1. A computer program product for selecting optimized code to be executed in a computing system comprising a first processor and a second processor, the computer program product comprising: a non-transitory computer-readable medium having computer-readable program code embodied therewith, the computer-readable program code configured to: compile a user-defined function included within a client job received by the computing system to create a first compiled code portion and a second compiled code portion, wherein the first compiled code portion is optimized based on an architecture implementation of the first processor and the second compiled code is optimized based on an architecture implementation of the second processor, wherein the respective architecture implementations of the first and second processors are different, and wherein compiling the function comprises: storing the first and second compiled code portions as first and second libraries; generating an executable comprising a first reference to the first library, a second reference to the second library, a third code portion optimized for the first processor, a fourth code portion optimized for the second processor, and selection code comprising instructions for selecting between the third and fourth optimized code portions based on a processor ID of a processor to which the executable is assigned, wherein the selection code is introduced into the executable by a compiler and does not correspond to any user-provided source code, and wherein the executable is separate from the first and second libraries; at execution of the executable, use a linker to select one of the first and second libraries, wherein the linker uses the processor ID to select one of the first and second libraries; upon determining that the executable is assigned to the first processor, load the first library into memory to be executed by the first processor and not the second library; and upon determining that the executable is assigned to the second processor, load the second library into memory to be executed by the second processor and not the first library.