MySQL 慢速查询连接两个表，每个表都有全文条件

Question

I need to join two tables (1M rows and 10M rows respectively)

Each table is filtered with a fulltext match condition:

SELECT SQL_NO_CACHE c.company_index
    
FROM dw.companies c INNER JOIN dw.people p 
ON c.company_index = p.company_index
    
WHERE MATCH ( c.tag ) AGAINST ( 'ecommerce' IN BOOLEAN MODE )
AND MATCH ( p.title ) AGAINST ( 'director' IN BOOLEAN MODE )
    
ORDER BY c.company_index DESC ;

Both tables have fulltext indexes (on "tag" and "title")

The query time is more than 1 mn with both conditions.

With only one of the two conditions, the query time is below 1 sec.

How could I optimize this query?

Answer 1

I think the problem is that FULLTEXT is very fast if it can be performed first , but very slow if not. With both of the tests in the original query, one MATCH can be first, but the other cannot be.

Here's a messy idea on how to work around the problem.

SELECT c.company_index
    FROM ( SELECT company_index FROM companies WHERE MATCH... ) AS c
    JOIN ( SELECT company_index FROM people    WHERE MATCH... ) AS p
       ON c.company_index
        = p.company_index
    ORDER BY ...

What version of MySQL are you using? Newer versions will automatically create an index on one of the 'derived' tables, thereby making the JOIN quite efficient.

Here's another approach:

SELECT c.company_index
    FROM ( SELECT company_index FROM companies WHERE MATCH... ) AS c
    WHERE EXISTS ( SELECT 1 FROM people WHERE MATCH...
                      AND company_index = c.company_index )
    ORDER BY ...

In both cases (I hope) one SELECT will use one FT index; the other will use the other, thereby getting the FT performance benefit.