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Postgres lock queue
Postgres lock queue







postgres lock queue

As mentioned before, CREATE INDEX (without CONCURRENTLY) acquires a lock ( SHARE) that conflicts with itself, CREATE INDEX, REFRESH MATERIALIZED VIEW CONCURRENTLY, and anything that gets the ACCESS EXCLUSIVE lock, AND also the ROW EXCLUSIVE table-level lock (so cannot do concurrent INSERT/ UPDATE/ DELETE).

#POSTGRES LOCK QUEUE UPDATE#

  • ANALYZE, VACUUM, CREATE INDEX CONCURRENTLY - gets a lock ( SHARE UPDATE EXCLUSIVE) that conflicts with itself, CREATE INDEX, REFRESH MATERIALIZED VIEW CONCURRENTLY, and anything that gets the ACCESS EXCLUSIVE lock.
  • postgres lock queue

    Other statements and the table level locks they get

    postgres lock queue

    So for all intents and purposes, SELECT FOR UPDATE and SELECT FOR SHARE get roughly the same table-level locks as plain SELECT.Ĭonflicts with: Only the ACCESS EXCLUSIVE table level lock. Note: There’s another table level lock called ROW SHARE which is gotten by the SELECT FOR UPDATE and SELECT FOR SHARE commands, with the only difference that this lock level additionally conflicts with the lock obtained by REFRESH MATERIALIZED VIEW CONCURRENTLY, which really only applies with the SELECT FOR SHARE statement. This is the weakest and most permissive table-level lock. Note that a potentially infinite number of transactions can hold a ROW EXCLUSIVE table-level lock simultaneously, which is good and what we expect. (Do note that despite having ROW in the name, it’s a TABLE level lock, there are row-level locks that are acquired by those statements as well).Ĭonflicts with: Anything that takes an ACCESS EXCLUSIVE lock, any materialized view refresh, and regular CREATE INDEX (hint: this is why when creating an index you do it CONCURRENTLY). UPDATE, DELETE, and INSERT acquire this lock. Use REFRESH MATERIALIZED VIEW CONCURRENTLY indicator_count instead since that allows reading the materialized view while it is being written.Whats happening here is that a PostgreSQL transaction takes an ExclusiveLock on its own transaction ID when it starts. This means that one transaction is waiting for another to commit/rollback before it can proceed. REFRESH MATERIALIZED VIEW indicator_count The key thing is that its a ShareLock on the transaction.Then it re-checks the WHERE clause on the outer query to make sure it still matches (in case the row was deleted, or was. PostgreSQL sees that someone else has a lock on that row, so it waits for that lock to be released. Anything to do with partition managementĪLTER TABLE ticket DROP CONSTRAINT ticket_idx_constraint, ADD CONSTRAINT ticket_unique_description UNIQUE ( description ) Otherwise what can happen is that the inner query finds the row, returns the ID, and you try to lock the row with that ID.ALTER TABLE ticket ALTER COLUMN severity SET NOT NULL / ALTER TABLE ticket ALTER COLUMN severity DROP NOT NULL.Some examples of things you might encounter

    postgres lock queue

    There are some other forms of ALTER TABLE that acquire different locks but we’re not likely to encounter them.

  • Most other forms of ALTER TABLE that we’re likely to encounter (except for adding a foreign key constraint which acquires a SHARE ROW EXCLUSIVE table level lock).
  • Note that this is very fast, but also note that Postgres won’t reclaim the disk space until you run a VACUUM FULL.
  • One caveat is that in Postgres >= 11 this only applies to non-volatile default-values, so now() would update all the rows.
  • ACCESS SHARE = 11, there’s an optimization that only updates the rows when they are accessed so you can do this much more easily.
  • The documentation () lists 8 different table level locks, I’m going to focus on the big 3 that are the most common : Row level locks are taken during a transaction (if needed) and released at the end of transaction. Table level locksĪs the name implies, these are at the table level. Goal: Not to get too deep into the exhaustive list of locks, but rather to build intuition. Now let's add a few jobs.DROP TABLE IF EXISTS store CREATE TABLE store ( k TEXT, v INTEGER ) INSERT INTO store ( k, v ) VALUES ( 'apple', 0 ), ( 'banana', 0 ) - Simple example #2 - entire table lockĭROP TABLE IF EXISTS store CREATE TABLE store ( k TEXT, v INTEGER ) BEGIN ALTER TABLE store ADD v2 INTEGER - acquires an ACCESS EXCLUSIVE table-level lock - ACCESS EXCLUSIVE conflicts with everything, including ACCESS SHARE - T2: SELECT * FROM store - attempts to acquire ACCESS SHARE table level lock, but has to wait COMMIT - Releases ACCESS EXCLUSIVE table-level lock - unblocks because acquires and then releases ACCESS SHARE table lock We will create simple jobs table with id and payload columns. So our example will be a simple job queue. The most obvious use for skip locked is multiple workers consuming jobs from a single source.









    Postgres lock queue