The following works reliably:

db, err := sql.Open("postgres", "dbname=my_db")
if err == nil {
  err = db.Ping()
}
// err should be nil if my_db exists

However the following will randomly use the value of the PGDATABASE environment variable instead of the given DSN.

db, err := sql.Open("postgres", "database=my_db")
if err == nil {
  err = db.Ping()
}
// err should be nil if my_db exists

This seems to be due to lib/pq implicitly relying on predictable map iteration internally, which Go does not provide.