Re: [HACKERS] proposal: Preference SQL

[Kevin Walker]

Yes, the preference clause can be rewritten using standard SQL.  The syntax to 
duplicate the example result set is listed below.  The syntax is not very 
flexible or easy to read.   

select id 
from computer
where (main_memory = (select max(main_memory) 
  from computer)
   and cpu_speed = (select max(cpu_speed) 
from computer
where cpu_speed < (select max(cpu_speed) from 
computer)))
   or (cpu_speed = (select max(cpu_speed) 
from computer)
   and   main_memory = (select max(main_memory) 
from computer
 where main_memory < (select max(main_memory) from 
computer)))
;
~ 
Kevin Walker

-Original Message-
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jan Urbanski
Sent: Saturday, May 31, 2008 7:34 AM
To: Stephen R. van den Berg
Cc: Postgres - Hackers
Subject: Re: [HACKERS] proposal: Preference SQL

Stephen R. van den Berg wrote:
> Jan Urbański wrote:
>> An example of a preference query would be (quoting the linked PDF):
> 
>> SELECT * FROM programmers PREFERRING exp IN ('java', 'C++'); or 
>> SELECT * FROM computers PREFERRING HIGHEST(main_memory) AND 
>> HIGHEST(cpu_speed);
> 
> Forgive my ignorance, but it appears that this can already be achieved 
> by using a properly weighted ORDER BY clause, as in:
> 
> SELECT * FROM computers
> ORDER BY HIGHEST(main_memory) DESC, HIGHEST(cpu_speed) DESC;

No, these are quite different. Consider a table with three columns: id, 
main_memory, cpu_speed containing four tuples:
   id  main_memorycpu_speed
---
comp1 100  80
comp2 80  100
comp3 100 70
comp4 6060

Now the result of a SELECT id FROM computers PREFERRING
HIGHEST(main_memory) AND HIGHEST(cpu_speed) would be:
id
-
comp1
comp2

This is because comp1 and comp2 are incomparable under the partial order 
defined by the preferences. comp1 has the largest main memory and comp2 the 
fastest CPU, but the preference states you like main memory just as much as CPU 
speed, so you get both tuples in the result. On the other hand, comp3 is not in 
the result set, because comp1 is greater than it under the preference partial 
order. The main_memory preference is satisfied by comp3 just as well as it is 
by comp1, but the cpu_speed preference is worse. The same goes for comp4.

And all this is significantly different from an ORDER BY, because first it 
doesn't throw away any rows and second it gives you a linear order, where every 
tuple can be compared with another. The clause you proposed (though it's not 
legal in PG, because there is no HIGHEST function,
right?) would, as I understand it, prefer main memory more than CPU speed.

There are still some issues about the exact meaning of a PREFERRING clause, but 
it is very different from a simple ORDER BY (and it has more options than just 
PREFERRING and AND).
Anyway, from what I've read most or all preference clauses can be rewritten to 
standard clauses, but sometimes it's difficult, and many times it's costly.

Cheers,
Jan

--
Jan Urbanski
GPG key ID: E583D7D2

ouden estin


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Re: [HACKERS] proposal: Preference SQL

[Kevin Walker]

Jan,

;-)  ... agree with your points.  Note I wasn't trying to duplicate the full 
functionality of the preferences clause.  ...just pointing out that the example 
presented could be duplicated with standard SQL with the result being both ugly 
and confusing.  

My intent was show that the syntax you proposed was easier to read and more 
flexible. 

Kevin



> Date: Sat, 31 May 2008 16:53:54 +0200
> From: [EMAIL PROTECTED]
> To: [EMAIL PROTECTED]
> CC: [EMAIL PROTECTED]; pgsql-hackers@postgresql.org
> Subject: Re: [HACKERS] proposal: Preference SQL
> 
> Kevin Walker wrote:
> > Yes, the preference clause can be rewritten using standard SQL.  The syntax 
> > to duplicate the example result set is listed below.  The syntax is not 
> > very flexible or easy to read.   
> > 
> > select id 
> > from computer
> > where (main_memory = (select max(main_memory) 
> >   from computer)
> >and cpu_speed = (select max(cpu_speed) 
> > from computer
> > where cpu_speed < (select max(cpu_speed) from 
> > computer)))
> >or (cpu_speed = (select max(cpu_speed) 
> > from computer)
> >and   main_memory = (select max(main_memory) 
> > from computer
> >  where main_memory < (select max(main_memory) from 
> > computer)))
> 
> Well, that's not 100% correct, but the idea is something like this. In 
> particular, if you'd have only one entry in the table, then this query 
> would not return any rows, which would be wrong. Also, if you had a 
> computer that has larger main memory and a faster CPU than any other 
> copmuter, it should be returned as the result, but the above query would 
> fail to do that.
> 
> The point is not rewriting that particular preference query into a 
> standard query. The point is whether it's worth having an automated 
> mechanism for executing arbitrary preference queries with complex 
> preferences (again: the syntax is richer, I didn't want to go into any 
> detail about it before getting some feedback).
> 
> Let me give you a more sophisticated example.
> You have a webpage that sells used cars. You have your typical search 
> form with car make, colour, engine power and so on. Normally, you would 
> make the search form input fields correspond to SQL WHERE clauses. So, 
> if I want a white Honda with a 180 hp engine and about 40k kilometers of 
> mileage I enter these parameters and hit the submit button. Now imagine 
> I don't get any results for my search. That could mean that you have no 
> Honda cars in stock, but it can also mean that you have my perfect Honda 
> at a bargain price, it's only that it's black. Or maybe you have a 
> Honda, but it has a 160 hp engine? Or is it just that the one perfect 
> Honda you have has a mileage of just over 41k km, and that's why I 
> didnt' get it in my result set? People seldom want a perfect match when 
> they are searching for something. They want the best match, they can 
> get. So, if I wanted to get a Honda with a decent engine and my 
> favourite color is white, I'd say:
> 
> SELECT * FROM cars WHERE make = 'Honda' PREFERRING (power = '180' AND 
> mileage AROUND 4) CASCADE color = 'white';
> 
> Remember, that an AND in a preference clause constructs a partial order. 
> The query says: I equally prefer having a 180 hp engine and having a car 
> that has a mileage of 40k km. Tha CASCADE clause intrudoces a less 
> important preference. It means that the color is not as important to me 
> as power and mileage, but if I had a choice I'd take the white one.
> 
> I'd strongly recommend skimming through the paper I mentioned in my 
> first email, it explains stuff much better than I do.
> 
> Preference SQL in Postgres could, as I see it, become one of the 
> distinct features that no other widespread database system has and that 
> could potentially be massively useful for online shops, social networks, 
> etc. - you name it.
> After hearing Bruce's keynote at PGCon and how Postgres now should be 
> aiming at more that just catching up with the big guys I just thought: 
> well, that's one neat feature that none of them has, that's useful, 
> that's kind of sexy *and* I could get my degree out of it.
> 
> Cheers,
> Jan
> 
> -- 
> Jan Urbanski
> GPG key ID: E583D7D2
> 
> ouden estin

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