When you read the index, you are doing a direct read. When on your second pass you are reading some other field as part of your select criteria, you are using the index, which is layed down sequentially, to access a non-sequential file, causing thrashing.
In other words, in order to determine if your zip code list also has the situation "foreign country" or whatever, you are reading each record, which you've picked up quickly from an index, but now you're drilling down to each individual record to check this second criteria. When you do that read, you are not doing it, in any manner like the way the records are layed on the disk. That causes thrashing. -----Original Message----- From: Steve Romanow <slestak...@gmail.com> To: U2 Users List <u2-users@listserver.u2ug.org> Sent: Tue, Oct 25, 2011 3:07 pm Subject: Re: [U2] UniBasic Question But you are potentially reducing your input set for the 2nd criteria y a large margin correct? I've never considered alt key + non-indexed usage to create disk thrash. When you select on an alternate key index, you are reading the file, ut (in Udt at least) an X_myfile binary file. So you are doing a irect read for the matching keys. It has no knowledge of the file ayout. On Tue, Oct 25, 2011 at 6:01 PM, Wjhonson <wjhon...@aol.com> wrote: Yes yes and no. If you execute a SSELECT on an indexed field, and don't specify NO.INDEX and o NOT refer at all to any other fields in the file, then index does NOT read he record to see if it even actually exists. It just retrieves the list of eys from the index. If you specific a SSELECT on an indexed field and ALSO specify some other ield criteria, then the select is *supposed* to use the index field FIRST to etrieve the list of keys and ONLY THEN read each record to see whether it atches the other criteria. So the "first pass" if you will should be lightning ast, but then IF your other criteria makes it traverse the majority of the file ou are in for big Trouble with a "T" that rhymes with "P" that stands for Pool". The reason you are in for trouble in this last case, is that traversing the ajority of the file causes Disk Thrashing which is bad very bad very very bad. It causes this because you are forcing the system to reference the records ut-of-disk-order. So it's jumping, jumping, jumping all over the disk in a elter skelter summer swelter. You want to avoid that. _______________________________________________ U2-Users mailing list U2-Users@listserver.u2ug.org http://listserver.u2ug.org/mailman/listinfo/u2-users ______________________________________________ 2-Users mailing list 2-us...@listserver.u2ug.org ttp://listserver.u2ug.org/mailman/listinfo/u2-users _______________________________________________ U2-Users mailing list U2-Users@listserver.u2ug.org http://listserver.u2ug.org/mailman/listinfo/u2-users
