code: func (col *Column) HashCode(_ *stmtctx.StatementContext) []byte {
  	if len(col.hashcode) != 0 {
  		return col.hashcode
  	}
  	col.hashcode = make([]byte, 0, 9)
  	col.hashcode = append(col.hashcode, columnFlag)
  	col.hashcode = codec.EncodeInt(col.hashcode, int64(col.UniqueID))
  	return col.hashcode
  }
docs: // HashCode implements Expression interface.