Get rows product (multiplication)

Question

SO,

The problem

I have an issue with rows multiplication. In SQL, there is a SUM() function which calculates sum for some field

Answer 1

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Precision

The exp(log(a)+log(b)) idea is a good one in itself. However, after reading "What Every Computer Scientist Should Know About Floating-Point Arithmetic", make sure you use DECIMAL or NUMERIC data types to be sure you are using Precision Math, or else your values will be surprisingly inaccurate. For a couple of million rows, errors can add up very quickly! DECIMAL (as per the MySQL doc) has a maximum of 65 digits precision, while for example 64bit IEEE754 floating point values have only up to 16 digits (log10(2^52) = 15.65) precision!

Overflow

As per the relevant part of the MySQL doc:

• Integer overflow results in silent wraparound.
• DECIMAL overflow results in a truncated result and a warning.
• Floating-point overflow produces a NULL result. Overflow for some operations can result in +INF, -INF, or NaN.

So you can detect floating point overflow if it would ever happen.

Sadly, if a series of operations would result in a correct value, fitting into the data type used, but at least one subresult in the process of calculations would not, then you won't get the correct value at the end.

Performance

Premature optimization is the root of all evil. Try it, and if it is slow, take the appropriate actions. Doing this might not be lightning quick, but still might be quicker than getting all the results, and doing it on the application server. Only measurements can decide which gets to be quicker...