Inline function much faster than UDF in C?

9 Dec 2013

      Hi everyone,

I have only been using MonetDB for a few weeks now and I have come across a very unexpected result when using User Defined Functions in C.

I have a query which does essentially this:

TABLE small_table (
    int col1;
    float p;
);

TABLE big_table (
    int col1;
    int col2;
    float p;
);

SELECT col1,1+SUM(MY_FUNCTION(big.p,small.p))
FROM small_table small
inner join big_table big on big.col2 == small.col1
group by big.col1;

My function is f(x,y) = 0.7213*(x*ln(x/x+y) + y*ln(y/x+y))

I wrote an optimised version of this function using sse intrinsics in C and compiled it into MonetDB.

To my surprise, I discovered that writing this function inline in the query like so:

1+SUM(0.7213475*(big.p*LOG(big.p/(big.p+small.p))+small.p*LOG(small.p/(big.p+small.p))))

is a lot faster!

The query takes approx 2 seconds with the C version and less than 0.5 seconds with the inline version in SQL!

Can anyone explain this strange result?

Best regards,

Alastair

Inline function much faster than UDF in C?

Alastair McKinley