As the title states, what is a fulltext index and when should I use it?
In databases indices are usually used to enhance performance when looking for something defined in your where clause. However when it comes to filtering some text, e.g. using something like
WHERE TextColumn LIKE '%searchstring%' then searches are slow, because the way regular database indices work are optimized for matches against the ‘whole content’ of a column and not just a part of it. In specific the LIKE search can not make use of any kind of index.
As mentioned in the comment below MySQL needs the
MATCH () ... AGAINST syntax to search within a fulltext index; BTW this varies depending on the database vendor. In MS SQL you can use
CONTAINS so keep this in mind when you plan to support other databases too.
Fulltext indices work better for regular text, because they are optimized for these type of columns. Very simplified: They split the text into words and make an index over the words and not the whole text. This works a lot faster for text searches when looking for specific words.
A full text index is an index you apply in a MySQL database to text fields that you plan to run a full text search on. A full text search uses the
match(field) against('text') syntax. If you want to run a full text search you must have a full text index on the columns you’ll be running it against.
There are three types of Full Text searches. I’ll quote the manual, because I think it says it best:
A boolean search interprets the search string using the rules of a
special query language. The string
contains the words to search for. It
can also contain operators that
specify requirements such that a word
must be present or absent in matching
rows, or that it should be weighted
higher or lower than usual. Common
words such as “some” or “then” are
stopwords and do not match if present
in the search string. The IN BOOLEAN
MODE modifier specifies a boolean
search. For more information, see
Section 11.9.2, “Boolean Full-Text
A natural language search interprets the search string as a
phrase in natural human language (a
phrase in free text). There are no
special operators. The stopword list
applies. In addition, words that are
present in 50% or more of the rows are
considered common and do not match.
Full-text searches are natural
language searches if no modifier is
A query expansion search is a modification of a natural language
search. The search string is used to
perform a natural language search.
Then words from the most relevant rows
returned by the search are added to
the search string and the search is
done again. The query returns the rows
from the second search. The WITH QUERY
EXPANSION modifier specifies a query
expansion search. For more
information, see Section 11.9.3,
“Full-Text Searches with Query
For more information take a gander at the Full Text Search Reference Page.