Where is PostgreSQL Getting That $%&^ing Configuration Parameter From?

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Photo by Steve Smith on Unsplash

Like a squirrel looking for a lost nut, finding where a configuration parameter is being set in PostgreSQL can be a pain in the (fluffy) tail.

For example, the log_min_duration_statement which is used to configure the threshold duration of a query before it is logged can be set in multiple ways. Here are 7 ways I know of to set the values:

1. Add log_min_duration_statement = 0 to postgresql.conf
2. Add log_min_duration_statement = 0 to a conf file declared in postgresql.conf with include =
3. Add log_min_duration_statement = 0 to a conf file in a folder declared in postgresql.conf with include_dir
4. Add the value to the command line when starting Postgres
5. Use an ALTER SYSTEM command, this will set the value in postgresql.auto.conf
6. Use an ALTER DATABASE command which will set the value for the current database
7. Use set_config to set the value for the current transaction or session

I believe I have these in the correct override order, so if the value was set in all 7 locations, the value in set_config would be used. Let me know in the comments if there are other ways of setting the value.

Look up the current value

So how do you know what the current values is of a particular configuration parameter?

There is a wonderful system view which stores where ALL the configuration parameters are coming from called pg_settings. Let’s take a look at a particular configuration parameter log_min_duration_statement to decipher how and where the value is being set:

Example 1

We’ll start with a non-typical way of assigning our example configuration parameter:

mydb=# select * from pg_settings where name = 'log_min_duration_statement';-[ RECORD 1 ]---+-----------------------------------------------------------------------
name            | log_min_duration_statement
setting         | 60000
unit            | ms
category        | Reporting and Logging / When to Log
short_desc      | Sets the minimum execution time above which statements will be logged.
extra_desc      | Zero prints all queries. -1 turns this feature off.
context         | superuser
vartype         | integer
source          | database
min_val         | -1
max_val         | 2147483647
enumvals        |
boot_val        | -1
reset_val       | 60000
sourcefile      |
sourceline      |
pending_restart | f

From the above we can see the current value is 60000ms (setting & unit), the value is an integer (vartype) and the source is database. database, in this context means the value was set with an ALTER DATABASE command (#6 in our list):

ALTER DATABASE mydb SET log_min_duration_statement TO 60000;

So any value set in postgresql.conf, config files, config folders, startup parameters and any ALTER SYSTEM settings were all ignored.

Hint: if you want to undo this customization run a RESET command while connected to the database:

ALTER DATABASE mydb RESET log_min_duration_statement;

You’ll need to reset your client connection to see the change take effect.

This configuration is per database, which can be a bit confusing, if you run the exact same query against a different database, such as the postgres database (see the next example), you may see that the value comes from a completely different source.

Example 2

In the next example the value is set in a configuration file:

postgres=# select * from pg_settings where name = 'log_min_duration_statement';
-[ RECORD 1 ]---+------------------------------------------------------------------------------------
name            | log_min_duration_statement
setting         | 120000
unit            | ms
category        | Reporting and Logging / When to Log
short_desc      | Sets the minimum execution time above which statements will be logged.
extra_desc      | Zero prints all queries. -1 turns this feature off.
context         | superuser
vartype         | integer
source          | configuration file
min_val         | -1
max_val         | 2147483647
enumvals        |
boot_val        | -1
reset_val       | 120000
sourcefile      | /var/postgres/bin/postgresql.conf
sourceline      | 623
pending_restart | f

Specifically the configuration value 120000ms (setting & unit) is coming from the file /var/postgres/bin/postgresql.conf (sourcefile) from line 623 (sourceline).

This type of configuration where the value is set in a configuration file such as postgresql.conf is the most common.

Example 3

This one is set in a configuration file, but with a twist:

postgres=# select * from pg_settings where name = 'log_min_duration_statement';
-[ RECORD 1 ]---+------------------------------------------------------------------------------------
name            | log_min_duration_statement
setting         | 30000
unit            | ms
category        | Reporting and Logging / When to Log
short_desc      | Sets the minimum execution time above which statements will be logged.
extra_desc      | Zero prints all queries. -1 turns this feature off.
context         | superuser
vartype         | integer
source          | configuration file
min_val         | -1
max_val         | 2147483647
enumvals        |
boot_val        | -1
reset_val       | 30000
sourcefile      | /var/postgres/bin/postgresql.auto.conf
sourceline      | 3
pending_restart | f

Our configuration parameter is listed as a configuration file (source) but the sourcefile points to a postgresql.auto.conf which means the value was set with an ALTER SYSTEM statement similar to:

ALTER SYSTEM SET log_min_duration_statement = 30000;

When changing this value you need to reset the client connection to see the change go into effect.

There is a slightly different RESET command to undo this customization:

ALTER SYSTEM RESET log_min_duration_statement;

Example 4

In the final example the configuration parameter is assigned with a set_config() function from pg_settings looks like:

postgres=# select * from pg_settings where name = 'log_min_duration_statement';                                                                                                                                                     -[ RECORD 1 ]---+-----------------------------------------------------------------------
name            | log_min_duration_statement
setting         | 180000
unit            | ms
category        | Reporting and Logging / When to Log
short_desc      | Sets the minimum execution time above which statements will be logged.
extra_desc      | Zero prints all queries. -1 turns this feature off.
context         | superuser
vartype         | integer
source          | session
min_val         | -1
max_val         | 2147483647
enumvals        |
boot_val        | -1
reset_val       | 30000
sourcefile      |
sourceline      |
pending_restart | f

The configuration value is listed as session and none of the other source* fields are populated which means it was set with a SQL query similar to:

SELECT set_config('log_min_duration_statement', '180000', false);

To reset the value simply terminate the current connection and reconnect the client.

Final Thoughts

The pg_settings view will show you the current configuration parameter values be used, hopefully this article helped to decipher how these values are being set!

~fin~