Comments section - Database/Index design

I'm looking for a DB structure designer, to design a comments section. There's no need to build the comments section functionality- only to design its DB/index structure: 1. What type of index to use, B-tree? B+ tree? R-tree? ... should we use multiple indexes for different purposes? 2. What the "insert comment" routine looks like? 3. What the "query comments" routine looks like? 4. Removal of comments I'm open to suggestions of existing engines, and to suggestions of customized data-stores and index libraries. The focus here is SCALABILITY, which means, being able to query the comments efficiently with high-volume of comments. The 'removal' of comments could be done by setting a flag. The size of the data-stores is insignificant- only the scalable properties matter. The comments structure must deal with recursion of SUB-comments: Comment #1 Comment #2 Comment #3 Comment #3 -> Comment #1 Comment #3 -> Comment #2 Comment #3 -> Comment #2 -> Comment #1 * Just in case: being SCALABLE is not the same as CACHING