code: func (col *Column) ResolveIndices(schema Schema) {
  	col.Index = schema.GetColumnIndex(col)
  	// If col's index equals to -1, it means a internal logic error happens.
  	if col.Index == -1 {
  		log.Printf("Can't find column %s in schema %s", col, schema)
  	}
  }
docs: // ResolveIndices implements Expression interface.