Field Options - FileMaker Pro 13: The Missing Manual - page 315

FileMaker's indexing feature takes its own computerized notes about the data in your fields
in advance , so that when you enter Find mode, the hard work has already been done, and
your finds go that much faster.
Just like a topic, you can have a field without an index. When FileMaker needs to find re-
cords based on a non-indexed field, it has to check every single record—a process that can
take noticeably longer in big databases. But indexing has tradeoffs, as explained in the box
on The Dark Side of Indexing .
Automatic Indexing
The good news is you almost never need to think about indexing. FileMaker has a really
smart way of dealing automatically with indexing: Every field starts out with no index at all,
to save space and keep things as lean as possible. Later, while you're working with your
database, if you do something that would be made faster with an index, FileMaker automat-
ically turns indexing on for you. Several actions trigger indexing, including using the field in
a find request or value list, turning on Unique or Existing validation, and using the field in a
relationship.
FREQUENTLY ASKED QUESTION: THE DARK SIDE OF INDEXING
If indexing makes searching faster, why not just index every field?
Indexes have disadvantages as well. An index is useful only if it's up-to-date. When you change a
field value, FileMaker has to store that new value in the table. If the field is indexed, FileMaker has
to change the data you've typed, and it also has to update the index so that it knows about the
changes to the field. Updating the index takes time (not much time, mind you, but it does take time).
When you're editing records in Browse mode, you never notice because updating the index for a
handful of fields takes FileMaker less time than it takes you to press Enter. But if you're entering
lots of information (like when you're importing records, as discussed in Chapter 21 ) , then the index
updating can slow things down noticeably.
Also, indexes take up space. A database file that has indexing turned on for every field could be
much larger than the same file without indexes. For most users, this isn't an issue, but if you have a
very large file, and saving space is a priority, you can turn off indexing where you don't need it.
That first find is slow since FileMaker looks through records one by one and builds the index
(showing a progress bar in really big files), but once the field is indexed, subsequent finds
happen quickly. You almost always want this automatic behavior.