32.8. The Fast-Path Interface#
Postgres Pro provides a fast-path interface to send simple function calls to the server.
Tip
This interface is somewhat obsolete, as one can achieve similar performance and greater functionality by setting up a prepared statement to define the function call. Then, executing the statement with binary transmission of parameters and results substitutes for a fast-path function call.
The functionPQfn requests execution of a server function via the fast-path interface:
PGresult *PQfn(PGconn *conn, int fnid, int *result_buf, int *result_len, int result_is_int, const PQArgBlock *args, int nargs);typedef struct{ int len; int isint; union { int *ptr; int integer; } u;} PQArgBlock; Thefnid argument is the OID of the function to be executed.args andnargs define the parameters to be passed to the function; they must match the declared function argument list. When theisint field of a parameter structure is true, theu.integer value is sent to the server as an integer of the indicated length (this must be 2 or 4 bytes); proper byte-swapping occurs. Whenisint is false, the indicated number of bytes at*u.ptr are sent with no processing; the data must be in the format expected by the server for binary transmission of the function's argument data type. (The declaration ofu.ptr as being of typeint * is historical; it would be better to consider itvoid *.)result_buf points to the buffer in which to place the function's return value. The caller must have allocated sufficient space to store the return value. (There is no check!) The actual result length in bytes will be returned in the integer pointed to byresult_len. If a 2- or 4-byte integer result is expected, setresult_is_int to 1, otherwise set it to 0. Settingresult_is_int to 1 causeslibpq to byte-swap the value if necessary, so that it is delivered as a properint value for the client machine; note that a 4-byte integer is delivered into*result_buf for either allowed result size. Whenresult_is_int is 0, the binary-format byte string sent by the server is returned unmodified. (In this case it's better to considerresult_buf as being of typevoid *.)
PQfn always returns a validPGresult pointer, with statusPGRES_COMMAND_OK for success orPGRES_FATAL_ERROR if some problem was encountered. The result status should be checked before the result is used. The caller is responsible for freeing thePGresult withPQclear when it is no longer needed.
To pass a NULL argument to the function, set thelen field of that parameter structure to-1; theisint andu fields are then irrelevant.
If the function returns NULL,*result_len is set to-1, and*result_buf is not modified.
Note that it is not possible to handle set-valued results when using this interface. Also, the function must be a plain function, not an aggregate, window function, or procedure.