对于MyISAM：

  FULLTEXT（`column_a`，`column_c`，`column_x`）
$ c对于InnoDB： 

  FULLTEXT（ （column_c`），
 FULLTEXT（`column_c`），
 FULLTEXT（`column_c`，`column_x`），
 FULLTEXT（`column_x`），
 
如果您使用的是5.6或更高版本，您应该转换为InnoDB。
 
I have an extremely large table (4M+ rows) with disk space of more than 40Gb (14Gb data and 28Gb index). I needed fulltext search on multiple fields both combined and separated, meaning that I needed to make it possible to fulltext search on both single columns and multiple columns together, like below:

for combined search
SELECT `column_a`, `column_b` FROM `table_1` WHERE MATCH (`column_a`, `column_c`, `column_x`) AGAINST ('+$search_quesry*' IN BOOLEAN MODE);

for separate search
SELECT `column_a`, `column_b` FROM `table_1` WHERE MATCH (`column_a`) AGAINST ('+search_query*' IN BOOLEAN MODE);
SELECT `column_a`, `column_b` FROM `table_1` WHERE MATCH (`column_c`) AGAINST ('+search_query*' IN BOOLEAN MODE);
SELECT `column_a`, `column_b` FROM `table_1` WHERE MATCH (`column_x`) AGAINST ('+search_query*' IN BOOLEAN MODE);

Here is my question. I have both following sets already defined as indexes, which cause 24Gb+ disk space. Did I do it right or one set is enough?
ALTER TABLE  `table_1` ADD FULLTEXT (`column_a`, `column_c`, `column_x`);

and/or
ALTER TABLE  `table_1` ADD FULLTEXT (`column_a`);
ALTER TABLE  `table_1` ADD FULLTEXT (`column_c`);
ALTER TABLE  `table_1` ADD FULLTEXT (`column_x`);

OR
ALTER TABLE  `table_1` ADD FULLTEXT (`column_a`);
ALTER TABLE  `table_1` ADD FULLTEXT (`column_c`, `column_x`);

This is mainly to reduced required disk space as well as better performance. Any better suggestion is more than welcome. Thanks :)

P.S. The cardinality numbers seem different for column_a when indexed combined and separated.

For MyISAM:
FULLTEXT (`column_a`, `column_c`, `column_x`)

For InnoDB:
FULLTEXT (`column_a`, `column_c`, `column_x`),
FULLTEXT (`column_a`),
FULLTEXT (`column_c`),
FULLTEXT (`column_x`)

If you have version 5.6 or later, you should convert to InnoDB.