Database Reference
In-Depth Information
CouchDB also lets you write your queries as
map
and
reduce
functions. If that sounds like a lot of effort, then
you're in good company; CouchDB has a somewhat severe learning curve. In fairness to CouchDB, an experienced
programmer can probably pick it up quite quickly; for most people, however, the learning curve is probably steep
enough that they won't bother with the tool.
Fortunately for us mere mortals, MongoDB is much easier to use. We'll cover how to use MongoDB in more
detail throughout the topic, but here's the short version: in MongoDB, you simply provide the parts of the document
you want to match against, and MongoDB does the rest. MongoDB can do much more, however. For example, you
won't find MongoDB lacking if you want to use
map
or
reduce
functions. At the same time, you can ease into using
MongoDB; you don't have to know all of the tool's advanced features up front.
Indexing Your Documents
MongoDB includes extensive support for indexing your documents, a feature that really comes in handy when
you're dealing with tens of thousands of documents. Without an index, MongoDB will have to look at each individual
document in turn to see whether it is something that you want to see. This is like asking a librarian for a particular
topic and watching as he works his way around the library looking at each and every topic. With an indexing system
(libraries tend to use the Dewey Decimal system), he can find the area where the topic you are looking for lives and
very quickly determine if it is there.
Unlike a library topic, all documents in MongoDB are automatically indexed on the
_id
key. This key is
considered a special case because you cannot delete it; the index is what ensures that each value is unique. One of
the benefits of this key is that you can be assured that each document is uniquely identifiable, something that isn't
guaranteed by an RDBMS.
When you create your own indexes, you can decide whether you want them to enforce uniqueness. If you do
decide to create a unique index, you can tell MongoDB to drop all the duplicates. This may or may not be what you
want, so you should think carefully before using this option because you might accidentally delete half your data. By
default, an error will be returned if you try to create a unique index on a key that has duplicate values.
There are many occasions where you will want to create an index that allows duplicates. For example, if your
application searches by lastname, it makes sense to build an index on the lastname key. Of course, you cannot
guarantee that each lastname will be unique; and in any database of a reasonable size, duplicates are practically
guaranteed.
MongoDB's indexing abilities don't end there, however. MongoDB can also create indexes on embedded
documents. For example, if you store numerous addresses in the address key, you can create an index on the ZIP or
postal code. This means that you can easily pull back a document based on any postal code—and do so very quickly.
MongoDB takes this a step further by allowing
composite indexes
. In a composite index, two or more keys are
used to build a given index. For example, you might build an index that combines both the
lastname
and
firstname
tags. A search for a full name would be very quick because MongoDB can quickly isolate the lastname and then, just
as quickly, isolate the firstname.
We will look at indexing in more depth in Chapter 10, but suffice it to say that MongoDB has you covered as far as
indexing is concerned.
Leveraging Geospatial Indexes
One form of indexing worthy of special mention is
geospatial indexing
. This new, specialized indexing technique was
introduced in MongoDB 1.4. You use this feature to index location-based data, enabling you to answer queries such as
how many items are within a certain distance from a given set of coordinates.
As an increasing number of web applications start making use of location-based data, this feature will play an
increasingly prominent role in everyday development. For now, though, geospatial indexing remains a somewhat
niche feature; nevertheless, you will be very glad it's there if you ever find that you need it.
