A SQL Server full-text index is actually more like a catalog than an index,
and its structure is not a B-tree.
The full-text index allows you to search by groups of keywords.
The full-text index is part of the Microsoft Search service;
it is used extensively in Web site search engines and in other text-based operations.
Unlike B-tree indexes,
a full-text index is stored outside the database but is maintained by the database. Because it is stored externally, the index can maintain its own structure.
The following restrictions apply to full-text indexes:
* A full-text index must include a column that uniquely identifies each row in the table.
* A full-text index also must include one or more character string columns in the table.
* Only one full-text index is allowed per table.
* A full-text index is not automatically updated as B-tree indexes are.
That is, in a B-tree index, a table insert, update, or delete operation will update the index.
With the full-text index, these operations on the table will not automatically update the index.
Updates must be scheduled or run manually.
The full-text index has a wealth of features that cannot be found in B-tree indexes.
Because this index is designed to be a text search engine,
it supports more than standard text-searching capabilities.
Using a full-text index,
you can search for words or phrases, single words or groups of words,
or words that are similar to each other.
Source:
Microsoft SQL Server 2000 Administrator's Companion eBook