sql: scan row(s) with unknown number of columns (select * from ...) - sql

Question

I have a table t containing a lot of columns, and my sql is like this: select * from t. Now I only want to scan one column or two from the wide returned row set. However, the sql.Scan accepts dest ...interface{} as arguments. Does it mean I have to scan everything and use only the column I needed?
I know I could change the sql from select * to select my_favorite_rows, however, in this case, I have no way to change the sql.

Answer 1

You can make use of Rows.Columns, e.g.
package main
import (
"database/sql"
"fmt"
"github.com/lib/pq"
)
type Vehicle struct {
Id int
Name string
Wheels int
}
// VehicleCol returns a reference for a column of a Vehicle
func VehicleCol(colname string, vh *Vehicle) interface{} {
switch colname {
case "id":
return &vh.Id
case "name":
return &vh.Name
case "wheels":
return &vh.Wheels
default:
panic("unknown column " + colname)
}
}
func panicOnErr(err error) {
if err != nil {
panic(err.Error())
}
}
func main() {
conn, err := pq.ParseURL(`postgres://docker:docker#172.17.0.2:5432/pgsqltest?schema=public`)
panicOnErr(err)
var db *sql.DB
db, err = sql.Open("postgres", conn)
panicOnErr(err)
var rows *sql.Rows
rows, err = db.Query("select * from vehicle")
panicOnErr(err)
// get the column names from the query
var columns []string
columns, err = rows.Columns()
panicOnErr(err)
colNum := len(columns)
all := []Vehicle{}
for rows.Next() {
vh := Vehicle{}
// make references for the cols with the aid of VehicleCol
cols := make([]interface{}, colNum)
for i := 0; i < colNum; i++ {
cols[i] = VehicleCol(columns[i], &vh)
}
err = rows.Scan(cols...)
panicOnErr(err)
all = append(all, vh)
}
fmt.Printf("%#v\n", all)
}

Answer 2

For unknown length of columns but if you're sure about their type,
cols, err := rows.Columns()
if err != nil {
log.Fatal(err.Error())
}
colLen := len(cols)
vals := make([]interface{}, colLen)
for rows.Next() {
for i := 0; i < len(colLen); i++ {
vals[i] = new(string)
}
err := rows.Scan(vals...)
if err != nil {
log.Fatal(err.Error()) // if wrong type
}
fmt.Printf("Column 1: %s\n", *(vals[0].(*string))) // will panic if wrong type
}
PS: Not recommended for prod