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Roshe Run Black Oreo

First, thank you very much for the clarifications and corrections. I wasn't on the engineering team that fixed the problem, and I'm not a Python guy; I just found the problem and explained the consequences of what was happening to the engineers. It's not open connections that interfere with vacuuming; it's open transactions. While a transaction is open, vacuuming can't reclaim dead tuples, if those tuples were live at the start of the transaction. A 2000 row table with 1600 byte rows took up a little over 400 (8KB) disk pages, and the autovacuum daemon was able to keep it at that size. Compare that to before, without autocommit: it would be that size after a VACUUM FULL, and weigh gibibytes within days, no matter how many autovacuum workers there were, how aggressively they were tuned, or how often you ran (regular) VACUUM manually. (To be fair, though, they were also using the database as a work queue, so it's probably reasonable to suggest they liked doing things sub optimally.) The high volume environments you allude to are probably going about things with more cognizance of the implications of implicitly transactional semantics. It's not. Transactional semantics are awesome, and as someone who gets paid for keeping peoples databases (particularly PostgreSQL) happy, I'm emphatically for them. in a bulk import, parsing, caching lookups and reference data outside the transaction, building an XML document to supply into a SP as a parameter which only used a transaction for a single INSERT from the XML).

Idea.[1] That said, my preferred backup strategy is to take a filesystem level snapshot of the db volumes, mount that, and start a second Postgres instance against it. ), and this is the approach I follow in my own work now.

Roshe Run Black Oreo

Roshe Run Black Oreo

Can you give any good reason why you need to leave transactions open for extended periods?Offhand, other than xid level consistency for a backup as mentioned by the sibling post, no. At a minimum, I think it places an unnecessary burden on engineers to have to "roll back" every time they even ask the db something. That's how memory leaks happen, too, and that's why we generally think garbage collected languages are a Good Nike Roshe Run Floral Blue

To put it simply, VACUUM has no problems with connections being open. What he is thinking about is indeed as he said transactions. Locks in PostgreSQL do also interfere with VACUUM but that is seldom the problem in practice since locks Roshe Run Kids Galaxy

Roshe Run Black Oreo

Roshe Run Black Oreo

jeltz 769 days ago link

Roshe Run Black Oreo

The only reason I can see is for taking consistent database backups. But then your transaction should also use a readonly snapshot (pg_dump will do this for you).

rosser 769 days ago link

Roshe Run Black Oreo

j_s 768 days ago link

Roshe Run Black Oreo

rosser 769 days ago link

are usually taken at the row level and reading rows does not require any row locks. Open transactions are the main culprit when it comes to vacuum problems and it does not matter much if they have taken locks or not.

thanks for the clarification. in practice, sure, we're all looking in our pg_stat_activity for "open in transaction" in general as something that shouldn't be hanging around. .

Roshe Run Black Oreo

all the way up to the 0.8 tip you won't see it. With Postgresql, you're usually using a DBAPI implementation known as psycopg2. The DBAPI is organized in such a way that transactions are implicit. This means, when you first get a DBAPI connection, it's per specification required to be in a transaction, or at least it has to be as soon as you do something with that connection. The DBAPI has a commit() method as well as a rollback(), but has no begin() method. It's also easy enough to set this flag when you're using psycopg2 via SQLAlchemy, and in fact things will work just fine unless you actually need some degree of transaction isolation and/or need ROLLBACK to actually work. SQLAlchemy has nothing to do with "BEGIN TRANSACTION" and 2. psycopg2 and all DBAPIs are required to maintain transactional scope by default when a connection is first procured. What about the supposed issues with VACUUM ?To put it simply, VACUUM has no problems with connections being open. What you're thinking of here are locks, and locks only occur once you're in a transaction and have accessed some table rows, which are now subject to various isolation rules. If you open a bunch of connections, and access/update a bunch of table rows, you'll have a lot of locks on hand, and that will get in the way of autovacuuming and such. However, as soon as you roll back the transactions, those locks are gone. When you use a database library like SQLAlchemy, a handful of connections are kept open in a pool, but the transactions are not. When you check out a connection, do a few things with it, then return it to the pool, any remaining transactional state is rolled back. Postgresql's auto vacuuming works just fine regardless. SQLAlchemy is just a client of psycopg2.

I mentioned Roshe Run Black Oreo something about this the last time a discussion involving Django and

jeltz 769 days ago link

Roshe Run Black Oreo

Physically, an UPDATE statement is an atomic INSERT/DELETE operation; each version of every row is stored on disk, and Postgres keeps track of which versions of which rows are "visible" in the context of which transactions. Obviously, that's not a sustainable approach, both in terms of performance, and resource consumption. They know which transactions are open, and which versions of rows were modified by which transactions. They were among the hottest tables in the system and performance, consequently, sucked. The only thing that kept them alive was that the people they had working on this stuff before me had a weekly maintenance window (site outage) where they ran a VACUUM FULL and REINDEX. What you're really looking for is Postgres backends with an xact_start current transaction start time that is (potentially significantly) out of line with how long your normal database operations should take. Feel free to grep for it, start at version 0.1.0 and go Nike Roshes Men Black And White

Can you point to any open source projects or other examples implementing this pattern? I think I understand where you're coming from on this but I have not seen many examples of this approach in the wild (except religiously using TransactionScope, eg. Most high level language modules built on top of it also work that way. Unfortunately python's DB API is not one of them, but you can just set a config option to "act like everyone expects".

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