Magento - 使用平台时需要哪些索引

I have a cronjob that does a reindexall for all indexes in my magento setup, and it takes more than 6 hours to finish the job, making the server slower to respond during that period.

Since flat tables are another way to store data, I wonder what I can do to optimize this process, and I have the following questions:

• While using flat tables, do I still need to do a reindex for all the indexes? or are there any unnecessary indexes?
• Is there a way that Magento only performs a re-index to modified/added/deleted products only? currently its doing re-index to all my data each time