Re: Standalone prefetch without main query?

Another of my vauge recollections...  :-)

A few weeks back Max Muller added some sort of batch fetching display group to 
Wonder.  I recall very little about it, but one thing sticks in my head.  He 
said he found a limit of about 300 OR conditions.  I don't recall why or if this 
was database specific - I think he was puzzled about the reason for this limitation.

Anyway, this might be worth looking into and you might find some code to assist.

Chuck


Jonathan Rochkind wrote:

> At 09:22 AM 11/6/2002 +0000, John Pollard wrote:
> 
>> One of the problems with the batchFetchRelationship approach is that 
>> it is built up with one OR per object in the NSArray passed to it. Am 
>> I right in thinking that most databases will limit the number of ORs 
>> in an SQL statement? In my case I happen to know the top level NSArray 
>> will not be too large, but the second level (Companies in your example 
>> above) could contain thousands of results.
> 
> 
> True, that's a good point.  In the case of my own database, MS SQL 
> Server, I believe the limit is around 3000.  So it's pretty high, but 
> could sometimes be exceeded. You could write the code to batch the 
> second-level fetch into groups of X, where X is the limit. (Ie, call the 
> second-level batchFetchRelationship multiple times, with no more than X 
> objects each time).  It's not ideal, and it's kind of a pain, but it's 
> better than nothing.
> 
>>  When prefetching as part of a fetch spec some clever code bases the 
>> query for Continents (in your example) on the criteria of the main 
>> query for People, not requiring an OR for each Company returned. I 
>> wish batchFetchRelationship() could be that clever.
> 
> 
> Yes.  You can figure out how to write an appropriate fetch spec that 
> will fetch the objects based on the joins, rather than a bunch of ORs.  
> The problem is getting the to-many relationship faults to be filled with 
> these results, rather than remain faults that will go to the db upon fetch.
> 
> I messed around with this a little, a long time ago.  In WO4.5, I think 
> I could get away with calling takeStoredValueForKey on an 
> EOGenericRecord, with a to-many relationship as the key. The EO would 
> then have my specified values as it's relationship, without ever going 
> to the db itself. In WO5, however, I believe that takeStoredValueForKey 
> will trigger willRead(), which will trip the fault resulting in a trip 
> to the db, before your new values are even accepted.  I was never very 
> comfortable with that solution anyway.
> 
> But if we could figure out how to tell a whole bunch of EOs: replace 
> your to-many fault for relationship X with this array of objects---don't 
> go to the db yourself... then we could write our own solution.
> 
> 
>> John
> 
> 
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-- 

Chuck Hill                                 [email protected]
Global Village Consulting Inc.             http://www.global-village.net