We are currently using Magento version 1.4.
We have a VERY LARGE number of products and reindexing after each product add is impractical.
That said, it would be nice to have a report on all unindexed products (based on new or even better of possible, both new and relevant edits like price). How would this query be expressed in SQL? Is there a method in native Magento code as well?
I would try async reindexing, some development parties have made extensions for this. Furthermore, calculating differences to find out what needs to be reindexed would be pretty darn hard and probably cause a tremendous amount of load. What you can do for example, is store product saves and start from there or use mysql triggers.
Another sidenote
What are you doing about XSS when using such an old Magento version? Also, newer Magento versions have index fixes :)
