Query large result sets

If you’re working with significantly large data sets, copying all matching records into a slice might be impractical for memory and performance reasons.

In this case, you might want to use Next to map all the records in the result-set one by one:

res := booksTable.Find().OrderBy("-id")

var book Book
for res.Next(&book) {
  // ...
}

Next will return true until there are no more records left to be read in the result set.

When handling results individually, you’ll also need to manually check for errors (with Err) and free locked resources (with Close).

res := booksTable.Find(...)
defer res.Close()

for res.Next(&book) {
  ...
}

if err := res.Err(); err != nil {
  ...
}

Calling Close is not required when using One or All, as they’re closed automatically.

< back
9/18
next >

              package main

import (
	"fmt"
	"log"

	"github.com/upper/db/v4/adapter/postgresql"
)

var settings = postgresql.ConnectionURL{
	Database: "booktown",
	Host:     "postgres",
	User:     "demo",
	Password: "b4dp4ss",
	Options: map[string]string{
		"sslmode": "disable", // Disable TLS
	},
}

type Book struct {
	ID        uint   `db:"id"`
	Title     string `db:"title"`
	AuthorID  uint   `db:"author_id"`
	SubjectID uint   `db:"subject_id"`

	IgnoredField string `db:"-"`
}

func main() {
	sess, err := postgresql.Open(settings)
	if err != nil {
		log.Fatal(err)
	}
	defer sess.Close()

	booksTable := sess.Collection("books")

	// Order by "id" (descending)
	res := booksTable.Find().OrderBy("-ID")
	defer res.Close() // Remember to close the result set.

	// Next goes over all records one by one. It proves useful when copying large
	// data sets into a slice is impractical.
	var book Book
	for res.Next(&book) {
		fmt.Printf("%d:\t%q\n", book.ID, book.Title)
	}

	// In the event of a problem Next returns false, that will break the loop and
	// generate an error (which can be retrieved by calling Err). On the other
	// hand, when the loop is successfully completed (even if the data set had no
	// records), Err will be nil.
	if err := res.Err(); err != nil {
		log.Printf("ERROR: %v", err)
		log.Fatalf(`SUGGESTION: change OrderBy("-ID") into OrderBy("id") and try again.`)
	}
}

            