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To get the most out of a C implementation, you need both a scalar version (one value at the time), and a bulk version (complete BATs as input). If you only have the former, the code generator will generate a slow loop in MAL calling the scalar function. And then it is very well possible that the bulk versions of the inline expression in your second attempt is a lot faster.
On 2013-12-09 13:47, Alastair McKinley wrote:
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
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- -- Sjoerd Mullender