检查sqlalchemy表是否为空 [英] Check whether sqlalchemy table is empty

问题描述

我需要知道是否任何给定的sqlalchemy表（sqlalchemy.schema.Table）都具有0行。我有很多表，每个表中都有很多行，并且这些表必须在启动时为所有表运行，因此我正在从运行时的角度寻找最有效的方法。因此，仅计算一项不是我想要的。如果数据库有任何更改，则该数据库为PostgreSQL

I need to know whether any given sqlalchemy table (sqlalchemy.schema.Table) has exactly 0 rows. I have a lot of tables with a lot of rows in each and this has to run for all of them at startup so I am looking for the most efficient way to do this from a runtime perspective. So, just doing a count is not what I am looking for. The database is PostgreSQL if that changes anything

推荐答案

SQLAlchemy将允许您执行原始查询。在PostgreSQL中，您可以查询元数据中的n_live_tup以获取记录数：

SQLAlchemy will allow you to do a "raw" query. In PostgreSQL, you can query the meta data for n_live_tup to get the record count:

SELECT 
  schemaname as schema_name,
  relname as table_name, 
  n_live_tup as record_count 
FROM pg_stat_user_tables WHERE n_live_tup > 0;

很显然，这为您提供了具有实时记录的所有表格。如果只希望没有，则将其更改为 WHERE n_live_tup = 0 ，如果要将其限制为仅某个表，请添加 table_name =< ;< your_table_name>>> 到where子句。

obviously, this gives you all of the tables with live records. If you only want the ones without, change the to WHERE n_live_tup = 0 and if you want to limit it to only a certain table add table_name = <<your_table_name>> to the where clause.

如果您没有具有权限的角色，而您只想看看表是否为空，那么您可以始终执行以下操作：

If you don't have a role with permissions, and you just want to see if the table is empty, then you can alway do this:

SELECT True FROM <<your_table_name>> LIMIT 1;

如果表中有记录，则返回True，否则返回NULL。

This will return True if the table has records or NULL if it doesn't.

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