I was going through this link and it is mentioned that PostgreSQL does not support undo log. So I am wondering how PostgreSQL rollback a transaction without undo log.
When a row is updated or deleted in PostgreSQL, it is not really updated or deleted. The old row version just remains in the table and is marked as removed by a certain transaction. That makes an update much like a delete of the old and insert of the new row version.
To roll back a transaction is nothing but to mark the transaction as aborted. Then the old row version automatically becomes the current row version again, without a need for undoing anything.
The Achilles' heel of this technique is that data modifications produce “dead row versions”, which have to be reclaimed later by a background procedure called “vacuuming”.
Related
I have been digging into Postgres' MVCC. I've watched videos regarding the behaviors of inserts, updates, and deletes. I keep seeing that a delete actually updates the xmax of the previous version and also possibly a commit flag and then inserts another record. I have yet to see any documentation or videos on what is actually inserted with a delete statement.
This seems very similar to Cassandra and its tombstone behavior, but have not been able to confirm. So my question is, what is actually inserted for a row when a delete occurs?
A DELETE does not insert a new version of the affected row, it only fills in the xmax field of the row thereby marking it as non-existent for all later transactions. (see this article for more details)
So the answer to your question
What is inserted when a DELETE executes?
is: Nothing is inserted. If you have any articles stating otherwise they are either wrong or unprecise.
I am using jdbi with postgres. And as i am running application in multiple instances locking is not an option on application code level. So i am using row level locking on database. So i am using select for update. But ideally i don't need any update to the row but i am going a head due to for update. So everytime to release a lock i fire an update query to update last updated time field,
Can i just lock it and once the transaction is done release the lock without any update. DO we have such an option?
When you commit or rollback your transaction the lock is automatically released, regardless whether you actually updated the row or not.
There is absolutely no need to UPDATE the row in order for the lock to be released.
I have following Quires:
How Do I check redo / un-committed data size in PostgreSQL ?
Looks like if I do multiple update in sequence, it slows down.
Like Update 1, update 2, .... update n; ...seem update n is slower than update 1. Does uncommitted data volume affects it ? How redo management works in PostgreSQL ?
How do I monitor current running SQL in stored function? pg_stat_activity just shows function call; at session level. How do I get current SQL under that function which is running now ?
~ Santosh
You're clearly coming from an Oracle background.
PostgreSQL does not have undo and redo logs, as such.
Uncommitted (in-progress or rolled back), live committed data and comimtted-then-deleted data are mixed together in the heap, i.e. the main table contents. The fraction used by rolled back transactions, old versions of updated rows and deleted rows is referred to as table bloat. See the wiki.
The closest thing to do the redo log is the write-ahead logs in pg_xlog. There's no SQL-level interface to getting the current xlog size.
The documentation discusses this in some more detail, but it's an area of PostgreSQL management that could really use more attention from interested contributors. Both better built-in monitoring tools and better documentation would be good. Patches are welcome.
As for your second question... you don't. There isn't currently a way to get a function call stack. One is being discussed, but hasn't been implemented as of 9.5.
I accidentally updated 71 rows of 1 column in table. It's production and I want to learn how I can revert back those changes. It's an PostgreSQL environment.
Just after updating I realized where condition is missing. I rollback it but:
db=# ROLLBACK;
WARNING: there is no transaction in progress
ROLLBACK
Not sure how it's not getting rolled back. I had no backup files for it. Else I would have copied files locally and stored in database. So, can someone suggest how else I can proceed with reverting those changes?
Is there any way PostgreSQL store logs and that can be used to restore my data?
If you have committed the transaction,you cannot rollback or undo the update.Please find the detailed answer here
Can I rollback a transaction I've already committed? (data loss)
I'm using PostgreSQL 9.2 in a Windows environment.
I'm in a 2PC (2 phase commit) environment using MSDTC.
I have a client application, that starts a transaction at the SERIALIZABLE isolation level, inserts a new row of data in a table for a specific foreign key value (there is an index on the column), and vote for completion of the transaction (The transaction is PREPARED). The transaction will be COMMITED by the Transaction Coordinator.
Immediatly after that, outside of a transaction, the same client requests all the rows for this same specific foreign key value.
Because there may be a delay before the previous transaction is really commited, the SELECT clause may return a previous snapshot of the data. In fact, it does happen sometimes, and this is problematic. Of course the application may be redesigned but until then, I'm looking for a lock solution. Advisory Lock ?
I already solved the problem while performing UPDATE on specific rows, then using SELECT...FOR SHARE, and it works well. The SELECT waits until the transaction commits and return old and new rows.
Now I'm trying to solve it for INSERT.
SELECT...FOR SHARE does not block and return immediatley.
There is no concurrency issue here as only one client deals with a specific set of rows. I already know about MVCC.
Any help appreciated.
To wait for a not-yet-committed INSERT you'd need to take a predicate lock. There's limited predicate locking in PostgreSQL for the serializable support, but it's not exposed directly to the user.
Simple SERIALIZABLE isolation won't help you here, because SERIALIZABLE only requires that there be an order in which the transactions could've occurred to produce a consistent result. In your case this ordering is SELECT followed by INSERT.
The only option I can think of is to take an ACCESS EXCLUSIVE lock on the table before INSERTing. This will only get released at COMMIT PREPARED or ROLLBACK PREPARED time, and in the mean time any other queries will wait for the lock. You can enforce this via a BEFORE trigger to avoid the need to change the app. You'll probably get the odd deadlock and rollback if you do it that way, though, because INSERT will take a lower lock then you'll attempt lock promotion in the trigger. If possible it's better to run the LOCK TABLE ... IN ACCESS EXCLUSIVE MODE command before the INSERT.
As you've alluded to, this is mostly an application mis-design problem. Expecting to see not-yet-committed rows doesn't really make any sense.