Creating a PostgreSQL procedural language – Part 2 – Embedding Julia

Julia provides an API so that Julia functions can be called from C.  PL/Julia will use this C API to execute Julia code from its user defined functions and stored procedures.

Julia’s documentation provides an example C program that starts up the Julia environment, evaluates the expression sqrt(2.0), displays the resulting value to the standard output, and cleans up the Julia environment.

We will integrate this example into PL/Julia, with some minor modifications, to demonstrate that a PostgreSQL extension can really execute Julia code.  This adapted example will send the results of the Julia expression to the PostgreSQL log instead of to the standard output.

jl_value_t *ret;
double ret_unboxed;

/* required: setup the Julia context */
jl_init();

/* run Julia command */
ret = jl_eval_string("sqrt(2.0)");
if (jl_typeis(ret, jl_float64_type))
{
    ret_unboxed = jl_unbox_float64(ret);
    elog(INFO, "sqrt(2.0) in C: %e", ret_unboxed);
}

/* strongly recommended: notify Julia that the
 * program is about to terminate. this allows Julia time to cleanup
 * pending write requests and run all finalizers
 */

jl_atexit_hook(0);

I’ll continue to gloss over the steps to create a user defined function or stored procedure in PL/Julia since the usefulness of the extension is still quite limited.

Backtracking a little bit, the Makefile also needs to be updated so that the PL/Julia extension is able to link up with the Julia C API.  Julia provides a script that helps generate the appropriate build flags:

JL_SHARE = $(shell julia -e 'print(joinpath(Sys.BINDIR, Base.DATAROOTDIR, "julia"))')
PG_CPPFLAGS += $(shell $(JL_SHARE)/julia-config.jl --cflags)
PG_LDFLAGS += $(shell $(JL_SHARE)/julia-config.jl --ldflags)
SHLIB_LINK += $(shell $(JL_SHARE)/julia-config.jl --ldlibs)

For another round of instant gratification, PL/Julia successfully builds and links to the Julia library.  Then if you decided to write your own PL/Julia function, it will execute Julia code to calculate the square root of 2 and emit the result to the Postgres log:

INFO:  sqrt(2.0) in C: 1.414214e+00

The full example is on GitHub.