Also note that hierarchical databases saw a revival in the 90s as web was taking over and web data was mostly hierarchical. A few groups started works on buildi

Also note that hierarchical databases saw a revival in the 90s as web was taking over and web data was mostly hierarchical. A few groups started works on building hierarchical database systems. That didn't go anywhere. Review a few years later show all those buggy, crashed frequently and worst -- slower than just storing the hierarchical data into an RDBMS.