SPI_prepare(query, nargs, argtypes)
SPI_prepare creates and returns an execution plan (parser+planner+optimizer) but doesn't execute the query. Should only be called from a connected procedure.
nargs is number of parameters ($1 ... $nargs - as in SQL-functions), and nargs may be 0 only if there is not any $1 in query.
Execution of prepared execution plans is sometimes much faster so this feature may be useful if the same query will be executed many times.
The plan returned by SPI_prepare may be used only in current invocation of the procedure since SPI_finish frees memory allocated for a plan. See SPI_saveplan.
If successful, a non-null pointer will be returned. Otherwise, you'll get a NULL plan. In both cases SPI_result will be set like the value returned by SPI_exec, except that it is set to SPI_ERROR_ARGUMENT if query is NULL or nargs < 0 or nargs > 0 && argtypes is NULL.