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From Thomas Fischer <tfisc...@apache.org>
Subject Re: LargeSelect and databases which do not support native limit/offset
Date Mon, 12 Sep 2005 06:06:45 GMT

On Mon, 12 Sep 2005, Scott Eade wrote:

>> Therefore I am inclined to follow the proposed solution and throw an
>> exception in the constructor of LargeSelect if the chosen DB does not
>> support native limit and offset.
>>         ...
>> Are there any objections ? Is everybody ok with throwing a
>> RuntimeException, or would it be better to add the TorqueException to
>> the throws clause, possibly breaking people's code because
>> TorqueException is not caught ?
> My only comment would be that as a heavy user of LargeSelect, if I were
> to ever switch to a database that did not support limit and offset I
> could not easily produce an executing (though obviously poorly
> performing when it comes to LargeSelect) version of my application.
> I would prefer us to add an appropriately worded statement in the docs
> and javadocs highlighting the issue with a recommendation that it not be
> used for the affected databases.

The comment already exists, though maybe one could highlight it more.  The 
problem is that the performance is much poorer 
than it need be, because fetching one page at a time is exactly the thing 
you should not do if you do not have native limit/offset.

Thinking over it again, the real solution would be to fetch the whole 
memoryLinit at once and do the offset by hand if the database does not 
support native limit/offset. The bad thing is that I do not know the 
LargeSelect code and have not the time to do it at the moment (sigh).

>> I would also guess that the problems with databases which do not
>> support native limit/offset have lead to the exclusion of the
>> LargeSelectTest from the runtimetest. Does anybody object to include
>> the LargeSelectTest and print an error but not execute the test if the
>> database dose not support native limit/offset ? Sample code would be
>> public class LargeSelectTest extends BaseRuntimeTestCase
>> {
>>     ....
>>     public void testLargeSelect() throws TorqueException
>>     {
>>         if (Torque.getDB(Torque.getDefaultDB()).getLimitStyle()
>>                 == DB.LIMIT_STYLE_NONE)
>>         {
>>             log.error("LargeSelect is known not to work for databases "
>>                     + "which do not support native limit/offset");
>>             return;
>>         }
>>     ....
>> If one adds this, the LargeSelectTest also runs for hsqldb which does
>> not support native limit/offset.
> Does the test case fail at present?

At the moment, the method LargeSelectTest.testLargeSelect() fails.

> If we continue to let the code code
> execute for the reason given above then the test case should at least be
> enabled to make sure that the simulated limit and offset are indeed
> compatible with the behaviour when the database supports them.

If we manage to do the limit/offset manually, one could take it out again.


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