Loop through database/sql sql.Rows multiple times?

Question

I need to loop through returned sql.Rows multiple times. Are my only two options:

1. to cache the returned results in a local data structure;
2. redo the database query?

In other words, there is no way to go back in sql.Rows (i.e., opposite of Rows.Next).

Answer 1

Another solution would be to use the decorator pattern:

// A RowsDecorator wraps sql.Rows and allows a callback to be called whenever Scan is called
type RowsDecorator struct {
    *sql.Rows
    OnScan func([]interface{}, error)
}

func Wrap(rows *sql.Rows, onScan func([]interface{}, error)) *RowsDecorator {
    return &RowsDecorator{Rows: rows, OnScan: onScan}
}

// Scan calls Rows.Scan and an optional callback
func (rows *RowsDecorator) Scan(dest ...interface{}) error {
    err := rows.Rows.Scan(dest...)
    if rows.OnScan != nil {
        rows.OnScan(dest, err)
    }
    return err
}

Used like this:

db.Exec(`CREATE TABLE example (id INTEGER, txt TEXT)`)
db.Exec(`INSERT INTO example (id, txt) VALUES (1, 'test-1'), (2, 'test-2'), (3, 'test-3') `)

rawrows, err := db.Query("SELECT id, txt FROM example")
if err != nil {
    log.Fatal(err)
}
defer rawrows.Close()

sum := 0
rows := Wrap(rawrows, func(dest []interface{}, err error) {
    if err == nil {
        sum += *dest[0].(*int)
    }
})
for rows.Next() {
    var id int
    var txt string
    err := rows.Scan(&id, &txt)
    if err != nil {
        log.Fatal(err)
    }
    log.Println(id, txt)
}
log.Println("sum", sum)

With this pattern you can write a custom function that is called as you iterate over the collection. By using an unnamed, embedded type all of the original methods (Next, Close, etc) can still be called.