尝试使用JOIN而不是子查询来获取max_date，并仔细检查索引：
SELECT spa_1.id as sid, spa_1.title, spa_1.doc_type, spa_1.inventory_id, inventory.unit_no as unit_no,spa_1.spa_status as spa_status,
spa_meta.*
FROM spa_meta
INNER JOIN (
SELECT s.spa_id, MAX(s.title) AS title, MAX(s.doc_type) AS doc_type, MAX(s.inventory_id) AS inventory_id,
MAX(s.spa_status) AS spa_status,
MAX(sm.created_at) as max_date
FROM spa s
INNER JOIN spa_meta sm
ON s.spa_id=sm.spa_id
WHERE s.spa_status LIKE 'completed%'
GROUP BY s.spa_id) AS spa_1
ON spa_1.max_date = spa_meta.created_at
AND spa_1.id = spa_meta.spa_id
LEFT JOIN inventory
ON spa_1.inventory_id = inventory.id;
感谢@PeterHe，感谢您的宝贵时间，我在结束时提出了另一个查询，它运行正常。