BigQuery：标量子查询产生多个元素

Question

这个查询完美地运行了好几个月，突然之间，开始抛出：标量子查询产生了多个元素

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

会是什么呢？

Answer 1

深入挖掘后发现。

这个查询不是问题，而是查询中使用的视图。 调试视图定义后，我发现它的子查询有问题。