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Is it reasonable to have index on DELETED()?
Message
From
21/03/1999 13:57:15
Walter Meester
HoogkarspelNetherlands
 
General information
Forum:
Visual FoxPro
Category:
Databases,Tables, Views, Indexing and SQL syntax
Miscellaneous
Thread ID:
00196021
Message ID:
00200297
Views:
21
Josh,

>What do you base that on? I've seen HUGE improvements in query performance achieved by adding a tag on DELETED().

This was probatly due to the fact that the SQL SELECT statment could be converted to a filter by the optimizer. If you set delete on and give the following type of command:

SELECT * FROM some_Table WHERE some_filter INTO CURSOR x

the optimzer just applies a filter rather then make a new cursor. The types of queries are fairly rare and most of the time unwanted.

Just see my reply to Davids example where david forgot to add NOFILTER after a query. In this statement the optimizer just applied a filter when it found a tag on DELETED().

But of course the type of queries could also be done by

USE some_Table
SET FILTER TO someexpression

Besides applying a filter there could be only an improvement of performance if the table (or tables for that matter) which are queried contains more than a few (5-10%) of deleted records

I did base this on the theory (with the use of some practical examples) which i did try to explain in this thread and in the tread 'TAG ON DELETED() ..... HUMBUG'

Just believe me, there is no HUGE improvement of queries by adding a tag on deleted. In theory it just CANNOT work.

Walter,
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