- POSTGRES DEADLOCK UPDATE
- POSTGRES DEADLOCK FULL
POSTGRES DEADLOCK UPDATE
However, locking a row might cause a disk write, e.g., SELECT FOR UPDATE modifies selected rows to mark them locked, and so will result in disk writes.
Postgres doesn’t remember any information about modified rows in memory, so there is no limit on the number of rows locked at one time. In Postgres 9.1 and 9.2 there are two row-level lock modes, but on Postgres 9.3 and 9.4 there are four modes. Here is the table which shows which modes are in conflict with each other: Row-level Locks But if a lock is acquired after establishing a savepoint, the lock is released immediately if the savepoint is rolled back to. When some lock mode is acquired it is held until end of transaction. It is also important to know that there are self-conflicting modes. Non-conflicting locks can be held concurrently by many transactions. Transaction is never in conflict with itself. Two transactions can’t hold locks on conflicting modes on the same table at the same time. The most important info for every mode is the list of modes which are in conflict with each other. It is important to know that all these locks are table-level locks, even if they have ROW in their name. This mode is default mode of LOCK command.
POSTGRES DEADLOCK FULL
ACCESS EXCLUSIVE – The ALTER TABLE, DROP TABLE, TRUNCATE, REINDEX, CLUSTER, and VACUUM FULL commands acquire lock on table referenced in query.It is not acquired implicitly by any command. EXCLUSIVE – This lock mode allows only reads to process in parallel with transaction that acquired this lock.SHARE ROW EXCLUSIVE – Not acquired implicitly by any command.SHARE – The CREATE INDEX command acquires lock on table referenced in query.SHARE UPDATE EXCLUSIVE – The VACUUM (without FULL), ANALYZE, CREATE INDEX CONCURRENTLY, and some forms of ALTER TABLE commands acquire this lock.General rule is that all queries that modify table acquire this lock.
ROW EXCLUSIVE – The UPDATE, INSERT and DELETE commands acquire this lock on target table (as well as ACCESS SHARE lock on all referenced tables in query). ROW SHARE – The SELECT FOR UPDATE and SELECT FOR SHARE commands acquire this lock on target table (as well as ACCESS SHARE lock on all referenced tables in query). General rule is that all queries that only read table acquire this lock. ACCESS SHARE – The SELECT command acquires this lock on table(s) referenced in query. Most of the table-level locks are acquired by built-in SQL commands, but they can also be acquired explicitly with LOCK command. Versions 9.1 and 9.2 are mainly the same, and versions 9.3 and 9.4 have some differences mainly connected to row-level locking. Locking mechanisms have changed in time, so I will cover locking in 9.x versions of Postgres. Application developers normally need not be concerned with page-level locks. These locks are released immediately after a row is fetched or updated. In addition to table and row locks, page-level share/exclusive locks are used to control read/write access to table pages in the shared buffer pool. Explicit locks are acquired on explicit user requests (with special queries) and implicit are acquired by standard SQL commands. Table and row level locks can be explicit or implicit. Generally in Postgres we have 3 mechanisms of locking: table-level, row-level and advisory locks. I will hereby try to present everything we should know in order to work with Postgres, having in mind locking mechanisms as well as faster resolution of locking issues. Despite its importance, locking mechanism in Postgres is poorly documented and doesn’t even work as expected in some cases (as documentation suggests). In most cases, these problems relate to deadlock or data inconsistency due to misunderstanding how locking works in Postgres. Locking is mostly addressed and researched when some problem arises. It should be familiar to every developer of DB applications (especially to those who are working on highly concurrent code). 08:07:45.Locking is a very important part of PostgreSQL (as well as most of other modern RDBMS). 08:07:45.697ĜONTEXT: while deleting tuple (1,5) in relation "device_queue" 08:07:45.697 HINT: See server log for query details. 08:07:45.697 Process 8421: delete from device_queue where dev_eui = $1 08:07:45.697 Process 8658 waits for ShareLock on transaction 6822711 blocked by process 8421. 08:07:45.697ĝETAIL: Process 8421 waits for ShareLock on transaction 6822714 blocked by process 8658. Is it a bug or there is a way to avoid this issue? 08:07:45.697ĞRROR: deadlock detected Hi, we faced an issue: regular deadlock in DB that is caused by the same combination of SQL queries with different device ids.
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