Why does rows.Next() depends on DB response rate?

Question

I'm using sqlx, pgx and postgresql. There are two DB`s. First - based on VPS server (slow one), second - installed locally on my PC (fast one). I have a question about this code:

var ordersSlice []OrdersModel    
start := time.Now()
query = `select * from get_all_orders();`
rows, err = db.Queryx(query)
log.Printf("Query time %s", time.Since(start)) // avg in slow DB - 62ms, avg in fast DB - 20ms

if rows == nil || err != nil {
    fmt.Println(err)
    fmt.Println("no result")
    response.WriteHeader(http.StatusInternalServerError)
    return
}

// db.Close() to check if rows.Next() depends on DB
start = time.Now()
for rows.Next() {
    var order OrdersModel

    err = rows.StructScan(&order)
    if err != nil {
        fmt.Println(err)
    }
    ordersSlice = append(ordersSlice, order)
}
log.Printf("Sturct scan time %s", time.Since(start)) // avg in slow DB - 14.4S, avg in fast DB - 9ms

I mean rows.Next() takes more time, than db.Queryx(query) with the slow DB. It takes 14.4 sec to handle result. Why so? The first part of code with db.Queryx(query), should depends on the db response rate. As I see, it is db.Queryx(query) should depend on the db response rate, since the query is executed here and the results are placed in rows. And in rows.Next(), the results are simply being processed. When I assumed rows.Next() somehow depends on DB, I closed connection before row.Next() loop execution, in this way db.Close(). But there was no error. Records was handled.

Cycle for rows.Next() don't communicate with DB, so why does it depends on the DB response rate?

Answer 1

Short answer is: YES, Rows.Next() communicates with DB.

From database/sql docs:

Rows is the result of a query. Its cursor starts before the first row of the result set

But, actually, implementation details are left to DB driver.

For example, in lib/pq, Query executes either simple query or extended query protocol (see postgres docs for more details) and after it receives RowDescription object, it parses it into internal structure(rowHeader). Then,rows.Next() uses it to fetch actual data.

You can see, that pgx does something similar. Query method executes one of protocols and saves data from RowDescription in ResultReader structure. Then rows.Next uses it to fetch data from DB.