问题
Let me open by saying: yes, I am aware of Determine if a transaction is active (Postgres)
Unfortunately the sole answer to that question is far too specific to the use case provided, and doesn't actually indicate whether or not a transaction is active.
The select txid_current(); trick suggested by How to check for pending operations in a PostgreSQL transaction doesn't appear to work - I always get the same transaction ID from adjacent calls to that function. Possibly this is because I'm trying to test it from pgAdmin, which is transparently starting transactions...? (Note: I don't actually care whether there are any pending changes or active locks, so looking at pg_locks isn't helpful - what if nothing's been touched since the transaction was started?)
So: How can I determine in PostgreSQL PL/pgSQL code if a transaction is currently active?
One possible use case is: the SP/FN in question will be doing its own explicit transaction management, and calling it with a transaction already active will greatly interfere with that. I want to raise an error so that the coding mistake of calling this SP/FN in a transaction can be corrected.
There are other use cases, though.
Ideally what I'm looking for is an equivalent to MSSQL's @@TRANCOUNT (though I don't really care how deeply the transactions may be nested...)
回答1:
Postgres runs PL/pgSQL inside the transaction. Thus you can't control transaction from inside PL/pgSQL. Code will look like:
begin;
select plpgsql_fn();
do '/*same any plpgsql*/';
end;
So answering your question:
If you have PL/pgSQL running ATM, you have your transaction active ATM...
Of course you can do some trick, like starting/ending work over dblink or such. but then you can check select txid_current(); over the dblink successfully...
回答2:
If you want to determine if there have been any data modifications in your transaction, call txid_current_if_assigned(). It returns NULL if nothing has been modified yet.
If you only want to know if you are inside some transaction, you can save yourself the trouble, because you always are.
Before PostgreSQL v11, you cannot use transaction control statements in a function.
来源：https://stackoverflow.com/questions/51567264/postgresql-how-to-determine-whether-a-transaction-is-active