perf(col_low_cardinality): use small value

proto/col_low_cardinality.go

@@ -286,17 +286,6 @@ func (c ColLowCardinality[T]) Rows() int {
 
 // Prepare column for ingestion.
 func (c *ColLowCardinality[T]) Prepare() error {
- // Select minimum possible size for key.
- if n := len(c.Values); n < math.MaxUint8 {
- c.key = KeyUInt8
- } else if n < math.MaxUint16 {
- c.key = KeyUInt16
- } else if uint32(n) < math.MaxUint32 {
- c.key = KeyUInt32
- } else {
- c.key = KeyUInt64
- }
-
  // Allocate keys slice.
  c.keys = append(c.keys[:0], make([]int, len(c.Values))...)
  if c.kv == nil {
@@ -317,6 +306,17 @@ func (c *ColLowCardinality[T]) Prepare() error {
  c.keys[i] = idx
  }
 
+ // Select minimum possible size for key.
+ if n := last; n < math.MaxUint8 {
+ c.key = KeyUInt8
+ } else if n < math.MaxUint16 {
+ c.key = KeyUInt16
+ } else if uint32(n) < math.MaxUint32 {
+ c.key = KeyUInt32
+ } else {
+ c.key = KeyUInt64
+ }
+
  // Fill key column with key indexes.
  switch c.key {
  case KeyUInt8: