BigQuery：標量子查詢產生多個元素

Question

這個查詢完美地運行了好幾個月，突然之間，開始拋出：標量子查詢產生了多個元素

select event_id, user_id, created_at, company_id, message_id, conversation_id, message_text, 'sent' as message_direction, created_at as message_read_at from message_sent

union all

select distinct e.event_id, e.user_id, e.created_at, e.company_id, 
e.message_id, e.conversation_id, 
e.message_text, 'received' as message_direction, 
min(c.created_at) as message_read_at
from message_received e
left outer join conversation_read c on (e.conversation_id=c.conversation_id  and c.created_at > e.created_at)
group by e.event_id, e.user_id, e.created_at, e.company_id, 
e.message_id, e.conversation_id, 
e.message_text, message_direction

會是什么呢？

Answer 1

深入挖掘后發現。

這個查詢不是問題，而是查詢中使用的視圖。 調試視圖定義后，我發現它的子查詢有問題。