mySQL MATCH across multiple tables

Question

I have a set of 4 tables that I want to search across. Each has a full text index. Can a query make use of every index?

CREATE TABLE `categories` (
  `id` in         


        

Answer 1

• You can't define fulltext indexes (or any kind of index) across multiple tables in MySQL. Each index definition references exactly one table. All columns in a given fulltext index must be from the same table.

• The columns named as arguments to the MATCH() function must be part of a single fulltext index. You can't use a single call to MATCH() to search all columns that are part of all fulltext indexes in your database.

• Fulltext indexes only index columns defined with CHAR, VARCHAR, and TEXT datatypes.

• You can define a fulltext index in each table.

Example:

CREATE TABLE categories (
  id SERIAL PRIMARY KEY,
  name VARCHAR(100),
  FULLTEXT INDEX ftcat (name)
);

CREATE TABLE host_types (
  id SERIAL PRIMARY KEY,
  category_id BIGINT UNSIGNED,
  name VARCHAR(100),
  FULLTEXT INDEX ftht (name)
);

CREATE TABLE hosts (
  id SERIAL PRIMARY KEY,
  host_id BIGINT UNSIGNED,
  category_id BIGINT UNSIGNED,
  name VARCHAR(100),
  FULLTEXT INDEX fthost (name)
);

CREATE TABLE products (
  id SERIAL PRIMARY KEY,
  name VARCHAR(100),
  keywords VARCHAR(100),
  uid VARCHAR(100),
  description VARCHAR(100),
  quantity INTEGER,
  price NUMERIC(9,2),
  host_id BIGINT UNSIGNED,
  FULLTEXT INDEX ftprod (name, keywords, description, uid)
);

And then you can write a query that uses each respective fulltext index:

SELECT ...
  MATCH(categories.name) AGAINST('search term') as cscore, 
  MATCH(host_types.name) AGAINST('search term') as htscore,
  MATCH(hosts.name) AGAINST('search term') as hscore,
  MATCH(products.name, products.keywords, products.description, products.uid)
    AGAINST('search term') as score
FROM products
LEFT JOIN hosts ON products.host_id = hosts.id
LEFT JOIN host_types ON hosts.host_id = host_types.id
LEFT JOIN categories ON host_types.category_id = categories.id
WHERE
  MATCH(categories.name) AGAINST('search term') OR
  MATCH(host_types.name) AGAINST('search term') OR
  MATCH(hosts.name) AGAINST('search term') OR
  MATCH(products.name, products.keywords, products.description, products.uid)
    AGAINST('search term')
ORDER BY score DESC;

Answer 2

Just to expand on the answer above. There are some things to consider. If you left join multiple rows and use GROUP to group those rows, you need to get the MAX(). Also, the above only orders by relevance of one of the tables, therefore disregarding the relevance of the other tables.

SELECT ...
  MAX(MATCH(categories.name) AGAINST('search term'))+ 
  MAX(MATCH(host_types.name) AGAINST('search term'))+
  MAX(MATCH(hosts.name) AGAINST('search term'))+
  MAX(MATCH(products.name, products.keywords, products.description, products.uid) AGAINST('search term')) as score
FROM products
LEFT JOIN hosts ON products.host_id = hosts.id
LEFT JOIN host_types ON hosts.host_id = host_types.id
LEFT JOIN categories ON host_types.category_id = categories.id
WHERE
  MATCH(categories.name) AGAINST('search term') OR
  MATCH(host_types.name) AGAINST('search term') OR
  MATCH(hosts.name) AGAINST('search term') OR
  MATCH(products.name, products.keywords, products.description, products.uid)
    AGAINST('search term')
ORDER BY score DESC;

This way you add the relevance from all the tables. The MAX() allows you to group your data. It also helps when you are LEFT joining multiple rows.