logBase generates invalid sql

[abigailalice]

I recently noticed attempting to use the logBase function causes an error with function log(real, real) does not exist or function log(double precision, double precision) does not exist, depending on whether I'm using Expr Float or Expr Double. This link seems to suggest the binary log function is only defined for numeric, though I'm not sure what (if any) Haskell type that would correspond to.

logBase should probably be implemented as log x / log b for the standard types.