Are there any gotchas when using $inc with float values in MongoDB?

Question

Firstly, is it fully supported to increment by floating values?

What level of floating-point precision is maintained?

Answer 1

MongoDB does support incrementing by floating point values. This will cause the stored value to be converted to floating point if it is currently some other numeric type, which can lead to rounding errors if the previous value was a very large 64-bit integer.

All of the standard caveats involving floating point numbers apply. MongoDB, like JavaScript interpreters in web browsers, stores IEEE double precision (64-bit) floating point values. These values have 53 bits of precision. Intermediate calculations may be computed using 80-bit long doubles, but the final result is always stored as a 64-bit double precision floating point value.

Answer 2

I've run into issues in the shell, when using very big numbers. JavaScript would round down, so the increment wouldn't happen and it would go into an infinite loop. I had to force it to use NumberLong(), to get around the problem. To be fair, I wasn't explicitly using a float type, but I believe that is the default in JavaScript--I was using it as a whole number (a count).

This shouldn't be an issue in a non-JavaScript language driver, as far as I know.