Performance Tuning - Core Data - page 60

memory when they are accessed. This causes atomic stores to have a larger
memory footprint than a SQLite store.
However, because they reside completely in memory while the application is
running, atomic stores can be very fast, since the disk is hit only when the
file is read into memory and when it is saved back out. SQLite, although still
considered a fast store, is slower when dealing with smaller data sets because
of its inherent disk access. The differences are measured in fractions of a
second, so we cannot expect a dramatic speed increase by using an atomic
store. But if fractions of a second matter, it may be something to consider.
SQLite Persistent Store
In the most common cases, SQLite is the store option to use for application
development. This is true on both iOS and OS X. SQLite is a software library
that implements a self-contained, server-less, zero-configuration, transactional
SQL database engine. SQLite is the most widely deployed SQL database engine
in the world. The source code for SQLite is in the public domain.
Better Scaling
By utilizing a relational database as the persistent store, we no longer need
to load the entire data set into memory to work with it. Because the data is
being stored in a relational database, our application can scale to a very large
size. SQLite itself has been tested with data sets measured in terabytes and
can handle just about anything that we can realistically develop. Since we
are loading only the data we want at a particular moment, SQLite keeps the
memory footprint of our application quite low. Likewise, SQLite makes efficient
use of its disk space and therefore has a small footprint on disk as well.
More Performance-Tuning Options
By working with a database instead of a flat file, we have access to many more
performance-tuning options. For example, we can index the columns within
our entities to enable faster predicates. We can also control exactly what gets
loaded into memory. It is possible to get just a count of the objects, just the
unique identifiers for objects, and so on. This flexibility allows us to tune the
performance of our application more than any other store type. Because the
SQLite store is the only format that is not fully loaded into memory, we get
to control the data flow. All of the other formats require that the entire data
file be loaded into memory before they can be used. The details of how to
utilize these features are discussed in Section 4.3, Fetching , on page 67 .