Introducing Cosette – a SQL solver for checking semantic equivalences of queries

https://medium.com/@uwdb/introducing-cosette-527898504bd6

by lx • 9 years ago

31 11 9 years ago

medium.com

LaPrometheus5• 9 years ago

Author here. Happy to answer questions.

lukaseder6• 9 years ago

Nice idea, but it breaks quite quickly (still). Tried to see if Cosette can prove equivalence of these two queries:

    /* define schema s1, 
       here s1 can contain any number of attributes, 
       but it has to at least contain integer attributes 
       x and y */
    schema s1(x:int, ya:int, ??);
    
    schema s2(yb:int, ??);        -- define schema s2
    
    table a(s1);            -- define table a using schema s1
    table b(s2);            -- define table b using schema s1
    
    query q1                -- define query q1 on tables a and b
    `select distinct x.x as ax from a x, b y
     where x.ya = y.yb`;
    
    query q2                -- define query q2 likewise
    `select x.x as ax from a x, a y, b z 
     where x.x = y.x and x.ya = z.yb
     union select 1 as ax from a x where 1 = 0`;
    
    verify q1 q2;           -- does q1 equal to q2?

Got a syntax error:

    Syntax Error. 
    ERROR: (line 19, column 8):
    unexpected "s"
    expecting "--" or "/*"

There are many other bugs up to the point where this is not really usable.

LaPrometheus5• 9 years ago

Thanks for trying out. The parser error report is not great now. The real error actually, it should be "union all" rather than "union".

I changed your query:

/* define schema s1, here s1 can contain any number of attributes, but it has to at least contain integer attributes x and y */ schema s1(x:int, ya:int, ??);

    schema s2(yb:int, ??);        -- define schema s2
    
    table a(s1);            -- define table a using schema s1
    table b(s2);            -- define table b using schema s1
    
    query q1                -- define query q1 on tables a and b
    `select distinct x.x as ax 
    from a x, b y
     where x.ya = y.yb`;
    
    query q2                -- define query q2 likewise
    `(select x.x as ax 
     from a x, a y, b z 
     where x.x = y.x and x.ya = z.yb)
     union all (select 1 as ax from a x where 1 = 0)`;
    
    verify q1 q2;           -- does q1 equal to q2?

The Rosette execution indeed returns a counterexample (since they are not equal). There is an error in Coq code generation part, we are fixing that now.

lukaseder6• 9 years ago

Me again - I really like the idea. I wish I had something like that at a previous job, where queries ran against some 10-level nested views, involving tons of unnecessary joins, projections, unions, sorts, distincts, and what not.

I'm pretty sure that Oracle figured it out all correctly, so the excessive SQL wasn't causing performance issues, but from a maintainability perspective, it would have been nice to have a tool that was able to reduce arbitrary SQL to a more minimal, semantically equivalent alternative.

Have you thought about this?

LaPrometheus5• 9 years ago

Exactly, SQL minimization is a real problem. While there is no canonical form of all SQL in general, it is still possible to reduce Query.

There is some existing work. You might want to look at this one: https://pdfs.semanticscholar.org/c243/25d76c3ba91388e16085c1...

One problem is that bag semantic chase is very complicated to implement. We are actually working on a new chase algorithm on our SQL formalization right now.

lukaseder6• 9 years ago

Another question: How do you plan on handling non-deterministic expressions, such as CURRENT_TIMESTAMP or ROWNUM, et al.?

LaPrometheus5• 9 years ago

CURRENT_TIMESTAMP is truly random. One thing we can do is to over approximate. For example, let CURRENT_TIMESTAMP be a symbolic variable, which essentially can be any value. If we still can prove the equivalence, then the equivalence is sound. But there might be some false negative.

ROWNUM is interesting. You can actually declare ROWNUM as an extra attribute of the table and then assert it is a key (We will add support of key constraints very soon). Then many cases will be covered.

Introducing Cosette – a SQL solver for checking semantic equivalences of queries

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