在多个表中查找最新的时间戳

Question

javascript 运行循环任务来监控数据库，任务（在 php 中）只是查询数据库的最新时间戳：

SELECT
    MAX(po.created)::timestamp last_purchase_order_created,
    MAX(po.modified)::timestamp last_purchase_order_modified,
    MAX(jobs.created)::timestamp last_job_created,
    MAX(jobs.modified)::timestamp last_job_modified,
    MAX(companies.created)::timestamp last_company_created,
    MAX(companies.modified)::timestamp last_company_modified,
    MAX(stock.created)::timestamp last_stock_created,
    MAX(stock.modified)::timestamp last_stock_modified
FROM purchase_orders po, jobs, companies, stock;

开始花了很长时间，因为 PHP 不是多线程的，它也阻塞了其他请求。有没有办法克服这个问题？修改后的查询，php中的不同方法，两者兼而有之？

这是问题的延续：Slow connection to Postgresql using PDO

Answer 1

当您这样做时，您会在所有这些表中创建一个笛卡尔积，这对您的数据库服务器要求很高。

如果您想在一个查询中执行此操作，请遵循此模式，这将为每个表提供一行：

select 'purchase_orders' as table_name, max(created) as last_created, max(modified) as last_modified 
  from purchase_orders
union all
select 'jobs', max(created), max(modified) 
  from jobs
union all
select 'companies', max(created), max(modified)
  from companies
union all
select 'stock', max(created), max(modified)
  from stock

我通常会以宿主语言（在您的情况下为 PHP）中进行转换，但如果您必须在查询中执行此操作，那么这样的事情应该可以工作：

with latest_dates as (
  select 'purchase_orders' as table_name, max(created) as last_created, max(modified) as last_modified 
    from purchase_orders
  union all
  select 'jobs', max(created), max(modified) 
    from jobs
  union all
  select 'companies', max(created), max(modified)
    from companies
  union all
  select 'stock', max(created), max(modified)
    from stock
)
select max(last_created) filter (where table_name = 'purchase_order') as last_purchase_order_created,
       max(last_modified) filter (where table_name = 'purchase_order') as last_purchase_order_modified,
       max(last_created) filter (where table_name = 'job') as last_job_created,
       max(last_modified) filter (where table_name = 'job') as last_job_modified,
       max(last_created) filter (where table_name = 'company') as last_company_created,
       max(last_modified) filter (where table_name = 'company') as last_company_modified,
       max(last_created) filter (where table_name = 'stock') as last_stock_created,
       max(last_modified) filter (where table_name = 'stock') as last_stock_modified
  from latest_dates;

Answer 2

像 Mike 推荐的 UNION 查询会更好，但如果您想要快速查询，则需要在 每个 列上创建一个索引，该列有一个 max。 p>

这似乎是一个经常运行的奇怪查询。因此可以替代使用序列更好的方法吗？

如果您确实经常需要这些最大值，您可以维护一个仅包含这些最大值的特殊表，并在您插入或更新一行时进行更新。这将为您节省许多昂贵的索引。