code: func ExplainColumnList(cols []*Column) []byte {
  	buffer := bytes.NewBufferString("")
  	for i, col := range cols {
  		buffer.WriteString(col.ExplainInfo())
  		if i+1 < len(cols) {
  			buffer.WriteString(", ")
  		}
  	}
  	return buffer.Bytes()
  }
docs: // ExplainColumnList generates explain information for a list of columns.