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:

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