sql/schemachanger/screl: optimize walk a tad

Before this change, walk would allocate once per struct field. Now it does not.

Release justification: bug fixes and low-risk updates to new functionality

Release note: None

pkg/sql/schemachanger/screl/walk.go

@@ -102,9 +102,11 @@ func walk(wantType reflect.Type, toWalk interface{}, f func(interface{}) error)
 			}
 		case reflect.Struct:
 			for i := 0; i < value.NumField(); i++ {
-				if f := value.Field(i); f.CanAddr() && value.Type().Field(i).IsExported() {
-					walk(f.Addr().Elem())
+				f := value.Field(i)
+				if !f.CanAddr() || !fieldIsExported(value.Type(), i) {
+					continue
 				}
+				walk(value.Field(i).Addr().Elem())
 			}
 		case reflect.Bool, reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
 			reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr,
@@ -123,3 +125,22 @@ func walk(wantType reflect.Type, toWalk interface{}, f func(interface{}) error)
 	walk(v.Elem())
 	return nil
 }
+
+// fieldIsExported caches whether a field is exported as an optimization to avoid
+// allocations due to (*reflect.Type).Field() calls.
+func fieldIsExported(typ reflect.Type, field int) bool {
+	k := fieldExportedCacheKey{typ: typ, field: field}
+	if exported, ok := fieldExportedCache[k]; ok {
+		return exported
+	}
+	exported := typ.Field(field).IsExported()
+	fieldExportedCache[k] = exported
+	return exported
+}
+
+type fieldExportedCacheKey struct {
+	typ   reflect.Type
+	field int
+}
+
+var fieldExportedCache = map[fieldExportedCacheKey]bool{}