> the topic breaking code into small functions and performance. I have
> read somewhere that *any* call whatoever, that is, methods, functions
> and such, involve a performance cost. Is that right?
> In the case it is true, the performance deterioration would be
> proportional with the number of calls being made, so the larger the
> number of iterations and the more function calls, the slower the code
> would run, is that correct?
Yes, it is correct. Worrying about it at this point is gross premature optimization. It
will only be a problem if you have many many function calls in a performance-critical loop.
First make working code whose design clearly expresses the intent of the code. If it is
too slow, then profile to find the hot spots and address them.
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