Improved ext/shopspring-numeric binary decoding performance

Before:
BenchmarkDecode/Zero-Binary-16       	 3944304	       292 ns/op	     128 B/op	       7 allocs/op
BenchmarkDecode/Small-Binary-16      	 2034132	       585 ns/op	     184 B/op	      13 allocs/op
BenchmarkDecode/Medium-Binary-16     	 1747191	       690 ns/op	     192 B/op	      12 allocs/op
BenchmarkDecode/Large-Binary-16      	 1334006	       899 ns/op	     304 B/op	      14 allocs/op
BenchmarkDecode/Huge-Binary-16       	  702382	      1590 ns/op	     584 B/op	      18 allocs/op

After:
BenchmarkDecode/Zero-Binary-16       	14592645	        80.1 ns/op	      64 B/op	       2 allocs/op
BenchmarkDecode/Small-Binary-16      	 5729318	       212 ns/op	     104 B/op	       7 allocs/op
BenchmarkDecode/Medium-Binary-16     	 4930009	       241 ns/op	      88 B/op	       5 allocs/op
BenchmarkDecode/Large-Binary-16      	 3369573	       344 ns/op	     144 B/op	       7 allocs/op
BenchmarkDecode/Huge-Binary-16       	 2587156	       453 ns/op	     216 B/op	       9 allocs/op

ext/shopspring-numeric/decimal.go

@@ -250,17 +250,7 @@ func (dst *Numeric) DecodeBinary(ci *pgtype.ConnInfo, src []byte) error {
 		return err
 	}
 
-	buf, err := num.EncodeText(ci, nil)
-	if err != nil {
-		return err
-	}
-
-	dec, err := decimal.NewFromString(string(buf))
-	if err != nil {
-		return err
-	}
-
-	*dst = Numeric{Decimal: dec, Status: pgtype.Present}
+	*dst = Numeric{Decimal: decimal.NewFromBigInt(num.Int, num.Exp), Status: pgtype.Present}
 
 	return nil
 }

ext/shopspring-numeric/decimal_test.go

@@ -11,6 +11,7 @@ import (
 	shopspring "github.com/jackc/pgtype/ext/shopspring-numeric"
 	"github.com/jackc/pgtype/testutil"
 	"github.com/shopspring/decimal"
+	"github.com/stretchr/testify/require"
 )
 
 func mustParseDecimal(t *testing.T, src string) decimal.Decimal {
@@ -284,3 +285,46 @@ func TestNumericAssignTo(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkDecode(b *testing.B) {
+	benchmarks := []struct {
+		name      string
+		numberStr string
+	}{
+		{"Zero", "0"},
+		{"Small", "12345"},
+		{"Medium", "12345.12345"},
+		{"Large", "123457890.1234567890"},
+		{"Huge", "123457890123457890123457890.1234567890123457890123457890"},
+	}
+
+	for _, bm := range benchmarks {
+		src := &shopspring.Numeric{}
+		err := src.Set(bm.numberStr)
+		require.NoError(b, err)
+		textFormat, err := src.EncodeText(nil, nil)
+		require.NoError(b, err)
+		binaryFormat, err := src.EncodeBinary(nil, nil)
+		require.NoError(b, err)
+
+		b.Run(fmt.Sprintf("%s-Text", bm.name), func(b *testing.B) {
+			dst := &shopspring.Numeric{}
+			for i := 0; i < b.N; i++ {
+				err := dst.DecodeText(nil, textFormat)
+				if err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+
+		b.Run(fmt.Sprintf("%s-Binary", bm.name), func(b *testing.B) {
+			dst := &shopspring.Numeric{}
+			for i := 0; i < b.N; i++ {
+				err := dst.DecodeBinary(nil, binaryFormat)
+				if err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}