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I understand what a B-Tree is (I already implemented a B-Tree in Java with insert and delete methods that preserve the invariant).

However I do not understand exactly how it is used for example for file systems or databases.

  1. How do you choose the keys in these 2 scenarios? And what is the data?

  2. Are data and keys the same?

  3. I can't think of a good key that is able to order the data in a way that it is easily accessible. If I want to find an entry containing some string.

  4. how useful would be a key that gives information about the size of each entry? or the alphabetical order? I don't exactly understand the concept.

I have to add that I don't understand much of file systems or databases which is probably why I'm confused.

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A B-Tree is a type of dictionary, no more and no less. It can be used to implement a set (e.g. see the interface for java.util.Set for the sort of operations we're talking about), but is most commonly used to implement a map (ditto for java.util.Map). So let's just look at maps for a moment.

If you think about a linguistic dictionary, it's ordered by "word", and associated with each word is a definition. You look up a word, and get a definition.

So the context of a map data structure is that you have keys ("words") and you want to map this to values ("definitions").

B-trees are like other kinds of search tree (e.g. binary search trees) in that keys can be accessed in sorted order. This has an advantage for certain types of query. For example you can do range queries, say if you want to find all entries where the key is between two values (e.g. all words in the dictionary starting with "q").

The main advantage of B-trees over other kinds of search tree is that it is particularly efficient when represented on disk. B-trees are page-structured (meaning they can be implemented on top of fixed-size disk pages; this also avoids fragmentation), and have a wide ply, which minimises the number of disk accesses needed to perform a query.

A typical use in a database server is to implement indexes. An index is, once again, much like the index in a paper book. Some books have multiple indexes (e.g. there might be a general index and an index of names of people). Given a key, the index in a book returns a set of pages on where you can find references to whatever is indexed (be it a concept, name, or whatever).

A database index typically maps a key to a set of database records. In a typical organisation, there is one data structure in which records are stored, and multiple indexes which map keys to record identifiers. To find records, you query the index, which gives you a collection of record identifiers. Then you can use those record identifiers to find the records in the store.

A typical use in a modern filesystem is to implement directories ("folders" if you're more used to GUIs). A directory is, logically, a map from names (file names or directory names) to filesystem objects. A filesystem object may be a file, or another directory, or what have you.

As has been pointed out elsewhere, you might want to do some reading on this. Any "introduction to databases" textbook will probably do.

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  • $\begingroup$ Nicely explained. The use of analogies to a real physical book really aided my understanding! $\endgroup$ – Frank Fu Oct 19 '15 at 23:08
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If you do proper research all you'r doubts will be clear. Let me try

how it is used for example for file systems or databases.

Take an example of a library books, And you want to search a specific book and you have kept all the books randomly how much time will it. If you use B-trees then you will divide the books in the racks with some order, So when you perform a search how much time will it take.

How do you choose the keys in these 2 scenarios? And what is the data?

the book name,or place will be the key and book will be the data.

2.Are data and keys the same?

No keys and data are different .The indexes will be act like keys and data will be the actual data(in data blocks) pointed by the keys.

  • index file: contains all the keys and the tree's topology is represented by the organization of data in this file.

  • data file: a file that contains all the objects and information stored by the “tree”. Objects contained here are referenced by block pointer references stored in the index file.

3.I can't think of a good key that is able to order the data in a way that it is easily accessible. If I want to find an entry containing some string.

If you keep all you'r specific books in specific Racks then the name of rack will become key and each rack may have different blocks each block will become the child of tree.

I have to add that I don't understand much of file systems or databases which is probably why I'm confused.

If you google you will find lot of information regarding this.

but in brief . filesystem is language dependent and architecture dependent. Data Base system is not.

Access mechanism is very fast in database system. Database is designed because of the need of large Data and Managing, storing, synchronization, security, etc of these data.

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