On 7/4/2007 10:56 AM, Christopher Browne wrote:
> Andrew Sullivan <ajs at crankycanuck.ca> writes:
>> On Wed, Jul 04, 2007 at 10:10:08AM -0400, Jan Wieck wrote:
>>> seems pretty deterministic, doesn't it? But the result of it depends on 
>>> the exact commit order and the transaction isolation level. We don't 
>>> capture the commit order of single transactions, nor do we care for it 
>>> anywhere in the Slony-I logic.
>>
>> I think this is key.  The current arrangement solves the problem
>> where the visibility rules as they were in force on the origin are
>> followed while applying on the replica.  You're going to need to do
>> quite a bit of theoretical work here to show that the agreeable order
>> rules are followed in any grouping approach you take.  Please see the
>> original concept paper on this exact point.  MVCC is hard.
> 
> The only change I'd propose in handling grouping is to
> opportunistically see if there are consecutive operations that may be
> trivially joined together.
> 
> In effect, if the *old* logic generated the sequence of queries:
> 
>   delete from my_table where id = 25;
>   delete from my_table where id = 82;
>   delete from another_table where id = 19;
>   delete from my_table where id = 45;
> 
> then there is only one "joining" possible, which is to combine the
> first two delete queries into one, so that the overall sequence of
> queries becomes thus:
> 
>   delete from my_table where id = 25 or id = 82;
>   delete from another_table where id = 19;
>   delete from my_table where id = 45;

Actually, only an insert or update to the same table would interrupt the 
possible grouping. But that is academic.

What I see here is that we are trying to come up with a special case 
optimization for mass-deletes. No mass insert or update operations will 
benefit from any of this. Do people do mass deletes that much that we 
really have to worry about them?


Jan

> 
> That doesn't change anything about "agreeable ordering" as far as I
> can see.
> 
> Of course, that example isn't much of a "win."  What would be way more
> interesting (from a performance perspective) is the case where there
> are 25 deletes in a row from my_table that could be folded together.


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