Custom Type Input/Output Functions

In this lesson, you will learn how to define custom binary input and output functions to ensure your database types communicate efficiently with external applications and low-level tools.

The Problem with Text-Only Serialization

By default, PostgreSQL often communicates custom types as text strings. While human-readable, text conversion is computationally expensive: the database must parse a string into memory, and the client must convert it back. When integrating with high-performance tools—like a C++ analytics engine or a specialized data pipeline—you need binary-compliant I/O. This allows your type to be sent across the wire as a raw byte array, bypassing expensive text parsing.

Implementing Binary I/O

To support binary communication, you must register a receive function (for binary input) and a send function (for binary output) when defining your type via CREATE TYPE.

1. receive function: Takes a internal type (the byte stream) and returns your custom type.
2. send function: Takes your custom type and returns a bytea (binary) representation.

Here is how you might structure these in a C-language extension (the standard for high-performance PostgreSQL internals):

/* Binary output function: convert internal struct to byte stream */
PG_FUNCTION_INFO_V1(my_type_send);
Datum my_type_send(PG_FUNCTION_ARGS) {
    MyType *ptr = (MyType *) PG_GETARG_POINTER(0);
    bytea *result = palloc(sizeof(MyType) + VARHDRSZ);
    
    // Copy raw memory into the buffer
    memcpy(VARDATA(result), ptr, sizeof(MyType));
    PG_RETURN_BYTEA_P(result);
}

Why Binary-Compliant Matters

When you register these functions, external tools can utilize the send function to fetch raw binary blocks directly from your table storage. This is critical for distributed systems where latency is minimized by passing structured objects rather than serialized strings. By implementing these, you essentially give your custom type an "API" for the wire protocol, making it a first-class citizen alongside native types like int4 or float8.

Key Takeaway: Binary I/O functions allow your custom types to bypass expensive text parsing, enabling high-speed, direct memory transfer between PostgreSQL and external high-performance applications.