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while .not. tbl.fetch_eof() not working

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  • #46
    Re: while .not. tbl.fetch_eof() not working

    Hi Cal,

    Originally posted by CALocklin View Post
    Actually, as best I recall, it's not the initial overhead; it's the final overhead. I believe an update operation changes all the records without updating the indexes as each record is completed. It rebuilds the indexes after the changes have all been completed. If there are many records to be changed, this is the fastest way. However, if there are only a couple records that need to be changed, the overhead of rebuilding all those indexes isn't worthwhile.
    What you said is true for both append and update operations. If the index have to be updated (always for an append operation), and if an update field is used in any index for an update operation). However, if another thread has the table open (e.g. a form is open) or another user is using the table, then the indexes are updated on the fly after processing each record.

    To guarantee that Alpha uses the latter, I always open a form based on the parent table of the operation, as in the code

    frm=form.load("tablename")
    update.run_silent("updateoperationname")
    frm.close()

    If you do it by the above code method, it will run much faster if your Update filter invokes LQO (if using a subset of the table). Tests on a 35000 record file had an update take about 11 seconds in the above method, versus 19 seconds for the index rebuilds happening at the end (index expressions are very complex for this table) when the table is available exclusively.

    As you and I recently conversed about, measuring code speed for short time events is difficult, and the profiler functions of A5 gave wildly inaccurate values to the execution speed differences in the case you and I discussed. The only way to get an accurate time result is to run short time code pieces many times over, and make sure the loop overhead is not significant. The second part can be difficult to do.

    The accurate speed timing button of my CSDA Code Utility for Alpha Five does just that.

    So to get to Tony's question, is the overhead of an update faster than parsing through the records?

    For large numbers of records where either the index rebuilds don't take long, or using the method shown above to update indexes on the fly, normally the answer is yes. For under 10 records, it is probably faster with code, with 10 to 100 records probably being the cross over point. It's even possible to use both methods, depending upon # of records to process, if the value changes dynamically.
    Regards,

    Ira J. Perlow
    Computer Systems Design


    CSDA A5 Products
    New - Free CSDA DiagInfo - v1.39, 30 Apr 2013
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    • #47
      Re: while .not. tbl.fetch_eof() not working

      Ira - Thanks for confirming my initial thoughts on the two methods.

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