How to Cancel Postgresql Query?

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To cancel a PostgreSQL query, you can use the command \q in the psql interactive terminal. This will terminate the current query and return you to the command prompt. Alternatively, you can use the keyboard shortcut Ctrl + C to cancel the query execution. This will stop the query and return you to the command prompt immediately. It is important to note that cancelling a query may not always work instantaneously, especially if the query is performing intensive operations or transactions. In such cases, you may need to wait for a few moments before the query is cancelled and returns you to the command prompt.


How to stop a long-running query in PostgreSQL?

There are a few ways to stop a long-running query in PostgreSQL:

  1. Using pg_terminate_backend: You can use the pg_terminate_backend function to terminate the backend process running the query. You will first need to identify the process ID (pid) of the backend process running the query by querying the pg_stat_activity view. Once you have the pid, you can use the following command to terminate the backend process:
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SELECT pg_terminate_backend(pid);


  1. Canceling the query from psql: If you are using psql, you can cancel a long-running query by pressing Ctrl+C. This will send a cancel request to the backend process running the query and terminate it.
  2. Using pg_cancel_backend: Alternatively, you can use the pg_cancel_backend function to send a cancel request to the backend process running the query. Similar to pg_terminate_backend, you will need to first identify the pid of the backend process and then use the following command to cancel the query:
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SELECT pg_cancel_backend(pid);


It's important to note that canceling or terminating a query may result in some incomplete work or transactions being rolled back. Make sure to carefully consider the consequences before terminating a query.


What is the consequence of canceling a query on ongoing transactions in PostgreSQL?

When a query is canceled in PostgreSQL, the ongoing transactions associated with that query will be rolled back. This means that any changes made as part of these transactions will be undone, and the database will be returned to its state before the transactions started. This can potentially lead to data inconsistencies or loss of data if the transactions were not completed properly before being canceled.


What is the role of pg_terminate_backend function in canceling queries in PostgreSQL?

The pg_terminate_backend function in PostgreSQL is used to terminate a backend process, which can be used to cancel queries that are currently running. By terminating the backend process associated with a running query, the query is effectively canceled and the resources it was using are released.


This function can be useful in situations where a long-running query needs to be canceled for some reason, such as if it is causing performance issues or if it is no longer needed. It allows administrators to quickly and efficiently cancel queries that are causing problems without having to restart the entire database server.


It is worth noting that using pg_terminate_backend to cancel queries should be done with caution, as it forcefully terminates the backend process and can potentially cause data loss or corruption if not used carefully. It is recommended that administrators only use this function when absolutely necessary and after considering the potential consequences.


What is the effect of canceling a query on the PostgreSQL query planner?

When a query is canceled in PostgreSQL, the query planner will stop processing the query and release any resources that were being used for that query. This can have a few potential effects:

  1. If the query was actively running, canceling it may free up resources and allow other queries to run more efficiently.
  2. If the query was in the planning stage, canceling it may prevent any further planning or execution of the query, saving resources.
  3. If the query was waiting on locks or other resources, canceling it may release those locks and allow other queries to proceed.


Overall, canceling a query can help improve performance by freeing up resources and preventing the query from consuming unnecessary resources.


How to cancel a query from a specific user in PostgreSQL?

To cancel a query from a specific user in PostgreSQL, you can use the following steps:

  1. Identify the process ID (PID) of the query you want to cancel. You can find the PID by querying the pg_stat_activity view:
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SELECT pid, usename, query
FROM pg_stat_activity
WHERE state = 'active'
AND usename = 'username'; -- replace 'username' with the specific user's username


  1. Once you have identified the PID of the query you want to cancel, you can use the pg_terminate_backend function to terminate the query:
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SELECT pg_terminate_backend(pid);


Replace 'pid' with the actual PID of the query you want to cancel.


After running the above command, the query from the specific user should be canceled immediately.

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