How to Properly Manage Resource Release with `defer` in Loops When Querying Databases?

Proper Resource Release with defer in Loops

Querying a Database Inside a Loop

In a loop where you need to make SQL queries to a database, you may structure your code as follows:

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

   // do something with `fields`

}

However, there are different ways to handle resource release using defer in such scenarios.

Moving defer Outside the Loop

One option is to move the defer statement outside the loop, as seen below:

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

   // do something with `fields`
}

defer fields.Close()

Utilizing Anonymous Functions

Another approach is to wrap the resource allocation code in an anonymous function and place the defer statement within that function:

for rows.Next() {
    func() {
        fields, err := db.Query(...)
        if err != nil {
            // Handle error and return
            return
        }
        defer fields.Close()

        // do something with `fields`
    }()
}

Error Handling in a Named Function

You can also create a named function to handle error reporting:

func foo(rs *db.Rows) error {
    fields, err := db.Query(...)
    if err != nil {
        return fmt.Errorf("db.Query error: %w", err)
    }
    defer fields.Close()

    // do something with `fields`
    return nil
}

for rows.Next() {
    if err := foo(rs); err != nil {
        // Handle error and return
        return
    }
}

Handling Rows.Close() Error

Since Rows.Close() returns an error, you may want to check it. This can be done using an anonymous function:

func foo(rs *db.Rows) (err error) {
    fields, err := db.Query(...)
    if err != nil {
        return fmt.Errorf("db.Query error: %w", err)
    }
    defer func() {
        if err = fields.Close(); err != nil {
            err = fmt.Errorf("Rows.Close() error: %w", err)
        }
    }()

    // do something with `fields`
    return nil
}

Conclusion

The correct approach to releasing resources with defer in a loop depends on your specific needs and error handling requirements. By carefully considering the options presented above, you can ensure proper resource management and prevent resource leaks.

The above is the detailed content of How to Properly Manage Resource Release with `defer` in Loops When Querying Databases?. For more information, please follow other related articles on the PHP Chinese website!