Say you have a large collection with n objects on disk and each one has a variable-sized string. What are common practices of efficient ways to make an index of those objects with plain string comparison. Storing the whole strings on the index would be prohibitive in the long rundue to size and I/O, but since disks have a high latency storing only references isn't a good idea, either.
I've been thinking on using a B-Tree-like design with tries but can't find any database implementation using this approach. In fact, it's hard to find how major databases implement indexes for strings (it probably gets lost in the vast results for SQL-level information.)
TIA!
EDIT: changed title from "Efficient external sorting and searching of stored objects with large strings" to "Efficient storage of external index of strings."
A "prefix B-tree" or "simple prefix B-tree" would probably be helpful here.
A "simple prefix B-tree" is a bit simpler, just storing the shortest prefix that separates two items, without trying to eliminate redundancy within those prefixes (e.g. for 'astronomy' and 'azimuth', it would store just 'as' and 'az', but not try to keep from duplicating the 'a').
A "prefix B-tree" is close to what you've described -- something like a trie, but in a B-tree structure to give good characteristics when stored primarily on disk. Nonetheless, it's intended to remove (most of) the redundancy within the prefixes that form the index.
There is one other question: do you really need to traverse the records in order, or do you just need to look up a specified record quickly? If the latter is adequate, you might be able to use extendible hashing instead. Extendible hashing has been around (in a number of different forms) for a few decades, and still works pretty well. The general idea is fairly simple: hash the strings to create keys of fixed length, then create some sort of tree of those fixed-length pseudo-keys. As with (almost) any hash, you have to be prepared to deal with collisions. As with other hash tables, the details of the hashing and collision resolution vary (though probably not quite as much with extendible hashing as in-memory hashing).
As for real use, major DBMS and DBMS-like systems use all of the above. B-tree variants are probably the most common in the general purpose DBMS market (e.g. Oracle or MS SQL Server). Extendible hashing is used in a fair number of more-specialized products (e.g., Lotus Domino Server).