public List<ColumnInfo> getColumnInfos() {
    return rowSchema.getSignature();
  }
Gets the column Info to tab_alias.col_alias type of a column reference. I
the tab_alias is not provided as can be the case with an non aliased
column, this function looks up the column in all the table aliases in this
row resolver and returns the match. It also throws an exception if the
column is found in multiple table aliases. If no match is found a null
values is returned.

This allows us to interpret both select t.c1 type of references and select
c1 kind of references. The later kind are what we call non aliased column
references in the query.

@param tab_alias
         The table alias to match (this is null if the column reference is
         non aliased)
@param col_alias
         The column name that is being searched for
@return ColumnInfo
@throws SemanticException
ql/src/java/org/apache/hadoop/hive/ql/parse/RowResolver.java