Database Reference
In-Depth Information
Every additional supported driver that you load when working with MongoDB (such as the PHP driver or the
Python driver) supports this special BSON datatype and uses it whenever new data is created. You can also invoke
ObjectId()
from the MongoDB shell to create a value for an
_id
key. Optionally, you can specify your own value by
using
ObjectId(
string
)
, where
string
represents the specified hex string.
Building Indexes
As mentioned in Chapter 1, an
index
is nothing more than a data structure that collects information about the values
of specified fields in the documents of a collection. This data structure is used by MongoDB's query optimizer to
quickly sort through and order the documents in a collection.
Remember that indexing ensures a quick lookup from data in your documents. Basically, you should view
an index as a predefined query that was executed and had its results stored. As you can imagine, this enhances
query-performance dramatically. The general rule of thumb in MongoDB is that you should create an index for the
same sort of scenarios where you would want to have an index in MySQL.
The biggest benefit of creating your own indexes is that querying for often-used information will be incredibly
fast because your query won't need to go through your entire database to collect this information.
Creating (or deleting) an index is relatively easy—once you get the hang of it, anyway. You will learn how to
do so in Chapter 4, which covers working with data. You will also learn some more advanced techniques for taking
advantage of indexing in Chapter 10, which covers how to maximize performance.
Impacting Performance with Indexes
You might wonder why you would ever need to delete an index, rebuild your indexes, or even delete all indexes within
a collection. The simple answer is that doing so lets you clean up some irregularities. For instance, sometimes the size
of a database can increase dramatically for no apparent reason. At other times, the space used by the indexes might
strike you as excessive.
Another good thing to keep in mind: you can have a maximum of 40 indexes per collection. Generally speaking,
this is far more than you should need, but you could potentially hit this limit someday.
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adding an index increases query speed, but it reduces insertion or deletion speed. It's best to consider only
adding indexes for collections where the number of reads is higher than the number of writes. When more writes occur
than reads, indexes may even prove to be counterproductive.
Note
Finally, all index information is stored in the
system.indexes
collection in your database. For example, you can
run the
db.indexes.find()
command to take a quick peek at the indexes that have been stored so far. To see the
indexes created for a specific collection, you can use the
getIndexes
command:
db.collection.getIndexes()
Implementing Geospatial Indexing
As Chapter 1briefly mentioned, MongoDB has implemented
geospatial indexing
since version 1.4. This means that,
in addition to normal indexes, MongoDB also supports geospatial indexes that are designed to work in an optimal
way with location-based queries. For example, you can use this feature to find a number of closest known items to the
user's current location. Or you might further refine your search to query for a specified number of restaurants near the
