Inserting an array into a Postgresql database

Implement database/sql/driver.Valuer with a custom type:

type int64array []int64

func (a int64array) Value() (driver.Value, error) {
    // Format a in PostgreSQL's array input format {1,2,3} and return it as as string or []byte.
}

Arrays are supported in github.com/lib/pq since 2016 Aug 6th. OP's statement could be written as:

_, err := db.Exec(
    "INSERT INTO history VALUES ($1, $2, $3, $4)", 
    rand.Int63(), 
    pq.Array(t.productids),   // <------- 
    card.cid, 
    t.salepoint,
)