Optimizing slow query

Question

For a few days now, i'm trying to optimize this query. But still I haven't found the right solution in how to fix this.

The contents table contains about 20 million records. As the connect table holds about 60 million.

The query runs in about 20 seconds:

SELECT *
FROM `contents` AS `n`
INNER JOIN `connect` AS `a` ON `a`.`mcded_primary_id_mcdno_id` = `n`.`mcdno_id`
INNER JOIN `connect` AS `b` ON `b`.`mcded_primary_id_mcdno_id` = `n`.`mcdno_id`
INNER JOIN `connect` AS `c` ON `c`.`mcded_primary_id_mcdno_id` = `n`.`mcdno_id`
WHERE `a`.`mcded_child_id_mcdno_id` = '1375'
  AND `a`.`mcded_structure_mcdes_id` = '85'
  AND `b`.`mcded_child_id_mcdno_id` = '13'
  AND `b`.`mcded_structure_mcdes_id` = '187'
  AND `c`.`mcded_child_id_mcdno_id` IN ('500065',
                                    '500066',
                                    '500067',
                                    '500068')
  AND `c`.`mcded_structure_mcdes_id` = '211'
  AND `n`.`mcdno_structure_mcdns_id` IN ('1')
GROUP BY `n`.`mcdno_id`
ORDER BY `n`.`mcdno_id` DESC LIMIT 14
OFFSET 42

But it uses temp table every time.

This is the EXPLAIN:

+----+-------------+------+---------------+---------------------------------------------------------------------------------------------------+--------------------------------------------------+-----+-----------------------------------------+-------+-----------------------------------------------------------------------------------------------------------------+
| id | select_type | type | possible_keys |                                                key                                                |                     key_len                      | ref |                  rows                   | Extra |                                                                                                                 |
+----+-------------+------+---------------+---------------------------------------------------------------------------------------------------+--------------------------------------------------+-----+-----------------------------------------+-------+-----------------------------------------------------------------------------------------------------------------+
|  1 | SIMPLE      | b    | index_merge   | UN_set,FK_primary_id_id,FK_structure_mcdns_id_id,FK_child_id_mcdno_id_id,IND_primary_id_structure | FK_structure_mcdns_id_id,FK_child_id_mcdno_id_id | 4,8 | NULL                                    | 37928 | Using intersect(FK_structure_mcdns_id_id,FK_child_id_mcdno_id_id); Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | c    | ref           | UN_set,FK_primary_id_id,FK_structure_mcdns_id_id,FK_child_id_mcdno_id_id,IND_primary_id_structure | IND_primary_id_structure                         | 12  | b.mcded_primary_id_mcdno_id,const       |     1 | Using where                                                                                                     |
|  1 | SIMPLE      | n    | eq_ref        | PRIMARY,FK_mcdno_structure_mcdns_id                                                               | PRIMARY                                          | 8   | c.mcded_primary_id_mcdno_id             |     1 | Using where                                                                                                     |
|  1 | SIMPLE      | a    | ref           | UN_set,FK_primary_id_id,FK_structure_mcdns_id_id,FK_child_id_mcdno_id_id,IND_primary_id_structure | UN_set                                           | 20  | b.mcded_primary_id_mcdno_id,const,const |     1 |                                                                                                                 |
+----+-------------+------+---------------+---------------------------------------------------------------------------------------------------+--------------------------------------------------+-----+-----------------------------------------+-------+-----------------------------------------------------------------------------------------------------------------+

I tried using connect (a) as primary table, but that is even worse.

Any suggestions?

-- CONTENTS

  CREATE TABLE `contents` (
  `mcdno_id` bigint(20) NOT NULL AUTO_INCREMENT,
  `mcdno_created_on` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `mcdno_changed_on` timestamp NULL DEFAULT NULL,
  `mcdno_structure_mcdns_id` int(11) NOT NULL,
  PRIMARY KEY (`mcdno_id`),
  KEY `FK_mcdno_structure_mcdns_id` (`mcdno_structure_mcdns_id`),
  KEY `IND_created_on` (`mcdno_created_on`),
  KEY `IND_changed_on` (`mcdno_changed_on`),
  KEY `IND_published_on` (`mcdno_published_on`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8

-- CONNECT

CREATE TABLE `connect` (
 `mcded_id` bigint(11) NOT NULL AUTO_INCREMENT,
 `mcded_primary_id_mcdno_id` bigint(20) NOT NULL,
 `mcded_child_id_mcdno_id` bigint(20) NOT NULL,
 `mcded_changed_on` datetime DEFAULT NULL,
 `mcded_structure_mcdes_id` int(11) NOT NULL
 PRIMARY KEY (`mcded_id`),
 UNIQUE KEY `UN_set` (`mcded_primary_id_mcdno_id`,`mcded_child_id_mcdno_id`,`mcded_structure_mcdes_id`),
 KEY `IND_created_on` (`mcded_created_on`),
 KEY `FK_primary_id_id` (`mcded_primary_id_mcdno_id`),
 KEY `FK_structure_mcdns_id_id` (`mcded_structure_mcdes_id`),
 KEY `FK_child_id_mcdno_id_id` (`mcded_child_id_mcdno_id`),
 KEY `IND_primary_id_structure`         (`mcded_primary_id_mcdno_id`,`mcded_structure_mcdes_id`)
 ),     ENGINE=InnoDB DEFAULT CHARSET=utf8

Answer 1

"Using intersect" usually implies that a composite index would be better:

 INDEX(mcded_child_id_mcdno_id, mcded_structure_mcdes_id) -- or the opposite order

Even better, rearrange UN_set so it starts with these two.

Don't use SELECT * unless you really need all the columns from all the joins. It inhibits use of "covering" indexes.

(Don't you find these long names hard to distinguish? And the similarity (mcded/mcdno/mcdes/mcdns; connect/content) makes it difficult for me to read. simply tossing "mcd" everywhere would help.)

Answer 2

SELECT * makes it difficult to guess what output you need, but here is a guess:

SELECT ...
FROM contents AS n
JOIN connect AS a 
    ON a.mcded_primary_id_mcdno_id = n.mcdno_id
   AND n.mcdno_structure_mcdns_id = '1'
JOIN (
    SELECT '1375' as x, '85' as y
    UNION ALL
    SELECT '13', '187'
    UNION ALL
    SELECT '500065', '211'
    UNION ALL 
    SELECT '500066', '211'
    UNION ALL
    SELECT '500067', '211'
    UNION ALL 
    SELECT '500068', '211'
) AS t
    ON a.mcded_child_id_mcdno_id = t.x
   AND a.mcded_structure_mcdes_id = t.y
ORDER BY n.mcdno_id DESC 
LIMIT 14 OFFSET 42;

You may want to use a temporary table instead of the derived table I used. The result will be spread over several rows, so you will have to fix that in the application.