Identifying Hang-Ups: Checking Active or Blocked Queries in PostgreSQL


In PostgreSQL, hang-ups refer to the conditions where a query or process is stuck and stops responding. Hang-ups can reduce system performance, cause data inconsistency, and pose a significant risk to business continuity. Identifying and resolving hang-ups quickly is critical to maintaining system stability.

Definition of Hang-Ups in PostgreSQL

Hang-ups in PostgreSQL represent situations where a particular process or thread stops working due to different reasons. These reasons may include locking conflicts, high CPU usage, network issues, slow disk I/O operations, insufficient memory allocation, etc. In short, hang-ups are when queries become unresponsive for extended periods.

Importance of Identifying Hang-Ups

Hang-ups can have significant consequences on the performance and security of your database systems if not addressed promptly. They can cause spikes in CPU utilization that affect other applications running on the same server negatively.

Additionally, queries that are hung up might hold locks on resources needed by other processes resulting in deadlocks that make it impossible for any process to proceed further until the problem is resolved. Therefore it’s crucial to identify which query is causing an issue and take corrective action immediately.

Purpose of the Article

The objective of this article is to help you understand what causes hangups in PostgreSQL databases and how you can check active or blocked queries using various tools available with PostgreSQL like pg_stat_activity table/view pg_locks table/view etc. We’ll also cover some common causes of hangups such as locking conflicts/high CPU usage/inefficient query plans plus steps needed before resolving these issues so that your database servers operate smoothly without any disruptions caused by these problems.

Overview of Active and Blocked Queries

PostgreSQL handles multiple client connections simultaneously. Each client connection can have one or more queries running at any given time. When a query is executed, PostgreSQL allocates resources to that query until it completes or is interrupted.

If a query takes too much time to execute or has a blocking conflict with another query, it may result in a hang-up. There are two types of queries that can be running on a PostgreSQL server: active queries and blocked queries.

Explanation of Active Queries

Active queries are those that are currently running and have been allocated resources by the PostgreSQL server. These queries can be monitored using various tools, such as the `pg_stat_activity` view or the `psql` command-line tool’s `\watch` command. Active queries can provide insights into system performance, as they show which parts of the system may be under heavy load and where resources may need to be optimized.

Explanation of Blocked Queries

Blocked queries are those that cannot execute due to conflicts with other transactions. When a transaction holds an exclusive lock on rows, other transactions attempting to read or modify those same rows will become blocked until the exclusive lock is removed.

Blocked transactions can cause application slowdowns, as they prevent other transactions from executing and completing their work in a timely manner. Proper identification and resolution of blocked transactions is critical for maintaining system performance.

Differences between Active and Blocked Queries

The main difference between active and blocked queries lies in their ability to execute. Active queries are currently executing without any hindrance from blocking locks on rows or tables.

Blocked queries, on the other hand, have encountered conflicts that prevent them from executing in their current state. They may need additional resources such as disk space or memory before they can continue executing.

Understanding the differences between active and blocked queries is crucial for identifying and resolving hang-ups in PostgreSQL. Active queries provide insight into system performance, while blocked queries can cause application slowdowns and must be addressed quickly.

Identifying Active Queries in PostgreSQL

Active queries are those that are currently running in the system. It is important to identify these queries to determine which ones may be causing performance issues and to monitor the overall health of the system. In PostgreSQL, there are several ways to identify active queries.

Steps to Identify Active Queries

The first step in identifying active queries is accessing the PostgreSQL server logs. The logs can provide information on running queries, including their start time, duration, and other useful metrics. These logs can be accessed through the terminal using a command such as: $ tail -f /var/log/postgresql/postgresql-12-main.log | grep duration

This command will show all running queries with their duration times. Other useful information such as query ID and client IP address can also be found in the logs.

Tools to Identify Active Queries

Another way to identify active queries is by using third-party monitoring tools. These tools provide a more comprehensive view of the system’s performance and allow for real-time monitoring of running processes.

One popular tool for monitoring PostgreSQL is pg_top. This tool provides a summary of all current processes and their resource usage, including CPU and memory utilization, allowing administrators to quickly identify any resource-intensive processes.

Examples of Identifying Active Queries

An example of identifying active queries using pg_top could look like this:


32529 postgres 20 0 22.8GiB 10.4GiB 1.70GiB S 24 9% 0:18.82 postgres: postgres my_db SELECT 32680 postgres 20 0 22.8GiB 10.4GiB 1.70GiB S 24 9% 0:18.73 postgres: postgres my_db SELECT

Here, we can see two active queries running on the ‘my_db’ database, both with a high percentage of CPU usage.


Identifying active queries is an important step in monitoring and optimizing the performance of a PostgreSQL system. By understanding how to identify these queries and using the right tools, administrators can quickly identify any bottlenecks or resource-intensive processes and take action to resolve them.

Identifying Blocked Queries in PostgreSQL

Blocked queries are a common cause of hang-ups in PostgreSQL. When a query is blocked, it means that it is waiting for another query to release a lock on the resource it needs.

This can happen for several reasons, such as conflicting locks or long-running transactions. Identifying blocked queries is crucial to resolving hang-ups in PostgreSQL.

Here are some steps you can take to identify them:

  1. Connect to your PostgreSQL database using psql or another client tool.
  2. Execute the following command: SELECT * FROM pg_stat_activity WHERE waiting=’t’;
  3. The above command will return a list of all queries that are currently waiting for a lock.

In addition to the above steps, there are several tools you can use to identify blocked queries:

  • pg_top: A utility that provides real-time monitoring of PostgreSQL processes. It can help you identify processes that are consuming resources and causing blockages.
  • PgAdmin: A popular GUI tool for managing PostgreSQL databases. It has an activity monitor that allows you to view and analyze running processes, including those that are blocked.
  • PgBadger: A log analyzer tool that parses PostgreSQL logs and provides detailed reports on database activity. It can help you identify long-running transactions and other bottlenecks that may be causing blockages.

Examples of Identifying Blocked Queries

To better illustrate how to identify blocked queries, let’s consider an example scenario: You have a web application running on top of a PostgreSQL database.

Users have reported slow response times and you suspect that there may be some blocked queries causing the issue. To investigate, you connect to your database using psql and execute the following command: SELECT * FROM pg_stat_activity WHERE waiting=’t’;

The above query returns a list of all processes that are currently waiting for a lock. You notice that one of the queries has been waiting for over 5 minutes. You also notice that this query is related to a long-running transaction.

You decide to use PgAdmin’s activity monitor to further investigate the issue. You navigate to the activity monitor and filter the results by blocked processes.

You see that there are several processes that are blocking each other, creating a deadlock situation. You use PgAdmin’s kill process feature to terminate one of the long-running transactions, which resolves the deadlock and improves response times for your users.

Common Causes for Hang-Ups in PostgreSQL

PostgreSQL is an efficient and reliable database management system, but it’s not immune to hang-ups. Identifying common causes of hang-ups is necessary to prevent their occurrence and to resolve them promptly. In this section, we will explore some of the most common reasons why PostgreSQL queries can get blocked and cause hang-ups.

Locking Conflicts

One of the most frequent causes of a hang-up in PostgreSQL is locking conflicts. Locks are used to protect data from concurrent access that could lead to inconsistencies or incorrect results.

There are several types of locks, including exclusive locks that prevent other transactions from accessing the locked resource and shared locks that allow multiple transactions read access but only one transaction write access. Locking conflicts occur when two or more queries compete for access to a locked resource.

For example, if query A holds an exclusive lock on a table while query B tries to acquire another exclusive lock on the same table, query B will be blocked until query A releases its lock. If query A doesn’t release its lock promptly, it can cause a hang-up and affect other queries waiting for resources held by query A.

High CPU Usage

Another reason why PostgreSQL queries can get blocked is high CPU usage. When a query consumes too much CPU time, it may cause other queries to wait indefinitely for CPU resources.

High CPU usage can be caused by many factors, such as inefficient algorithms or poorly optimized SQL statements. For example, if a SELECT statement returns millions of rows and takes too long to execute due to an inefficient query plan or lack of indexes, it can consume all available CPU resources and block other queries until it completes its execution.

Inefficient Query Plans

Inefficient query plans are one of the primary causes of performance issues in PostgreSQL databases. The optimizer generates execution plans based on the available statistics and indexes, but sometimes these plans can be suboptimal, resulting in slow queries and high resource consumption. Query plans can be inefficient due to several reasons, such as outdated statistics or missing indexes.

For example, if a query joins two large tables without proper indexing, it may generate a Cartesian product that requires a massive amount of resources and time to complete. Inefficient query plans can cause hang-ups by blocking other queries while waiting for resources to become available.

Identifying and addressing the common causes of hang-ups is crucial for maintaining optimal performance in PostgreSQL databases. In the next section, we will explore some solutions for resolving hang-ups.

How to Resolve Hang-Ups in PostgreSQL

Hang-ups in PostgreSQL can cause major issues if not resolved quickly. In this section, we will discuss the methods and techniques that can be used to resolve hang-ups in PostgreSQL. We will cover killing processes, optimizing query plans, and tuning system configuration.

Killing Processes

One of the most common causes of hang-ups in PostgreSQL is locking conflicts between processes. When two or more processes try to access the same resource simultaneously, it can cause a deadlock or blockage, which can lead to a hang-up.

Killing these conflicting processes is a simple yet effective way of resolving this issue. The first step is to identify the problematic process by using the “pg_locks” view or any other available tool that lists all active locks.

Once identified, you can use either of these two methods:

  • Kill Signal: Use the “kill” command followed by the process ID (PID) and signal number (e.g., -9 for SIGKILL). This method immediately terminates the process but may result in data loss or corruption.
  • Cancellation Request: use the “pg_cancel_backend” function followed by the pid to send a cancellation request to a backend process that is running a query.

Optimizing Query Plans

Inefficient query plans can also lead to hang-ups in PostgreSQL. Query plans are generated by Postgres’ query optimizer based on various factors such as available indexes and table statistics. It’s important to regularly check for inefficient queries and optimize them since poor performance impacts overall application response times.

The first step towards optimizing query performance is identifying problematic queries that consume excessive resources such as CPU or memory. Postgres provides various tools such as the “pg_stat_activity” view or “pgBadger” utility which can be used to identify slow running queries and resource consumption patterns.

Once you have identified problematic queries, you can use the following techniques to optimize query plans:

  • Create indexes: Indexes are a way of improving query performance by speeding up data retrieval. It’s essential to create indexes on columns that are frequently used in WHERE clauses or join conditions.
  • Analyze tables: Running the “ANALYZE” command updates table statistics, which helps Postgres generate more accurate and efficient query plans. It’s important to run this command regularly on all tables.
  • Tune configuration parameters: PostgreSQL provides various configuration parameters that can be tuned to improve query performance based on system resources and workload patterns. Some commonly used parameters for tuning query performance are shared_buffers, work_mem, and effective_cache_size.

Tuning System Configuration

Tuning system configuration is another effective way to resolve hang-ups in PostgreSQL by optimizing resource usage based on workload patterns. System configuration parameters impact various aspects of database operations such as memory allocation, disk I/O, and network bandwidth utilization. The first step towards tuning system configuration is identifying bottlenecks using tools like “top” or “sar”.

Once identified, you can adjust the following system parameters:

  • Memory usage: allocate appropriate amounts of ram for shared_buffers、work_mem、and maintenance_work_mem based on available physical memory and expected workload patterns.
  • Disk I/O settings: tune disk-related parameters like fsync、synchronous_commit,and checkpoint settings based on the available storage hardware and expected workload patterns.
  • Network settings: adjust network-related parameters such as max_connections、listen_addresses,and port settings to optimize network bandwidth usage and connection management.

These tuning techniques should be approached with care, as modifications to system configuration can have significant performance impacts. Therefore, it’s recommended to test new configurations on a staging environment before deploying them to production.


Identifying and resolving hang-ups in PostgreSQL is critical for maintaining stable application performance and providing a good user experience. Killing processes, optimizing query plans, and tuning system configuration are effective methods for resolving hang-ups in PostgreSQL. By following best practices for monitoring Postgres performance metrics, identifying problematic queries, and making necessary changes to Postgres configuration parameters,you can ensure that your database is always performing at its best.


Identifying and resolving hang-ups in PostgreSQL is crucial for maintaining a high-performance database. Through this article, we have explored the definition of hang-ups, the importance of identifying them, and the process of checking for active and blocked queries. We have also discussed common causes for hang-ups and provided tips on how to resolve them.

Regularly monitoring your database for hang-ups can help you prevent potential problems before they cause significant damage. By identifying active queries, blocked queries, and common causes of hang-ups, you can take proactive measures to optimize your query plans and tune your system configuration to improve overall performance.

Summary of the Article

In this article, we have covered the basics of identifying hang-ups in PostgreSQL. We started with an overview of active and blocked queries and their differences.

Then we discussed how to identify both types using various tools available in PostgreSQL. We also explored common causes for hang-ups such as locking conflicts, high CPU usage, and inefficient query plans.

We covered some tips on how to resolve these issues including killing processes, optimizing query plans, and tuning system configuration. Overall this article provides a comprehensive guide on how to maintain healthy database performance by regularly checking for hang-ups.

Importance of Regularly Checking for Hang-Ups

Regularly checking for hang-ups is essential because it helps maintain healthy database performance. If left unchecked over time, these issues can lead to significant performance problems that are difficult to resolve quickly without causing other complications.

By regularly monitoring your PostgreSQL instance for hanging queries or locked transactions you will be able to address potential problems before they cause harm to your business or application users. This practice helps ensure that your database remains efficient at all times while minimizing downtime caused by complex issues.

Final Thoughts on Resolving Hang-Ups

Resolving hanging queries or blocking transactions is a complex process that requires a deep understanding of your database schema, query structure, and system configuration. However, with the right tools and expertise, these issues can be identified and resolved quickly, minimizing downtime and preventing future problems.

While regular maintenance is essential to preventing potential hang-ups in PostgreSQL databases, it is equally important to have experts on hand who can provide support when needed. With the right team in place to monitor your database performance, you can ensure a reliable system for your business or application users.

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