Matthew-
> I'm not saying Unique indexes are great or anything, just
> saying that I've used them... don't recall ever using them in
> a program, but they have their uses at the command line....
>
> Or do you need to educate me about something?
Alrighty. In a nutshell, UNIQUE index tags can give you misleading results.
Woody has illustrated the single most brilliant and useful trick for UNIQUE,
but that's strictly a command line or ad hoc sort of occasion. It's been a
helluva long time since I've needed it, but it's really great if your
massaging a lot of data conversions. However, as a permanent index...like I
said...bad dog. Here's an example of how a unique index can bite you.
* Begin code
SET DELETED ON
CLEAR
* Create a sample table
CREATE TABLE UniqueEx (I1 N(1,0), C1 C(10))
INSERT INTO UniqueEx VALUES (RECCOUNT('UniqueEx'),"Goodbye")
INSERT INTO UniqueEx VALUES (RECCOUNT('UniqueEx'),"Hello")
INSERT INTO UniqueEx VALUES (RECCOUNT('UniqueEx'),"Hello")
LIST I1, C1 && We see all three records, as expected
* Create (and activate) a duplicate index
INDEX ON C1 TAG C1Unique UNIQUE
LIST I1, C1 && Two records, as expected
* Delete one of the duplicates
GO BOTTOM
DELETE
LIST I1, C1 && Just one record (neither "Hello")
* Reindexing doesn't show us the second "Hello"
REINDEX
LIST I1, C1
* Even setting order off doesn't show "Hello" #2
SET ORDER TO
LIST I1, C1
* If we reopen the table, then we can see "Hello" #2
USE UniqueEx
LIST I1, C1
* End code
Regards-
Nancy Folsom
Pixel Dust Industries
Author _Debugging_Visual_FoxPro_Applications_
http://www.hentzenwerke.com/catalogpricelists/debugvfp.htm
©2004 Nancy Folsom |
