Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?
Date: 4 Mar 2003 09:57:45 +0100
Lauri Pietarinen wrote:
>Jan Hidders wrote:
>>>OK, point being that if SQL had not had duplicates from the start the
>>>motivation for such research would have been much stronger, and we would
>>>have got results sooner.
>>Could be, but note that this would then be because you made things *harder*
>>for the implementors and forced them to think about these issues.
>Exactly, and as a result we would have better products. I would also
>affect how people thought and used products.
But you have been arguing all the time that we should keep things as simple as possible for the implementors. Have you now changed your mind? Could it not be that adding bags is also going to give us extra optimizations that would not be considered in a set-only approach?
>>Meaning that in this case you might be right that for this particular query
>>the bag-based approach makes optimization a bit harder. However, after
>>giving it a bit more thought I doubt that even this is true. The problem is
>>just as difficult in a set-only as in a bag-based approach. In both cases
>>you can optimize a join followed by a project that projects out a certain
>>table by using only the index for the "invisible" table.
>Or ignoring it completely...
That would be semantic query optimization and that is already hard in a set-only approach.
>Your point is that if the user gives the 'DISTINCT' keyword,
>it works as if it was a set-based system.
Yes, it should and it could.
- Jan Hidders