Index probably not used correctly on simple SQL query - mysql

Question

Size:
Campaigns: 3k rows (200 with campaigns.is_active = 1)
Links: 20k rows (4k with links.status = 1 // 500 with links.status = 1 AND campaigns.is_active = 1)
Clicks: 10mln rows (50k with created > '2020-10-25 00:00:00')
This query runs 2 seconds
SELECT links.id, COUNT(clicks.id)
FROM links
INNER JOIN campaigns ON campaigns.id = links.campaign_id
AND campaigns.is_active = 1
LEFT JOIN clicks ON clicks.link_id = links.id
WHERE links.status = 1
AND clicks.created > '2020-10-25 00:00:00'
GROUP BY links.id
When I remove the following line, it runs just 0.13 seconds (15 times faster)
AND campaigns.is_active = 1
There is an INDEX on campaigns.is_active.
Also tried to set an index on 2 columns (campaigns.id + campaigns.is_active) but didn't help.
"campaigns.is_active" contains simply 0 or 1. The campaigns table is small, the campaigns.is_active condition actually reduces the amount of rows. So it should speed up the query instead.
Why does it take so much longer because of this condition and how to fix it?
If I would remove the JOIN to campaigns and instead add links.campaign_id to the SELECT fields and then query every single of the returned campaign_id's in an additional query like "SELECT is_active FROM campaigns WHERE id = ?" it would still be faster, because such a query is 0.000x. From my experience when something is faster in 2 queries, it usually means the first query isn't optimized to its full extent.
Explain-Select
Structure
CREATE TABLE `campaigns` (
`id` int(11) UNSIGNED NOT NULL,
`is_active` tinyint(4) NOT NULL DEFAULT 0
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `clicks` (
`id` int(11) UNSIGNED NOT NULL,
`link_id` int(11) UNSIGNED NOT NULL,
`created` datetime NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `links` (
`id` int(11) UNSIGNED NOT NULL,
`campaign_id` int(8) UNSIGNED NOT NULL,
`status` tinyint(4) NOT NULL DEFAULT 0
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
ALTER TABLE `campaigns`
ADD PRIMARY KEY (`id`),
ADD UNIQUE KEY `id_isactive` (`id`,`is_active`),
ADD KEY `is_active` (`is_active`)
ALTER TABLE `clicks`
ADD PRIMARY KEY (`id`),
ADD KEY `link_id` (`link_id`),
ADD KEY `created` (`created`)
ALTER TABLE `links`
ADD PRIMARY KEY (`id`),
ADD KEY `campaign_id` (`campaign_id`),

Answer 1

How long does this take?
SELECT l.id,
(SELECT COUNT(*)
FROM clicks cl
WHERE cl.link_id = l.id AND
cl.created > '2020-10-25'
)
FROM links l JOIN
campaigns ca
ca.id = l.campaign_id
WHERE l.status = 1 AND ca.is_active = 1;
EDIT:
Hmmm, with an order by, you can try:
SELECT l.id,
(SELECT COUNT(*)
FROM clicks cl
WHERE cl.link_id = l.id AND
cl.created > '2020-10-25'
)
FROM links l
WHERE EXISTS (SELECT 1
FROM campaigns ca
WHERE ca.id = l.campaign_id AND ca.is_active = 1
)
WHERE l.status = 1
ORDER BY l.id;
For this, you want an index on links(status, id) and campaigns(campaign_id, is_active).

Answer 2

Question... If a campaign is not currently active, you don't want any output for it, correct? Furthermore, there won't be any clicks for inactive campaigns, correct? Then why bother checking is_active?
Even if my analysis is wrong, it may be faster to ignore is_active until after the counts have been tallied.
Please don't use LEFT when it is not functional. You have a simple JOIN.
Use COUNT(*); COUNT(x) tests x for being not null.
SELECT links.id, COUNT(*)
FROM links
JOIN clicks ON clicks.link_id = links.id
WHERE links.status = 1
AND clicks.created > '2020-10-25 00:00:00'
GROUP BY links.id
This is redundant:
ADD UNIQUE KEY `id_isactive` (`id`,`is_active`),
since PRIMARY KEY(id) declares id to be an index and unique.

Answer 3

I prefer not to fight the database engine optimizer.
SELECT links.id, campaigns.is_active, COUNT(clicks.id)
FROM links
INNER JOIN campaigns ON campaigns.id = links.campaign_id
LEFT JOIN clicks ON clicks.link_id = links.id
WHERE links.status = 1
AND clicks.created > '2020-10-25 00:00:00'
GROUP BY links.id, campaigns.is_active
HAVING campaigns.is_active = 1;
Second variant!
-- Second Variant
EXPLAIN
SELECT links.id AS LinksId
, COUNT(clicks.id) AS ClickCount
FROM links
LEFT JOIN clicks
ON links.id = clicks.link_id
WHERE links.status = 1
AND clicks.created > '2020-10-25 00:00:00'
AND links.campaign_id IN (SELECT campaign_id
FROM campaigns
WHERE is_active = 1)
GROUP BY links.id;
Third time is the charm! Using CTEs due to the published cardinalities.
-- Third time is the charm
WITH ActiveCampaigns
AS
(SELECT *
FROM campaigns
WHERE is_active = 1)
SELECT links.id, COUNT(clicks.id)
FROM links
INNER JOIN ActiveCampaigns
ON ActiveCampaigns.id = links.campaign_id
LEFT JOIN clicks
ON clicks.link_id = links.id
WHERE links.status = 1
AND clicks.created > '2020-10-25 00:00:00'
GROUP BY links.id;