#Postgres lock queue update
#Postgres lock queue code

Queries in low-priority queues may be delayed while higher priority queries are run. Queries or statements in queues with higher priority levels will receive a larger share of available CPU resources in case of contention. Sets the priority of queries associated with a resource queue. MIN_COST to have no limit, enter a value of MIN_COST is specified as a floating point number (for example 100.0) or can also be specified as an exponent (for example 1e+2). Cost is measured in units of disk page fetches 1.0 equals one sequential disk page read. Queries with a cost under this limit will not be queued and run immediately. IfĬOST_OVERCOMMIT=FALSE is specified, queries that exceed the cost limit will always be rejected and never allowed to run. This means that a query that exceeds the allowed cost threshold will be allowed to run but only when the system is idle.

If a resource queue is limited based on query cost, then the administrator can allow cost overcommit (ĬOST_OVERCOMMIT=TRUE, the default). MAX_COST to have no limit, enter a value of MAX_COST is specified as a floating point number (for example 100.0) or can also be specified as an exponent (for example 1e+2). The total query optimizer cost of statements submitted from users in this resource queue allowed on the system at any one time. There is no maximum however the upper boundary at query execution time is limited by the physical memory of a segment host.

The minimum memory quota for a resource queue is 10MB. Memory units can be specified in kB, MB or GB. Sets the total memory quota for all statements submitted from users in this resource queue. To resetĪCTIVE_STATEMENTS to have no limit, enter a value of The value forĪCTIVE_STATEMENTS should be an integer greater than 0. The number of active statements submitted from users in this resource queue allowed on the system at any one time. The name of the resource queue whose limits are to be altered. Do not remove both these queue_attributes from a resource queue. A resource queue must have either an ACTIVE_STATEMENTS or a MAX_COST value. You can also set or reset priority for a resource queue to control the relative share of available CPU resources used by queries associated with the queue, or memory limit of a resource queue to control the amount of memory that all queries submitted through the queue can consume on a segment host.ĪLTER RESOURCE QUEUE WITHOUT removes the specified limits on a resource that were previously set. A resource queue must have either an ACTIVE_STATEMENTS or a MAX_COST value (or it can have both). Only a superuser can alter a resource queue. DescriptionĪLTER RESOURCE QUEUE changes the limits of a resource queue. Where queue_attribute is: ACTIVE_STATEMENTSĪ resource queue must have either an ACTIVE_STATEMENTS or a MAX_COST value. Where queue_attribute is: ACTIVE_STATEMENTS=ĪLTER RESOURCE QUEUE WITHOUT ( ) See PSQL Row Locking.Changes the limits of a resource queue.

#Postgres lock queue update

So if two queries try to SELECT FOR UPDATE on the same row, the second query will need to wait until the first one relinquishes the lock. The SELECT FOR UPDATE command acquires a row level FOR UPDATE lock which prevents other FOR UPDATE locks from being acquired on that row. The UPDATE command acquires a ROW EXCLUSIVE lock which conflicts with other ROW EXCLUSIVE locks on the row but not SELECT’s which acquires a ACCESS SHARE lock.

#Postgres lock queue code

The resources used to develop the talk and original code can be found at The idea and the queries in this post are from a company technical talk by Nibir Bora and Tim Higgins. By using Postgres for our job queue, we can implement more complex logic for how priority of jobs should be calculated compared to a message broker. Takeawaysīy using the SELECT FOR UPDATE SKIP LOCKED command, we can implement a job queue in Postgres which assigns the most important job at all times and can be used concurrently. If Query 3 with sleep fails, then the next consumer to get work would pick up job 1. This doesn’t block so if we run Query 3 with sleep which locks job 1 and then Query 3, Query 3 will complete job 2 without waiting for other jobs to complete. Begin update jobs set is_complete = true where chunk_idx = ( select chunk_idx from jobs where not is_complete order by chunk_idx for update skip locked limit 1 ) pg_sleep ( 10 ) returning chunk_idx commit