How do FULLTEXT INDEXES on multiple columns work?

Question

When adding a FULLTEXT INDEX on 3 columns, does that add 1 single index on 3 columns, or does it add 3 separate indexes?

I ask this, because at the moment I\'m using

Answer 1

Only one index, of type FULLTEXT, will be created. That index will "span" around multiple columns if necessary. However, the indexes themselves are not indexing the columns as much as their words. From the documentation:

InnoDB FULLTEXT indexes have an inverted index design. Inverted indexes store a list of words, and for each word, a list of documents that the word appears in. [...]

If you create one FULLTEXT grouping three columns, or three FULLTEXT grouping only one, these columns will in effect consist of entries in the FULLTEXT meta-data tables.

When incoming documents are tokenized, the individual words (also referred to as “tokens”) are inserted into the index tables along with position information and the associated Document ID (DOC_ID).

Basically, it seems multi-column FULLTEXT vs multiple single-column FULLTEXT indexes work the same, as the storage of the words is abstracted from the original separation of the columns.