延迟中的“延迟”-有什么更好的选择？

Question

I need to make SQL queries to database in the loop:

for rows.Next() {

   fields, err := db.Query(.....)
   if err != nil {
      // ...
   }
   defer fields.Close()

   // do something with `fields`

}

What will be better: leave all as is or move defer after loop:

for rows.Next() {

   fields, err := db.Query(.....)
   if err != nil {
      // ...
   }

   // do something with `fields`
}

defer fields.Close()

Or something else ?

Answer 1

The whole point of defer is that it does not execute until the function returns, so the appropriate place to put it would be immediately after the resource you want to close is opened. However, since you're creating the resource inside the loop, you should not use defer at all - otherwise, you're not going to close any of the resources created inside the loop until the function exits, so they'll pile up until then. Instead, you should close them at the end of each loop iteration, without defer:

for rows.Next() {

   fields, err := db.Query(.....)
   if err != nil {
      // ...
   }

   // do something with `fields`

   fields.Close()
}