Right Query to Get the Current Number of Connections in a Postgresql Db

Right query to get the current number of connections in a PostgreSQL DB

Those two requires aren't equivalent. The equivalent version of the first one would be:

SELECT sum(numbackends) FROM pg_stat_database;

In that case, I would expect that version to be slightly faster than the second one, simply because it has fewer rows to count. But you are not likely going to be able to measure a difference.

Both queries are based on exactly the same data, so they will be equally accurate.

How to list active connections on PostgreSQL?

Oh, I just found that command on PostgreSQL forum:

SELECT * FROM pg_stat_activity;

Tool to find the number of open postgresql connection in the application

Try this

select * from pg_stat_activity;

How to increase the max connections in postgres?

Just increasing max_connections is bad idea. You need to increase shared_buffers and kernel.shmmax as well.

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Considerations

max_connections determines the maximum number of concurrent connections to the database server. The default is typically 100 connections.

Before increasing your connection count you might need to scale up your deployment. But before that, you should consider whether you really need an increased connection limit.

Each PostgreSQL connection consumes RAM for managing the connection or the client using it. The more connections you have, the more RAM you will be using that could instead be used to run the database.

A well-written app typically doesn't need a large number of connections. If you have an app that does need a large number of connections then consider using a tool such as pg_bouncer which can pool connections for you. As each connection consumes RAM, you should be looking to minimize their use.

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How to increase max connections

1. Increase max_connection and shared_buffers

in /var/lib/pgsql/{version_number}/data/postgresql.conf

change

max_connections = 100
shared_buffers = 24MB

to

max_connections = 300
shared_buffers = 80MB

The shared_buffers configuration parameter determines how much memory is dedicated to PostgreSQL to use for caching data.

• If you have a system with 1GB or more of RAM, a reasonable starting
  value for shared_buffers is 1/4 of the memory in your system.
• it's unlikely you'll find using more than 40% of RAM to work better
  than a smaller amount (like 25%)
• Be aware that if your system or PostgreSQL build is 32-bit, it might
  not be practical to set shared_buffers above 2 ~ 2.5GB.
• Note that on Windows, large values for shared_buffers aren't as
  effective, and you may find better results keeping it relatively low
  and using the OS cache more instead. On Windows the useful range is
  64MB to 512MB.

2. Change kernel.shmmax

You would need to increase kernel max segment size to be slightly larger
than the shared_buffers.

In file /etc/sysctl.conf set the parameter as shown below. It will take effect when postgresql reboots (The following line makes the kernel max to 96Mb)

kernel.shmmax=100663296

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References

Postgres Max Connections And Shared Buffers

Tuning Your PostgreSQL Server

Get DB stats (number of updates/deletes/inserts) over a period of time - PostgreSQL

You will have to sum up the table statistics for all tables in the database, as there are no such statistics collected on the database level.

Where to define the maximum number of connections in Postgres?

1. max_connections minus superuser_reserved_connections is the maximum for the sum of all non-superuser connections to all databases in the cluster.

2. If you want to limit the number of users per database, setting the limit on the database seems like the obvious choice, right?

3. If you end up setting max_connections to a high value, consider using a connection pool instead.

Get ID and name of each customer and the total sum (sum of quantity of products purchased)

If you just want the sum of quantity of products:

SELECT c.client_id, c.first_name, SUM(o.units_sold)
FROM
    client c
    INNER JOIN orders o ON o.client_id = c.client_id
GROUP BY c.client_id, c.first_name;

But if you also want to see the price:

SELECT c.client_id, c.first_name, SUM(o.units_sold), 
SUM(o.units_sold*p.price)
FROM
    client c
    INNER JOIN orders o ON o.client_id = c.client_id
    INNER JOIN products p ON o.product_id = p.product_id
GROUP BY c.client_id, c.first_name;

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