The ordering needs to be unambiguous, and stable over time, so a generated primary key is ideal (i.e. More generally, as long as all processes agree to follow the same ordering when acquiring locks, it's guaranteed that at least one of them is always making progress if you only ever try to acquire locks which are "higher" than the ones you already hold, then whoever holds the "highest" lock will never be waiting on anyone.
POSTGRES DEADLOCK FREE
And at this point, the processes will be waiting on each other forever (or rather, until the server notices, and kills one of them off).īut if both processes had agreed ahead of time to lock row 1 and then row 2, then this wouldn't have happened one process would still be waiting on the other, but the other is free to proceed.
Process A requests a lock on row 2 (and waits for B to release it).I am still not clear even after explicit locking and being within transaction why is deadlock occurring.Īs Laurenz said, in this simple case you should be able to eliminate the possibility of deadlock with an ORDER BY in your locking query. Help me figure out what I am missing or another workaround for this.
POSTGRES DEADLOCK UPDATE
This table is used in highly concurrent and distributed application (100's in parallel with same ref_id) and thats why i wanted to avoid distributed lock by having select and then update in same transaction.But i am facing with this deadlock error I don't know why explicit locking is not working.Įxpected behaviour is that any other job with same reference ID must wait if any one else with same reference ID has acquired the lock The database you want to monitor should connect from. Process 28384: select * from jobs where ref_id ='a840a8bd-b8a7-45b2-a474-47e2f68e702d' and is_active is true for update It monitors Postgres DB agent metrics like database deadlocks, database conflict deadlocks. Process 28454: select * from jobs where ref_id ='a840a8bd-b8a7-45b2-a474-47e2f68e702d' and is_active is true for update Process 28297 waits for AccessExclusiveLock on tuple (113628,5) of relation 16817 of database 16384 blocked by process 28454. (Exactly which transaction will be aborted is difficult to predict and should not be relied upon. Process 28384 waits for ShareLock on transaction 4810092 blocked by process 28297. PostgreSQL automatically detects deadlock situations and resolves them by aborting one of the transactions involved, allowing the other (s) to complete. Process 28296: select * from jobs where ref_id ='a840a8bd-b8a7-45b2-a474-47e2f68e702d' and is_active is true for ERROR: deadlock DETAIL: Process 28454 waits for ShareLock on transaction 4810111 blocked by process 28384. Process 28297: select * from jobs where ref_id ='a840a8bd-b8a7-45b2-a474-47e2f68e702d' and is_active is true for update
Process 28296 waits for ShareLock on transaction 4809502 blocked by process 28297. ISSUE ERROR: deadlock DETAIL: Process 28297 waits for ShareLock on transaction 4809510 blocked by process 28296. UPDATE table set status = $1 where id =$2 ġ) Select query result will be used to lock all the rows with provided ref ID and that result is used for some business logicĢ) Update query to update the STATUS of a row which is part of same ref ID Select * from table where ref_id = $1 and is_active is true for update Or better yet, avoid blocking updates and delete for a long time by updating in small batches, i.e.I am using following query to select and update BEGIN select and insert go through, some updates and deletes block while the table is rewritten In the meantime, all queries will block, so your database will be unavailable.ĭon't do this: - reads and writes block until it is fully rewritten (hours?)ĪLTER TABLE items ADD COLUMN last_update timestamptz DEFAULT now() ĭo this instead: - select, update, insert, and delete block until the catalog is update (milliseconds)ĪLTER TABLE items ADD COLUMN last_update timestamptz
POSTGRES DEADLOCK HOW TO
If you add a column with a default, PostgreSQL will rewrite the whole table to fill in the default for every row, which can take hours on large tables. with PostgreSQL, so I was wondering if there is any chance to switch the database backend or if someone has another hint how to mitigate the deadlocks. Never Add a Column With a Default ValueĪ golden rule of PostgreSQL is: when you add a column to a table in production, never specify a default.Īdding a column takes a very aggressive lock on the table, which blocks read and write. These are some of the important dos and don'ts that we've seen as helpful when working with users to migrate from their single node Postgres database to Citus or when building new real-time analytics apps on Citus. With Postgres, it is possible to shoot yourself in the foot, but Postgres also offers you a way to stay on target. Of course, after the diagnosis, you may also want a cure. Recently, I wrote about locking behavior in Postgres, which commands block each other, and how you can diagnose blocked commands.
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