On Tiger, Core Data only creates indices for relationships (joins).
On Leopard, in the modeling tool, you can also specify that an
attribute should have a binary index.
Binary indices can't be used for string matching predicates
(contains, like, etc) or case/diacritic insensitivity. Literal
string equality and prefix searching can use binary indices. As of
Leopard, Core Data does not support custom collations.
For queries that cannot use an index (or an index does not exist),
Core Data relies on SQLite. Core Data does not fetch everything into
memory (instantiate objects) to perform a table scan. Obviously,
SQLite walks the table's entire b-tree for a table scan.
Complex text queries (like case insensitive LIKE) can be very
expensive as Core Data handles unicode and locale issues. It can be
helpful to denormalize some data to make it eligible for a literal
string comparison. Some cleverness with literal string operations
such as = and < and >= upon a canonical literal string (i.e. a search
column v.s. a display column) can produce performance improvements of
several orders of magnitude.
It can also be helpful to organize your predicate in order of maximum
discrimination to reduce the number of rows SQLite examines. That
is, for compound where clauses, put the simplest or most
discriminating subexpressions first, and expensive text expressions
Both SQLite and Core Data have made significant performance
improvements since Tiger (3 years ago). SQLite now has a (primitive)
query optimizer. Core Data fetches are much faster, large fetches
are implicitly multi-threaded, and memory usage is significantly
Also, the array controller has a "lazy fetching" option in entity
mode, which is a bit like a cursor. This only fetches the identity
(PK) of the rows for the entire fetch request, and the actual data
for the rows in use (displayed) by the array controller.
Finally, the tools on Leopard should make debugging performance
problems easier. Instruments has a standard Core Data template, and
the SQL logging includes annotations for durations.
Please file bug reports with bugreport.apple.com early and often.
Performance problems are bugs. Deciding not to use a technology
because it crashes or it's too slow amounts to the same thing.