Add example of CompositeType handling with ScanRowValue helper

composite_test.go

@@ -0,0 +1,77 @@
+package pgtype_test
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/jackc/pgtype"
+	pgx "github.com/jackc/pgx/v4"
+	errors "golang.org/x/xerrors"
+)
+
+type MyType struct {
+	a int32   // NULL will cause decoding error
+	b *string // there can be NULL in this position in SQL
+}
+
+func (dst *MyType) DecodeBinary(ci *pgtype.ConnInfo, src []byte) error {
+	if src == nil {
+		return errors.New("NULL values can't be decoded. Scan into a &*MyType to handle NULLs")
+	}
+
+	a := pgtype.Int4{}
+	b := pgtype.Text{}
+
+	if err := pgtype.ScanRowValue(ci, src, &a, &b); err != nil {
+		return err
+	}
+
+	// type compatibility is checked by AssignTo
+	// only lossless assignments will succeed
+	if err := a.AssignTo(&dst.a); err != nil {
+		return err
+	}
+
+	// AssignTo also deals with null value handling
+	if err := b.AssignTo(&dst.b); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func Example_compositeTypes() {
+	conn, err := pgx.Connect(context.Background(), os.Getenv("PGX_TEST_DATABASE"))
+	if err != nil {
+		panic(err)
+	}
+	defer conn.Close(context.Background())
+	_, err = conn.Exec(context.Background(), `drop type if exists mytype;
+
+create type mytype as (
+  a int4,
+  b text
+);`)
+	if err != nil {
+		panic(err)
+	}
+	defer conn.Exec(context.Background(), "drop type mytype")
+
+	var result *MyType
+	if err = conn.QueryRow(context.Background(), "select (1,'foo')::mytype", pgx.QueryResultFormats{pgx.BinaryFormatCode}).Scan(&result); err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("First row: a=%d b=%s\n", result.a, *result.b)
+
+	// Because we scan into &*MyType, NULLs are handled generically by assigning nil to result
+	if err = conn.QueryRow(context.Background(), "select NULL::mytype", pgx.QueryResultFormats{pgx.BinaryFormatCode}).Scan(&result); err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("Second row: %v\n", result)
+	// Output:
+	// First row: a=1 b=foo
+	// Second row: <nil>
+}