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# PostgreSQL

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PostgreSQL is an open source, community driven, standard compliant object-relational database system.

## Installation

Warning: See #Upgrading PostgreSQL for necessary steps before upgrading to new versions of the PostgreSQL packages.

Install the postgresql package. It will also create a system user called postgres.

You can now switch to the postgres user using a privilege elevation program.

## Initial configuration

Before PostgreSQL can function correctly, the database cluster must be initialized:

    [postgres]$ initdb -D /var/lib/postgres/data

Where `-D` is the default location where the database cluster must be stored (see #Change default data directory if you want to use a different one). `initdb` accepts a number of extra arguments:

This article or section needs expansion.

Reason: PostgreSQL also supports ICU locales.[1] (Discuss in Talk:PostgreSQL)

- By default, the locale and the encoding for the database cluster are derived from your current environment (using $LANG value). If this is not what you want, you can override the defaults using `--locale=locale` (where locale is to be chosen amongst the system's available locales) and `--encoding=encoding` (which must match the chosen locale). (Once the database is up, you can check which values were used with `[postgres]$ psql -l`.)
  Note: Using a locale other than `C.UTF-8`, `C`, `POSIX` or `ucs_basic` can result in a collation version mismatch that will require reindexing if the library providing the locale (glibc or icu) gets updated.
- If your data directory resides on a file system without data checksumming, you may wish to enable PostgreSQL's built-in checksumming for increased integrity guarantees - add the `--data-checksums` argument to do so. Read #Enable data checksumming for more information. (Once the database is up, you can check if it is enabled with `[postgres]$ psql --tuples-only -c "SHOW data_checksums"`.)

  Note: The `/var/lib/postgres/data/` directory has the `C` (`No_COW`) file attribute set. [2] This disables checksumming in Btrfs.

- The trust authentication method is used by default, meaning that anyone on the host can connect as any database user. You can use `--auth-local=peer --auth-host=scram-sha-256` for safer authentication methods.
- The `-c`/`--set` option can be used to set any `postgresql.conf` parameter avoiding the need to manually edit `postgresql.conf`.
- For more options, see `initdb --help` and official documentation.

Example:

    [postgres]$ initdb --locale=C.UTF-8 --encoding=UTF8 -D /var/lib/postgres/data --data-checksums

Many lines should now appear on the screen with several ending by `... ok`:

    The files belonging to this database system will be owned by user "postgres".
    This user must also own the server process.

    The database cluster will be initialized with locale "C.UTF-8".
    The default text search configuration will be set to "english".

    Data page checksums are enabled.

    creating directory /var/lib/postgres/data ... ok
    creating subdirectories ... ok
    selecting dynamic shared memory implementation ... posix
    selecting default max_connections ... 100
    selecting default shared_buffers ... 128MB
    selecting default time zone ... UTC
    creating configuration files ... ok
    running bootstrap script ... ok
    performing post-bootstrap initialization ... ok
    syncing data to disk ... ok

    initdb: warning: enabling "trust" authentication for local connections
    initdb: hint: You can change this by editing pg_hba.conf or using the option -A, or --auth-local and --auth-host, the next time you run initdb.

    Success. You can now start the database server using:

        pg_ctl -D /var/lib/postgres/data -l logfile start

If these are the kind of lines you see, then the process succeeded. Return to the regular user using `exit`.

Warning:

- To read more about this initdb warning, see #Restricts access rights to the database superuser by default.
- If the database resides on a Btrfs file system, you should consider disabling Copy-on-Write for the directory before creating any database.
- If the database resides on a ZFS file system, you should consult ZFS#Databases before creating any database.

Tip: If you change the root to something other than `/var/lib/postgres`, you will have to edit the service file. If the root is under `home`, make sure to set `ProtectHome` to false.

Finally, start and enable the `postgresql.service`.

## Create your first database/user

Tip: If you create a PostgreSQL role/user with the same name as your Linux username, it allows you to access the PostgreSQL database shell without having to specify a user to login (which makes it quite convenient).

Become the postgres user. Add a new database role/user using the createuser command:

    [postgres]$ createuser --interactive

Create a new database over which the above user has read/write privileges using the createdb command (execute this command from your login shell if the database user has the same name as your Linux user, otherwise add `-O database-username` to the following command):

    $ createdb myDatabaseName

Tip: If you did not grant your new user database creation privileges, add `-U postgres` to the previous command.

## Familiarize with PostgreSQL

### Access the database shell

Become the postgres user. Start the primary database shell, psql, where you can do all your creation of databases/tables, deletion, set permissions, and run raw SQL commands. Use the `-d` option to connect to the database you created (without specifying a database, `psql` will try to access a database that matches your username).

    [postgres]$ psql -d myDatabaseName

Some helpful commands:

Get help:

    => \help

List all databases:

    => \l

Connect to a particular database:

    => \c database

List all users and their permission levels:

    => \du

Show summary information about all tables in the current database:

    => \dt

Exit/quit the `psql` shell:

    => \q

or press `Ctrl+d`.

There are of course many more meta-commands, but these should help you get started. To see all meta-commands run:

    => \?

## Optional configuration

The PostgreSQL database server configuration file is `postgresql.conf`. This file is located in the data directory of the server, typically `/var/lib/postgres/data`. This folder also houses the other main configuration files, including the `pg_hba.conf` which defines authentication settings, for both local users and other hosts ones.

Note: By default, this folder will not be browsable or searchable by a regular user. This is why `find` and `locate` are not finding the configuration files.

### Restricts access rights to the database superuser by default

The defaults `pg_hba.conf` allow any local user to connect as any database user, including the database superuser. This is likely not what you want, so in order to restrict global access to the postgres user, change the following line:

    /var/lib/postgres/data/pg_hba.conf

    # TYPE  DATABASE        USER            ADDRESS                 METHOD

    # "local" is for Unix domain socket connections only
    local   all             all                                     trust

To:

    /var/lib/postgres/data/pg_hba.conf

    # TYPE  DATABASE        USER            ADDRESS                 METHOD

    # "local" is for Unix domain socket connections only
    local   all             postgres                                peer

You might later add additional lines depending on your needs or software ones.

### Require password for login

Edit `/var/lib/postgres/data/pg_hba.conf` and set the authentication method for each user (or `all` to affect all users) to `scram-sha-256`:

    /var/lib/postgres/data/pg_hba.conf

    # TYPE  DATABASE        USER            ADDRESS                 METHOD

    # "local" is for Unix domain socket connections only
    local   all             user                                    scram-sha-256

Note: Changing the authentication method in `pg_hba.conf` does not update the hashed passwords stored in the database [3]. To migrate from `md5` to `scram-sha-256`, you need to set a new password for each database user.

Restart `postgresql.service`, and then re-add each user's password using `ALTER USER user WITH ENCRYPTED PASSWORD 'password';`.

### Configure PostgreSQL to be accessible exclusively through UNIX Sockets

When initially creating the cluster, append `-c listen_addresses=''` to the initdb command.

For an existing cluster, edit `postgresql.conf` and in the connections and authentication section set:

    /var/lib/postgres/data/postgresql.conf

    listen_addresses = ''

This will disable network listening completely. After this you should restart `postgresql.service` for the changes to take effect.

### Configure PostgreSQL to be accessible from remote hosts

In the connections and authentications section, set the `listen_addresses` line to your needs:

    /var/lib/postgres/data/postgresql.conf

    listen_addresses = 'localhost,my_local_ip_address'

You can use `'*'` to listen on all available addresses.

Note: PostgreSQL uses TCP port `5432` by default for remote connections. Make sure this port is open in your firewall and able to receive incoming connections. You can also change it in the configuration file, right below `listen_addresses`

Then add a line like the following to the authentication config:

    /var/lib/postgres/data/pg_hba.conf

    # TYPE  DATABASE        USER            ADDRESS                 METHOD
    # IPv4 local connections:
    host    all             all             ip_address/32           scram-sha-256

where `ip_address` is the IP address of the remote client.

See the documentation for pg_hba.conf.

Note:

The factual accuracy of this article or section is disputed.

Reason: The official documentation says that `md5` uses challenge-response authentication which "prevents password sniffing". Perhaps it should not be considered as insecure as sending the password in clear-text. (Discuss in Talk:PostgreSQL)
Neither sending your plain password nor the md5 hash over the Internet is secure if it is not done over an SSL-secured connection. See Secure TCP/IP Connections with SSL for how to configure PostgreSQL with SSL.

After this you should restart `postgresql.service` for the changes to take effect.

For troubleshooting take a look in the server log file:

    # journalctl -u postgresql.service

### Configure PostgreSQL authenticate against PAM

PostgreSQL offers a number of authentication methods. If you would like to allow users to authenticate with their system password, additional steps are necessary. First you need to enable PAM for the connection.

For example, the same configuration as above, but with PAM enabled:

    /var/lib/postgres/data/pg_hba.conf

    # IPv4 local connections:
    host   all   all   my_remote_client_ip_address/32   pam

The PostgreSQL server is however running without root privileges and will not be able to access `/etc/shadow`. We can work around that by allowing the postgres group to access this file:

    # setfacl -m g:postgres:r /etc/shadow

### Change default data directory

The default directory where all your newly created databases will be stored is `/var/lib/postgres/data`. To change this, follow these steps:

Create the new directory and make the postgres user its owner:

    # mkdir -p /pathto/pgroot/data
    # chown -R postgres:postgres /pathto/pgroot

Become the postgres user, and initialize the new cluster:

    [postgres]$ initdb -D /pathto/pgroot/data

Edit `postgresql.service` to create a drop-in file and override the `Environment` and `PIDFile` settings. For example:

    /etc/systemd/system/postgresql.service.d/PGROOT.conf

    [Service]
    Environment=PGROOT=/pathto/pgroot
    PIDFile=/pathto/pgroot/data/postmaster.pid

If you want to use `/home` directory for default directory or for tablespaces, add one more line in this file:

    ProtectHome=false

### Change default encoding of new databases to UTF-8

Note: If you ran `initdb` with `-E UTF8` or while using an UTF-8 locale, these steps are not required.

When creating a new database (e.g. with `createdb blog`) PostgreSQL actually copies a template database. There are two predefined templates: `template0` is vanilla, while `template1` is meant as an on-site template changeable by the administrator and is used by default. In order to change the encoding of a new database, one of the options is to change on-site `template1`. To do this, log into PostgreSQL shell (`psql`) and execute the following:

First, we need to drop `template1`. Templates cannot be dropped, so we first modify it so it is an ordinary database:

    UPDATE pg_database SET datistemplate = FALSE WHERE datname = 'template1';

Now we can drop it:

    DROP DATABASE template1;

The next step is to create a new database from `template0`, with a new default encoding:

    CREATE DATABASE template1 WITH TEMPLATE = template0 ENCODING = 'UNICODE';

Now modify `template1` so it is actually a template:

    UPDATE pg_database SET datistemplate = TRUE WHERE datname = 'template1';

Optionally, if you do not want anyone connecting to this template, set `datallowconn` to `FALSE`:

    UPDATE pg_database SET datallowconn = FALSE WHERE datname = 'template1';

Note: This last step can create problems when upgrading via `pg_upgrade`.

Now you can create a new database:

    [postgres]$ createdb blog

If you log back in to `psql` and check the databases, you should see the proper encoding of your new database:

    \l

                                  List of databases
      Name    |  Owner   | Encoding  | Collation | Ctype |   Access privileges
    -----------+----------+-----------+-----------+-------+----------------------
    blog      | postgres | UTF8      | C         | C     |
    postgres  | postgres | SQL_ASCII | C         | C     |
    template0 | postgres | SQL_ASCII | C         | C     | =c/postgres
                                                         : postgres=CTc/postgres
    template1 | postgres | UTF8      | C         | C     |

### Enable data checksumming

If your database files reside on a file system without checksumming, its data is suspectible to silent data corruption due to bit rot and broken hardware. While those events are rare, you might want to enable PostgreSQL's built-in data checksumming if you care about data integrity. This feature must be enabled on the cluster level, not per-database or per-table.

Note: This feature has a number of caveats:

- There is a minimal performance impact, especially while reading large datasets from disk. In-memory operations are not affected.
- PostgreSQL is unable to repair corrupt data - it will only abort transactions reading from corrupt pages to prevent further damage or invalid execution results.
- Checksums cover on-disk data (row) pages only, not metadata or control structures. In-memory pages are not checksummed. Error-corrected storage and ECC memory is still beneficial.

- To enable checksumming during cluster creation, add the `--data-checksums` argument to `initdb`.
- To verify whenever checksumming is enabled, run `[postgres]$ psql --tuples-only -c "SHOW data_checksums"` (which should print `off` or `on`).
- To toggle checksumming on an existing cluster:

1. Stop `postgresql.service`.
2. Run `[postgres]$ pg_checksums --pgdata /var/lib/postgres/data --enable` (or `--disable` if you no longer want checksumming). Enabling checksums will rewrite all database pages, which will take a while for large database instances.
3. Start `postgresql.service`.

## Graphical tools

- phpPgAdmin — Web-based administration tool for PostgreSQL.

  https://github.com/phppgadmin/phppgadmin || phppgadminAUR

- pgAdmin-desktop — The desktop user interface for pgAdmin, a comprehensive design and management GUI for PostgreSQL.

  https://www.pgadmin.org/ || pgadmin4-desktopAUR

- pgAdmin — Comprehensive design and management GUI for PostgreSQL.

  https://www.pgadmin.org/ || pgadmin4[broken link: package not found]

- pgModeler — Graphical schema designer for PostgreSQL.

  https://pgmodeler.io/ || pgmodelerAUR

- Postbird — Cross-platform PostgreSQL GUI client, written in JavaScript, runs with Electron.

  https://github.com/paxa/postbird || postbird-binAUR

- rainfrog — Database management TUI for Postgres.

  https://github.com/achristmascarl/rainfrog || rainfrog

- pgweb — Cross-platform Web client for PostgreSQL databases.

  https://sosedoff.github.io/pgweb || pgweb-binAUR

For tools supporting multiple DBMSs, see List of applications/Documents#Database tools.

## Set up backups

It is recommended to set up backups for databases containing valuable data. See the Backup and Restore chapter in the PostgreSQL documentation. There is also a list of backup tools in the PostgreSQL wiki, though it may not be up-to-date or complete. Remember that a backup system cannot be trusted unless you perform a test restore from time to time!

## Upgrading PostgreSQL

This article or section needs expansion.

Reason: How to upgrade when using third party extensions? (Discuss in Talk:PostgreSQL#pg_upgrade problem if extensions (like postgis) are used)

Upgrading major PostgreSQL versions (e.g. version 14.x to version 15.y) requires some extra maintenance.

Note: Official PostgreSQL upgrade documentation should be followed.
Warning: The following instructions could cause data loss. Do not run the commands below blindly, without understanding what they do. Make a backup first.

Get the currently used database version via

    # cat /var/lib/postgres/data/PG_VERSION

To ensure you do not accidentally upgrade the database to an incompatible version, it is recommended to skip updates to the PostgreSQL packages.

Minor version upgrades are safe to perform. However, if you do an accidental upgrade to a different major version, you might not be able to access any of your data. Always check the PostgreSQL home page to be sure of what steps are required for each upgrade. For a bit about why this is the case, see the versioning policy.

Note: If you use extensions check #PostgreSQL database unable to start after package update when using extensions and #Failing to start a PostgreSQL server with the older version of the database while upgrading to the newer version with extensions.

There are two main ways to upgrade your PostgreSQL database. Read the official documentation for details.

### pg_upgrade

The `pg_upgrade` utility attempts to copy over as much compatible data as possible between clusters and upgrading everything else. It is generally the fastest method to upgrade most instances, although it requires access to binaries for both source and target PostgreSQL versions. Read the pg_upgrade(1) man page to understand what actions it performs. For non-trivial instances (e.g. with streaming replication or log-shipping), read the upstream documentation first.

For those wishing to use `pg_upgrade`, a postgresql-old-upgrade package is available that will always run one major version behind the real PostgreSQL package. This can be installed side-by-side with the new version of PostgreSQL. To upgrade from older versions of PostgreSQL there are AUR packages available, e.g. postgresql-12-upgradeAUR. (You must use the `pg_upgrade` version packaged with the PostgreSQL version you are upgrading to.)

Note that the database cluster directory does not change from version to version, so before running `pg_upgrade`, it is necessary to rename your existing data directory and migrate into a new directory. The new database cluster must be initialized using the same parameters as the old one.

When you are ready to begin the upgrade:

1. While the old database cluster is still online, collect the `initdb` arguments used to create it. Refer to #Initial configuration for more information.
2. Stop `postgresql.service`. Check the unit status to be sure that PostgresSQL was stopped correctly. If it failed, `pg_upgrade` will fail with `The source cluster was not shut down cleanly`.
3. Upgrade postgresql, postgresql-libs, and postgresql-old-upgrade.
4. Make sure that `/var/lib/postgres/olddata` does not exist. If you did not delete it after a previous upgrade, do it now.
5. Rename the old cluster directory, then create a new cluster and temporary working directory:

       # mv /var/lib/postgres/data /var/lib/postgres/olddata
       # mkdir /var/lib/postgres/data /var/lib/postgres/tmp
       # chown postgres:postgres /var/lib/postgres/data /var/lib/postgres/tmp
       [postgres]$ cd /var/lib/postgres/tmp

6. Initialize the new cluster using the same `initdb` arguments as were used for the old cluster:

       [postgres]$ initdb -D /var/lib/postgres/data --locale=C.UTF-8 --encoding=UTF8 --data-checksums

7. Upgrade the cluster, replacing `PG_VERSION` below, with the old PostgreSQL version number (e.g. `15`):

       [postgres]$ pg_upgrade -b /opt/pgsql-PG_VERSION/bin -B /usr/bin -d /var/lib/postgres/olddata -D /var/lib/postgres/data

   Tip: On file systems that support reflinks (e.g. Btrfs and XFS), append the `--clone` option to speed up file copying.
   If necessary, adjust the configuration files of new cluster (e.g. `pg_hba.conf` and `postgresql.conf`) to match the old cluster.
8. Start `postgresql.service` again.
9. Optional: Run `[postgres]$ vacuumdb --all --analyze-in-stages` to recalculate query analyzer statistics, which should improve query performance shortly after the upgrade. (Adding `--jobs=NUMBER_OF_CPU_CORES` argument may improve this command's performance.)
10. Optional: Back up the `/var/lib/postgres/olddata` directory in case you need to restore a previous PostgreSQL version.
11. Delete the `/var/lib/postgres/olddata` directory with old cluster data.
12. Delete the `/var/lib/postgres/tmp` directory.
13. If you use pgbackrest, run the stanza-upgrade command.

### Manual dump and reload

You could also do something like this (after the upgrade and install of postgresql-old-upgrade).

Note:

- Below are the commands for upgrading from PostgreSQL 14. You can find similar commands in `/opt/` for your version of PostgreSQL cluster, provided you have matching version of postgresql-old-upgrade package installed.
- If you had customized your `pg_hba.conf` file, you may have to temporarily modify it to allow full access to old database cluster from local system. After upgrade is complete set your customization to new database cluster as well and restart `postgresql.service`.

Stop `postgresql.service`

    # mv /var/lib/postgres/data /var/lib/postgres/olddata
    # mkdir /var/lib/postgres/data
    # chown postgres:postgres /var/lib/postgres/data
    [postgres]$ initdb -D /var/lib/postgres/data --locale=C.UTF-8 --encoding=UTF8 --data-checksums
    [postgres]$ /opt/pgsql-14/bin/pg_ctl -D /var/lib/postgres/olddata/ start
    # cp /usr/lib/postgresql/postgis-3.so /opt/pgsql-14/lib/ # Only if postgis installed
    [postgres]$ pg_dumpall -h /tmp -f /tmp/old_backup.sql
    [postgres]$ /opt/pgsql-14/bin/pg_ctl -D /var/lib/postgres/olddata/ stop

Start `postgresql.service`

    [postgres]$ psql -f /tmp/old_backup.sql postgres

## Troubleshooting

### Improve performance of small transactions

If you are using PostgresSQL on a local machine for development and it seems slow, you could try turning synchronous_commit off in the configuration. Beware of the caveats, however.

    /var/lib/postgres/data/postgresql.conf

    synchronous_commit = off

### PostgreSQL database unable to start after package update when using extensions

The cause in this case is mostly the existing package is not compiled for the newer version (and it may be up-to-date), the solution is rebuilding the package either manually or waiting for an update to the extension package.

### Failing to start a PostgreSQL server with the older version of the database while upgrading to the newer version with extensions

This is caused because the old version of postgres from the package postgresql-old-upgrade does not have the required extensions (.so files) in its lib directory, the current solution is dirty, and might cause a lot of problems so keep a backup of the database just in case, basically copy the required extension .so files from `/usr/lib/postgresql/` to `/opt/pgsql-XX/lib/` (remember to replace XX with the major version of postgresql-old-upgrade).

For example, for timescaledb

    # cp /usr/lib/postgresql/timescaledb*.so /opt/pgsql-13/lib/

Warning: while copying the .so files was enough for me, it might be required to copy more files to the correct directories under `/opt/pgsql-XX/`.

to know the exact files to copy, check the contant of the package of the extension using :

    $ pacman -Ql package_name

Warning: This is a very dirty solution that may break or cause data loss in the database, so keep a backup.

### WARNING: database "postgres" has a collation version mismatch

You might see something like this:

    WARNING:  database "postgres" has a collation version mismatch
    DETAIL:  The database was created using collation version X.YY, but the operating system provides version X.ZZ.
    HINT:  Rebuild all objects in this database that use the default collation and run ALTER DATABASE postgres REFRESH COLLATION VERSION, or build PostgreSQL with the right library version.

That means collation provider library (glibc or icu) was updated which might have made some indexes invalid. So that means need to reindex those databases.

You can do that with:

    [postgres]$ psql -c 'REINDEX DATABASE' postgres
    [postgres]$ psql -c 'ALTER DATABASE postgres REFRESH COLLATION VERSION'

Repeat this above for all other databases by replacing postgres with respective DB name.

Tip: This issue can be avoided by using the `C.UTF-8`, `C`, `POSIX` or `ucs_basic` locale for the database cluster.
Category:

- Relational DBMSs

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