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...)
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...
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.
I haven't found a clean way to do that, but you can always call BEGIN and if it succeeds it means there is no transaction in progress (don't forget to rollback). If it fails with "there is already a transaction in progress" this means you are within transaction (better not to rollback then).
Related
As far as I know, we can't use start transaction within functions, thus we can't use COMMIT and ROLLBACK in functions.
But how then we ROLLBACK by some if-condition?
How then we can perform a sequence of statements in a specific level of isolation? I mean a situation when an application wants to call a SQL (plpgsql) function and that function really needs to be run in a transaction with a certain isolation level. What to do in such a case?
In which cases then it is really practical to run ROLLBACK? Only when we manually write a script, check something and then ROLLBACK manually if we don't like the result. And in the same case, I see the practicality of savepoints. However, I feel like it is a serious constraint.
If you want to rollback the complete transaction, RAISE an exception.
If you only want to roll back part of your work, start a new block with a BEGIN at the point to which you want to roll back and add an EXCEPTION clause to the block.
Since the transaction is started outside the function, the isolation level already has to be set properly when you are in the function.
You can query
SELECT current_setting('transaction_isolation', TRUE);
and throw an error if the setting is not correct.
is too general or too simple to answer.
You roll back a transaction if you have reached a point in your processing where you want to undo everything you have done so far in the transaction.
Often, that happens implicitly rather than explicitly by throwing an error.
In PostgreSQL, are DEFERRED triggers executed before (within) the completion of the transaction or just after it?
The documentation says:
DEFERRABLE
NOT DEFERRABLE
This controls whether the constraint can be deferred. A constraint
that is not deferrable will be checked immediately after every
command. Checking of constraints that are deferrable can be postponed
until the end of the transaction (using the SET CONSTRAINTS command).
It doesn't specify if it is still inside the transaction or out. My personal experience says that it is inside the transaction and I need it to be outside!
Are DEFERRED (or INITIALLY DEFERRED) triggers executed inside of the transaction? And if they are, how can I postpone their execution to the time when the transaction is completed?
To give you a hint what I'm after, I'm using pg_notify and RabbitMQ (PostgreSQL LISTEN Exchange) to send out messages. I process such messages in an external application. Right now I have a trigger which notifies the external app of the newly inserted records by including the record's id in the message. But in a non-deterministic way, once in a while, when I try to select a record by its id at hand, the record can not be found. That's because the transaction is not complete yet and the record is not actually added to the table. If I can only postpone the execution of the trigger for after the completion of the transaction, everything will work out.
In order to get better answers let me explain the situation even closer to the real world. The actual scenario is a little more complicated than what I explained before. The source code can be found here if anyone's interested. Becuase of reasons that I'm not gonna dig into, I have to send the notification from another database so the notification is actually sent like:
PERFORM * FROM dblink('hq','SELECT pg_notify(''' || channel || ''', ''' || payload || ''')');
Which I'm sure makes the whole situation much more complicated.
Triggers (including all sorts of deferred triggers) fire inside the transaction.
But that is not the problem here, because notifications are delivered between transactions anyway.
The manual on NOTIFY:
NOTIFY interacts with SQL transactions in some important ways.
Firstly, if a NOTIFY is executed inside a transaction, the notify
events are not delivered until and unless the transaction is
committed. This is appropriate, since if the transaction is aborted,
all the commands within it have had no effect, including NOTIFY. But
it can be disconcerting if one is expecting the notification events to
be delivered immediately. Secondly, if a listening session receives a
notification signal while it is within a transaction, the notification
event will not be delivered to its connected client until just after
the transaction is completed (either committed or aborted). Again, the
reasoning is that if a notification were delivered within a
transaction that was later aborted, one would want the notification to
be undone somehow — but the server cannot "take back" a notification
once it has sent it to the client. So notification events are only
delivered between transactions. The upshot of this is that
applications using NOTIFY for real-time signaling should try to keep
their transactions short.
Bold emphasis mine.
pg_notify() is just a convenient wrapper function for the SQL NOTIFY command.
If some rows cannot be found after a notification has been received, there must be a different cause! Go find it. Likely candidates:
Concurrent transactions interfering
Triggers doing something more or different than you think they do.
All sorts of programming errors.
Either way, like the manual suggests, keep transactions that send notifications short.
dblink
Update: Transaction control in a PROCEDURE or DO statement in Postgres 11 or later makes this a lot simpler. Just COMMIT; to (also) send waiting notifications.
Original answer (mostly for Postgres 10 or older):
PERFORM * FROM dblink('hq','SELECT pg_notify(''' || channel || ''', ''' || payload || ''')');
... which should be rewritten with format() to simplify and make the syntax secure:
PRERFORM dblink('hq', format('NOTIFY %I, %L', channel, payload));
dblink is a game-changer here, because it opens a separate transaction in the other database. This is sometimes used to fake autonomous transaction.
Does Postgres support nested or autonomous transactions?
How do I do large non-blocking updates in PostgreSQL?
dblink() waits for the remote command to finish. So the remote transaction will most probably commit first. The manual:
The function returns the row(s) produced by the query.
If you can send notification from the same transaction instead, that would be a clean solution.
Workaround for dblink
If notifications have to be sent from a different transaction, there is a workaround with dblink_send_query():
dblink_send_query sends a query to be executed asynchronously, that is, without immediately waiting for the result.
DO -- or plpgsql function
$$
BEGIN
-- do stuff
PERFORM dblink_connect ('hq', 'your_connstr_or_foreign_server_here');
PERFORM dblink_send_query('con1', format('SELECT pg_sleep(3); NOTIFY %I, %L ', 'Channel', 'payload'));
PERFORM dblink_disconnect('con1');
END
$$;
If you do this right before the end of the transaction, your local transaction gets 3 seconds (pg_sleep(3)) head start to commit. Chose an appropriate number of seconds.
There is an inherent uncertainty to this approach, since you get no error message if anything goes wrong. For a secure solution you need a different design. After successfully sending the command, chances for it to still fail are extremely slim, though. The chance that successful notifications are missed seem much higher, but that's built into your current solution already.
Safe alternative
A safer alternative would be to write to a queue table and poll it like discussed in #Bohemian's answer. This related answer demonstrates how to poll safely:
Postgres UPDATE … LIMIT 1
I'm posting this as an answer, assuming the actual problem you are trying to solve is deferring execution of an external process until after the transaction is completed (rather than the X-Y "problem" you're trying to solve using trigger Kung Fu).
Having the database tell an app to do something is a broken pattern. It's broken because:
There's no fallback if the app doesn't get the message, eg because it's down, network explodes, whatever. Even the app replying with an acknowledgment (which it can't), wouldn't fix this problem (see next point)
There's no sensible way to retry the work if the app gets the message but fails to complete it (for any of lots of reasons)
In contrast, using the database as a persistant queue, and having the app poll it for work, and take the work off the queue when work is complete, has none of the above problems.
There are lots of ways to achieve this. The one I prefer is to have some process (usually trigger on insert, update and delete) put data into a "queue" table. Have another process poll that table for work to do, and delete from the table when work is complete.
It also adds some other benefits:
The production and consumption of work is decoupled, which means you can safely kill and restart your app (which must happen from time to time, eg deploying) - the queue table will happily grow while the app is down, and will drain when the app is back up. You can even replace the app with an entirely new one
If for whatever reason you want to initiate processing of certain items, you can just manually insert rows into the queue table. I used this technique myself to initiate the processing of all items in a database that needed initialising by being put on the queue once. Importantly, I didn't need to do a perfunctory update to every row just to fire the trigger
Getting to your question, a slight delay can be introduced by adding a timestamp column to the queue table and having the poll query only select rows that are older than (say) 1 second, which gives the database time to complete its transaction
You can't overload the app. The app will read only as much work as it can handle. If your queue is growing, you need a faster app, or more apps If multiple consumers are operating, concurrency can be solved by (for example) adding a "token" column to the queue table
Queues that are backed by database tables is the basis of how persistent queues are implemented in commercial grade queue-based platforms, so the pattern is well tested, used and understood.
Leave the database to do what it does best, and the only thing it does well: Manage data. Don't try to make your database server into an app server.
According to postgresql docs;
CREATE DATABASE cannot be executed inside a transaction block.
Is there a technical reason for this?
When you try it, you get the error:
ERROR: CREATE DATABASE cannot run inside a transaction block
This comes from src/backend/access/transam/xact.c (line 3023 on my sources, but varies by version), in PreventTransactionChain(...).
The comment there explains that:
This routine is to be called by statements that must not run inside
a transaction block, typically because they have non-rollback-able
side effects or do internal commits.
For CREATE DATABASE it's called from src/backend/tcop/utility.c in standard_ProcessUtility under the case for T_CreatedbStmt, but unfortunately there isn't any informative comment that says why specifically CREATE DATABASE isn't safe to run in a transaction.
Looking at the sources, I can see that for one thing it forces a checkpoint.
Overall, though, I don't see anything that really screams out "we can't do this transactionally". It's more "we haven't implemented the functionality to do this transactionally".
It's conceptual reason: files creation has no relation to DB transaction and there is no guaranty that during the rollback they will be deleted.
I have a problem with existing database code (a trigger) that call a function trigger that use the NOTIFY command, which is not supported in the context of a prepared transaction.
My question is simple : from the function trigger, is there a way to detect that we are in the context of a prepared transaction ?
Thanks in advance.
There is no way to detect that the current transaction will be committed using prepared transactions and two-phase commit, because you haven't PREPAREd the transaction yet; the transaction has no idea it's going to be subjected to two-phase commit until after your trigger runs. PostgreSQL doesn't require that you BEGIN TRANSACTION FOR TWO PHASE COMMIT (imaginary syntax) or anything like that.
You can test for max_prepared_transactions > 0 in pg_settings to see if prepared transactions are enabled, but there's no way to know if 2PC will be used until it happens.
I have a session (SQLAlchemy) on PostgreSQL, with an active uncommitted transaction. I have just passed the session to some call tree that may or may not have issued SQL INSERT/UPDATE/DELETE statements, through sqlalchemy.orm or directly through the underlying connection.
Is there a way to check whether there are any pending data-modifying statements in this transaction? I.e. whether commit would be a no-op or not, and whether rollback would discard something or not?
I've seen people point out v$transaction in Oracle for the same thing (see this SO question). I'm looking for something similar to use on PostgreSQL.
Start by checking into system view pg_locks.
http://www.postgresql.org/docs/8.4/interactive/view-pg-locks.html
Consider the following sequence of statements:
select txid_current();
begin;
select txid_current();
If the transaction id returned by the two selects is equal, then there is an open transaction. If not then there wasn't, (but now is).
If the numbers are different, then as a side effect you will just have opened a transaction, which you will probably want to close.
UPDATE: In fact, as #r2evans points out (thanks for the insight!), you don't need the "begin" -- txid_current() will return the same number just if you are in a transaction.
Since Postgres 10:
select txid_current_if_assigned();
will return null if there is no current transaction.
If a Start Transaction has been issued, it will still return null if there have been no updates.
No, not from the database level, really. Perhaps you can add some tracing at the sqlalchemy level to track it?
Also, how do you define a no-op? What if you updated a value to the same value it had before, is that a no-op or not? From the databases perspective, if it had one, it would not be a no-op. But from the application perspective, it probably would.