How to stream binary data into a PostgreSQL BYTEA column using the Golang lib/pq API?

If one would be willing to switch to github.com/jackc/pgx, Large Objects (PostgreSQL docs) can be streamed. The pgx.LargeObject type implements:

io.Writer
io.Reader
io.Seeker
io.Closer

Large Objects are stored in a system table, there is no Large Object type which can be used in a column of a table. Large Objects are referenced by their object identifier. So a separate table needs to be maintained with file metadata and oid mapping.

Example program:

package main

import (
    "context"
    "io"
    "log"
    "os"
    "time"

    "github.com/jackc/pgx/v4"
)

const (
    // files table maps Large Object oid to file names
    createFileTable = `CREATE TABLE files (
        id oid primary key,
        name varchar,
        unique(name)
    );`
)

func main() {
    ctx, cancel := context.WithTimeout(context.TODO(), time.Minute)
    defer cancel()

    conn, err := pgx.Connect(ctx, "user=postgres host=/run/postgresql dbname=postgres")
    if err != nil {
        panic(err)
    }
    defer conn.Close(ctx)

    if _, err = conn.Exec(ctx, createFileTable); err != nil {
        panic(err)
    }

    written, err := storeFile(ctx, conn, "somefile.bin")
    log.Printf("storeFile written: %d", written)
    if err != nil {
        panic(err)
    }

    read, err := loadFile(ctx, conn, "somefile.bin")
    log.Printf("loadFile read: %d", read)
    if err != nil {
        panic(err)
    }
}

// storeFile as Large Object in the database.
// The resulting object identifier is stored along with the file name in the files table.
// The amount of written bytes and an erorr is returned, if one occured.
func storeFile(ctx context.Context, conn *pgx.Conn, name string) (written int64, err error) {
    file, err := os.Open(name)
    if err != nil {
        return 0, err
    }
    defer file.Close()

    // LargeObjects can only operate on an active TX
    tx, err := conn.Begin(ctx)
    if err != nil {
        return 0, err
    }
    defer tx.Rollback(ctx)

    lobs := tx.LargeObjects()

    // Create a new Large Object.
    // We pass 0, so the DB can pick an available oid for us.
    oid, err := lobs.Create(ctx, 0)
    if err != nil {
        return 0, err
    }

    // record the oid and filename in the files table
    _, err = tx.Exec(ctx, "INSERT INTO files (id, name) VALUES ($1, $2)", oid, name)
    if err != nil {
        return 0, err
    }

    // Open the new Object for writing.
    obj, err := lobs.Open(ctx, oid, pgx.LargeObjectModeWrite)
    if err != nil {
        return 0, err
    }

    // Copy the file stream to the Large Object stream
    written, err = io.Copy(obj, file)
    if err != nil {
        return written, err
    }

    err = tx.Commit(ctx)
    return written, err
}

// loadFile loads the file identified by name as Large Object
// and writes the contents to a local file by the same name.
// The amount of bytes read or an error is returned.
func loadFile(ctx context.Context, conn *pgx.Conn, name string) (read int64, err error) {
    tx, err := conn.Begin(ctx)
    if err != nil {
        return 0, err
    }
    defer tx.Rollback(ctx)

    var oid uint32
    err = conn.QueryRow(ctx, "SELECT id FROM files WHERE name = $1", name).Scan(&oid)
    if err != nil {
        return 0, err
    }

    file, err := os.Create(name)
    if err != nil {
        return 0, err
    }

    lobs := tx.LargeObjects()
    obj, err := lobs.Open(ctx, oid, pgx.LargeObjectModeRead)
    if err != nil {
        return 0, err
    }

    return io.Copy(file, obj)
}