MySQL slow query with two inner joins

Question

The following query takes 4 seconds, and I don't know why, because the index results are low. It is not possible to change the way it works (since this is a compatibility request to get the application name from two different systems)

Do you have any ideas?

SELECT runList.appName
FROM projectList
INNER JOIN varMeta ON (projectList.id = varMeta.projectId)
INNER JOIN runList ON (projectList.projectName = runList.appName)
WHERE varMeta.htmlvar_content = "example-app-name"
ORDER BY runList.id DESC
LIMIT 1;

+----+-------------+-------------+-------+--------------------+-------------+---------+---------------------------+------+-------------+
| id | select_type | table       | type  | possible_keys      | key         | key_len | ref                       | rows | Extra       |
+----+-------------+-------------+-------+--------------------+-------------+---------+---------------------------+------+-------------+
|  1 | SIMPLE      | runList     | index | env,appName,all    | PRIMARY     | 8       | NULL                      |    3 | Using where |
|  1 | SIMPLE      | projectList | ref   | PRIMARY,appNameIDX | appNameIDX  | 138     | compat.runList.appName    |    1 | Using where |
|  1 | SIMPLE      | varMeta     | ref   | varMetaIDX         | varMetaIDX  | 5       | compat.projectList.id     |   63 | Using where |
+----+-------------+-------------+-------+--------------------+-------------+---------+---------------------------+------+-------------+

This is the profile for this request:

+--------------------+----------+
| Status             | Duration |
+--------------------+----------+
| starting           | 0.000215 |
| Opening tables     | 0.000043 |
| System lock        | 0.000034 |
| Table lock         | 0.000015 |
| init               | 0.000081 |
| optimizing         | 0.000029 |
| statistics         | 0.000345 |
| preparing          | 0.000033 |
| executing          | 0.000009 |
| Sorting result     | 0.000009 |
| Sending data       | 3.023702 |
| end                | 0.000018 |
| query end          | 0.000004 |
| freeing items      | 0.000223 |
| logging slow query | 0.000004 |
| cleaning up        | 0.000005 |
+--------------------+----------+

Answer 1

I'm going to guess here because you did not tell us which fields or indexes you have, and you did not specify the number of rows in each table. (So ​​it could be trash).

varMeta.html_varcontent, varMeta.projectID. (html_varcontent, projectID) varMeta. ? , MySQL . MySQL , , . .

, ORDER BY id DESC LIMIT 1 runList. , (appName, id) runList. appName, id. .

, projectList : (id, appName) (appName, id). , MySQL , .