MySQL Join 1 to 1 in N items table - mysql

Question

I need some help on my shop database structure. To fetch items from category, there are already some joins, but i would like to add an image to an item, from other table. Main items table doesn't have an image id, so we just get one from other table by it's weight = 1.
So, there is a structure:
CREATE TABLE IF NOT EXISTS `categories` (
`category_id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`name` VARCHAR(255)
CHARACTER SET utf8mb4 DEFAULT NULL
COMMENT 'Category title',
`parent_id` INT(11) NOT NULL DEFAULT '0'
COMMENT 'Category Parent ID',
`status` TINYINT(1) DEFAULT '0'
COMMENT 'Category active status',
`weight` INT(11) DEFAULT '0',
`slug` VARCHAR(255)
CHARACTER SET utf8mb4 DEFAULT NULL
COMMENT 'Category url alias',
PRIMARY KEY (`category_id`)
)
ENGINE = InnoDB
DEFAULT CHARSET = utf8mb4
COLLATE = utf8mb4_unicode_ci;
CREATE TABLE IF NOT EXISTS `items` (
`item_id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`name` VARCHAR(255)
CHARACTER SET utf8mb4 NOT NULL DEFAULT ''
COMMENT 'Item name',
`description` TEXT CHARACTER SET utf8mb4 NOT NULL
COMMENT 'Item description',
`user_id` INT(11) UNSIGNED NOT NULL DEFAULT '0'
COMMENT 'User id',
`category_id` INT(11) UNSIGNED NOT NULL DEFAULT '0'
COMMENT 'Category id',
`price` DECIMAL(10, 2) NOT NULL DEFAULT '0.00'
COMMENT 'Item price',
`status` INT(1) UNSIGNED NOT NULL DEFAULT '1'
COMMENT 'Item status',
`deleted` INT(1) UNSIGNED NOT NULL DEFAULT '0'
COMMENT 'Delite status',
`blocked` INT(1) UNSIGNED NOT NULL DEFAULT '0'
COMMENT 'Block status',
PRIMARY KEY (`item_id`)
)
ENGINE = InnoDB
DEFAULT CHARSET = utf8mb4
COLLATE = utf8mb4_unicode_ci;
CREATE TABLE IF NOT EXISTS `items_images` (
`id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`item_id` INT(10) UNSIGNED NOT NULL DEFAULT '0',
`file` VARCHAR(255)
CHARACTER SET utf8mb4 NOT NULL DEFAULT '',
`weight` INT(11) UNSIGNED NOT NULL DEFAULT '0',
`status` TINYINT(1) UNSIGNED NOT NULL DEFAULT '0',
`deleted` TINYINT(1) UNSIGNED NOT NULL DEFAULT '0',
`created` INT(11) UNSIGNED NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
)
ENGINE = InnoDB
DEFAULT CHARSET = utf8mb4
COLLATE = utf8mb4_unicode_ci
ROW_FORMAT = COMPACT;
As you can see from there, category structure also contain parent_id inside, so we can always get the category tree.
Using this query is working as well and really fast.
SELECT
`i`.*,
`c1`.`name` AS `category_name`,
`c1`.`slug` AS `category_slug`,
`c2`.`name` AS `subcategory_name`,
`c2`.`slug` AS `subcategory_slug`
FROM `items` AS `i`
LEFT JOIN `categories` AS `c2` ON `c2`.`category_id` = `i`.`category_id`
LEFT JOIN `categories` AS `c1` ON `c1`.`category_id` = `c2`.`parent_id`
WHERE `i`.`deleted` = 0 AND `i`.`blocked` = 0 AND `i`.`status` = 1
ORDER BY `i`.`created` DESC
LIMIT 40
But, if i join items_images like:
SELECT
`i`.*,
`ii`.`file` AS `image`,
`c1`.`name` AS `category_name`,
`c1`.`slug` AS `category_slug`,
`c2`.`name` AS `subcategory_name`,
`c2`.`slug` AS `subcategory_slug`
FROM `items` AS `i`
LEFT JOIN `items_images` AS `ii`
ON `i`.`item_id` = `ii`.`item_id` AND `ii`.`weight` = 1
AND `ii`.`status` = 1 AND `ii`.`deleted` = 0
LEFT JOIN `categories` AS `c2` ON `c2`.`category_id` = `i`.`category_id`
LEFT JOIN `categories` AS `c1` ON `c1`.`category_id` = `c2`.`parent_id`
WHERE `i`.`deleted` = 0 AND `i`.`blocked` = 0 AND `i`.`status` = 1
ORDER BY `i`.`created` DESC LIMIT 40
Sometimes it takes up to 1 minute on 14k items with 40k images.
Is there something i can improve?! Please note, there might be items without images too. That is not requirement.
Small addition. Even single images join on items makes the query run over a minute. here a sample:
SELECT `i`.*, `ii`.`file` AS `image` FROM `items` AS `i`
LEFT JOIN `items_images` AS `ii` ON `i`.`item_id` = `ii`.`item_id`
AND `ii`.`weight` = 1 AND `ii`.`status` = 1 AND `ii`.`deleted` = 0
WHERE `i`.`deleted` =0 AND `i`.`sold` =0 AND `i`.`blocked` =0 AND `i`.`status` = 1
ORDER BY `i`.`created` DESC

Answer 1

You have inconsistency in naming convention.
Index parent_id
ALTER TABLE categories ADD INDEX parent_id_ind (parent_id ASC) ;
Try to run your query again.
In databases you would use indexes to improve the speed of data retrieval. An index is typically created on columns used in JOIN, WHERE, and ORDER BY clauses.

Answer 2

I suggest you to improve your structure, you can take it or leave it,
Here my suggestion:
DROP TABLE IF EXISTS `category`;
CREATE TABLE `category` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) CHARACTER SET utf8mb4 DEFAULT NULL COMMENT 'Category title',
`category_id` int(11) NOT NULL DEFAULT '0' COMMENT 'Category Parent ID',
`status` enum('status1','status2') COLLATE utf8mb4_unicode_ci DEFAULT NULL COMMENT 'Category active status',
`weight` int(11) DEFAULT '0',
`slug` varchar(255) CHARACTER SET utf8mb4 DEFAULT NULL COMMENT 'Category url alias',
PRIMARY KEY (`id`),
KEY `category_id_ind` (`category_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
DROP TABLE IF EXISTS `item`;
CREATE TABLE `item` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) CHARACTER SET utf8mb4 NOT NULL DEFAULT '' COMMENT 'Item name',
`description` text CHARACTER SET utf8mb4 NOT NULL COMMENT 'Item description',
`user_id` int(11) unsigned NOT NULL DEFAULT '0' COMMENT 'User id',
`category_id` int(11) unsigned NOT NULL DEFAULT '0' COMMENT 'Category id',
`price` decimal(10,2) NOT NULL DEFAULT '0.00' COMMENT 'Item price',
`status` enum('status1','status2') COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT 'status1' COMMENT 'Item status',
`is_deleted` tinyint(1) unsigned NOT NULL DEFAULT '0' COMMENT 'Delite status',
`is_blocked` tinyint(1) unsigned NOT NULL DEFAULT '0' COMMENT 'Block status',
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
DROP TABLE IF EXISTS `item_images`;
CREATE TABLE `item_images` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`item_id` int(10) unsigned NOT NULL DEFAULT '0',
`file_path` varchar(255) CHARACTER SET utf8mb4 DEFAULT NULL ,
`weight` int(11) unsigned NOT NULL DEFAULT '0',
`status` tinyint(1) unsigned NOT NULL DEFAULT '0',
`is_deleted` tinyint(1) unsigned NOT NULL DEFAULT '0',
`creation_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci ROW_FORMAT=COMPACT;

Answer 3

Add composite indexes:
items: INDEX(deleted, blocked, status, created) -- created must be last
items_images: INDEX(item_id, weight, status, deleted)
categories: INDEX(category_id, deleted, blocked, status)
Except as noted, the order of columns is not important.
Remove LEFT unless you see a need for it.
Please provide EXPLAIN SELECT ....
The likely reason for the sluggishness is having to check lots of image rows for weight=1. On top of that, the lack of any useful index means a costly table scan.