具有多个联接的SQL查询会影响性能

Question

I have a relational MySQL database with almost 4000 records. The contacts table is related to both the keywords and notes tables with manny-to-many relationships. I wrote a query (with PHP) that would retrieve each contact record and, for each contact, all related notes and keywords in a group concat function. If I only grab the contacts, the query performs relatively quickly but with the two left joins and group concat, it takes almost 30 seconds. Is there a way to speed this up?

Here is my query:

SELECT  c.*, GROUP_CONCAT(DISTINCT n.id, '[-]', n.value, '' SEPARATOR '---') as notes,
        GROUP_CONCAT(DISTINCT kk.id, '[-]', kk.value) as keywords
    FROM  contacts c
    LEFT JOIN  notes n  ON c.id LIKE n._contactID
    LEFT JOIN  
        ( SELECT  k.*, kc._contactID as contactID
            FROM  keywords k
            INNER JOIN  keywords_contacts kc  ON k.id LIKE kc._keywordID
        ) kk  ON kk.contactID LIKE c.id
    GROUP BY  c.id
    ORDER BY  c.`Last Name`, c.`First Name`

Answer 1

I your query I can see c.id LIKE n._contactID , here LIKE is making performace low so use = operator and give index to foreign key field _contactID .

Same as for k.id LIKE kc._keywordID use = operator instead of LIKE and apply index on field _keywordID .

Answer 2

Yes, use = instead of LIKE ; keep LIKE for when you need wild cards, such as first_name LIKE 'H%' .

After changing to = , you may still find the query slow. Consider these:

If you don't need LEFT , don't use it. Normally it is faster to start with the 'table' that is a subquery (see LEFT JOIN ( SELECT ...) ), but the use of LEFT tends to inhibit such.

If the GROUP BY and ORDER BY are the same, you can avoid a temp table and sort:

GROUP BY c.`Last Name`, c.`First Name`, c.id
ORDER BY c.`Last Name`, c.`First Name`, c.id

Please provide SHOW CREATE TABLE and EXPLAIN SELECT ... , so we can critique the indexes.