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Chapter 4. Using PostgreSQL

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The PostgreSQL server is an open source robust and highly-extensible database server based on the SQL language. The PostgreSQL server provides an object-relational database system that can manage extensive datasets and a high number of concurrent users. For these reasons, PostgreSQL servers can be used in clusters to manage high amounts of data.

The PostgreSQL server includes features for ensuring data integrity, building fault-tolerant environments and applications. With the PostgreSQL server, you can extend a database with your own data types, custom functions, or code from different programming languages without the need to recompile the database.

Learn how to install and configure PostgreSQL on a RHEL system, how to back up PostgreSQL data, and how to migrate from an earlier PostgreSQL version.

4.1. Installing PostgreSQL

RHEL 9 provides PostgreSQL 13 as the initial version of this Application Stream, which you can install easily as an RPM package.

Additional PostgreSQL versions are provided as modules with a shorter life cycle in minor releases of RHEL 9:

  • RHEL 9.2 introduced PostgreSQL 15 as the postgresql:15 module stream
  • RHEL 9.4 introduced PostgreSQL 16 as the postgresql:16 module stream

To install PostgreSQL, use the following procedure.

Note

By design, it is impossible to install more than one version (stream) of the same module in parallel. Therefore, you must choose only one of the available streams from the postgresql module. You can use different versions of the PostgreSQL database server in containers, see Running multiple PostgreSQL versions in containers.

Procedure

  1. Install the PostgreSQL server packages:

    • For PostgreSQL 13 from the RPM package:

      # dnf install postgresql-server
    • For PostgreSQL 15 or PostgreSQL 16 by selecting stream (version) 15 or 16 from the postgresql module and specifying the server profile, for example:

      # dnf module install postgresql:16/server

      The postgres superuser is created automatically.

  2. Initialize the database cluster:

    # postgresql-setup --initdb

    Red Hat recommends storing the data in the default /var/lib/pgsql/data directory.

  3. Start the postgresql service:

    # systemctl start postgresql.service
  4. Enable the postgresql service to start at boot:

    # systemctl enable postgresql.service
Important

If you want to upgrade from an earlier postgresql stream within RHEL 9, follow both procedures described in Switching to a later stream and in Migrating to a RHEL 9 version of PostgreSQL.

4.1.1. Running multiple PostgreSQL versions in containers

To run different versions of PostgreSQL on the same host, run them in containers because you cannot install multiple versions (streams) of the same module in parallel.

This procedure includes PostgreSQL 13 and PostgreSQL 15 as examples but you can use any PostgreSQL container version available in the Red Hat Ecosystem Catalog.

Prerequisites

  • The container-tools meta-package is installed.

Procedure

  1. Use your Red Hat Customer Portal account to authenticate to the registry.redhat.io registry:

    # podman login registry.redhat.io

    Skip this step if you are already logged in to the container registry.

  2. Run PostgreSQL 13 in a container:

    $ podman run -d --name <container_name> -e POSTGRESQL_USER=<user_name> -e POSTGRESQL_PASSWORD=<password> -e POSTGRESQL_DATABASE=<database_name> -p <host_port_1>:5432 rhel9/postgresql-13

    For more information about the usage of this container image, see the Red Hat Ecosystem Catalog.

  3. Run PostgreSQL 15 in a container:

    $ podman run -d --name <container_name> -e POSTGRESQL_USER=<user_name> -e POSTGRESQL_PASSWORD=<password> -e POSTGRESQL_DATABASE=<database_name> -p <host_port_2>:5432 rhel9/postgresql-15

    For more information about the usage of this container image, see the Red Hat Ecosystem Catalog.

  4. Run PostgreSQL 16 in a container:

    $ podman run -d --name <container_name> -e POSTGRESQL_USER=<user_name> -e POSTGRESQL_PASSWORD=<password> -e POSTGRESQL_DATABASE=<database_name> -p <host_port_3>:5432 rhel9/postgresql-16

    For more information about the usage of this container image, see the Red Hat Ecosystem Catalog.

    Note

    The container names and host ports of the two database servers must differ.

  5. To ensure that clients can access the database server on the network, open the host ports in the firewall:

    # firewall-cmd --permanent --add-port={<host_port_1>/tcp,<host_port_2>/tcp,<host_port_3>/tcp,...}
    # firewall-cmd --reload

Verification steps

  1. Display information about running containers:

    $ podman ps
  2. Connect to the database server and log in as root:

    # psql -u postgres -p -h localhost -P <host_port> --protocol tcp

4.2. Creating PostgreSQL users

PostgreSQL users are of the following types:

  • The postgres UNIX system user - should be used only to run the PostgreSQL server and client applications, such as pg_dump. Do not use the postgres system user for any interactive work on PostgreSQL administration, such as database creation and user management.
  • A database superuser - the default postgres PostgreSQL superuser is not related to the postgres system user. You can limit access of the postgres superuser in the pg_hba.conf file, otherwise no other permission limitations exist. You can also create other database superusers.
  • A role with specific database access permissions:

    • A database user - has a permission to log in by default
    • A group of users - enables managing permissions for the group as a whole

Roles can own database objects (for example, tables and functions) and can assign object privileges to other roles using SQL commands.

Standard database management privileges include SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, TRIGGER, CREATE, CONNECT, TEMPORARY, EXECUTE, and USAGE.

Role attributes are special privileges, such as LOGIN, SUPERUSER, CREATEDB, and CREATEROLE.

Important

Red Hat recommends performing most tasks as a role that is not a superuser. A common practice is to create a role that has the CREATEDB and CREATEROLE privileges and use this role for all routine management of databases and roles.

Prerequisites

  • The PostgreSQL server is installed.
  • The database cluster is initialized.

Procedure

  • To create a user, set a password for the user, and assign the user the CREATEROLE and CREATEDB permissions:

    postgres=# CREATE USER mydbuser WITH PASSWORD 'mypasswd' CREATEROLE CREATEDB;

    Replace mydbuser with the username and mypasswd with the user’s password.

Example 4.1. Initializing, creating, and connecting to a PostgreSQL database

This example demonstrates how to initialize a PostgreSQL database, create a database user with routine database management privileges, and how to create a database that is accessible from any system account through the database user with management privileges.

  1. Install the PosgreSQL server:

    # dnf install postgresql-server
  2. Initialize the database cluster:

    # postgresql-setup --initdb
    * Initializing database in '/var/lib/pgsql/data'
    * Initialized, logs are in /var/lib/pgsql/initdb_postgresql.log
  3. Set the password hashing algorithm to scram-sha-256.

    1. In the /var/lib/pgsql/data/postgresql.conf file, change the following line:

      #password_encryption = md5              # md5 or scram-sha-256

      to:

      password_encryption = scram-sha-256
    2. In the /var/lib/pgsql/data/pg_hba.conf file, change the following line for the IPv4 local connections:

      host    all             all             127.0.0.1/32            ident

      to:

      host    all             all             127.0.0.1/32            scram-sha-256
  4. Start the postgresql service:

    # systemctl start postgresql.service
  5. Log in as the system user named postgres:

    # su - postgres
  6. Start the PostgreSQL interactive terminal:

    $ psql
    psql (13.7)
    Type "help" for help.
    
    postgres=#
  7. Optional: Obtain information about the current database connection:

    postgres=# \conninfo
    You are connected to database "postgres" as user "postgres" via socket in "/var/run/postgresql" at port "5432".
  8. Create a user named mydbuser, set a password for mydbuser, and assign mydbuser the CREATEROLE and CREATEDB permissions:

    postgres=# CREATE USER mydbuser WITH PASSWORD 'mypasswd' CREATEROLE CREATEDB;
    CREATE ROLE

    The mydbuser user now can perform routine database management operations: create databases and manage user indexes.

  9. Log out of the interactive terminal by using the \q meta command:

    postgres=# \q
  10. Log out of the postgres user session:

    $ logout
  11. Log in to the PostgreSQL terminal as mydbuser, specify the hostname, and connect to the default postgres database, which was created during initialization:

    # psql -U mydbuser -h 127.0.0.1 -d postgres
    Password for user mydbuser:
    Type the password.
    psql (13.7)
    Type "help" for help.
    
    postgres=>
  12. Create a database named mydatabase:

    postgres=> CREATE DATABASE mydatabase;
    CREATE DATABASE
    postgres=>
  13. Log out of the session:

    postgres=# \q
  14. Connect to mydatabase as mydbuser:

    # psql -U mydbuser -h 127.0.0.1 -d mydatabase
    Password for user mydbuser:
    psql (13.7)
    Type "help" for help.
    mydatabase=>
  15. Optional: Obtain information about the current database connection:

    mydatabase=> \conninfo
    You are connected to database "mydatabase" as user "mydbuser" on host "127.0.0.1" at port "5432".

4.3. Configuring PostgreSQL

In a PostgreSQL database, all data and configuration files are stored in a single directory called a database cluster. Red Hat recommends storing all data, including configuration files, in the default /var/lib/pgsql/data/ directory.

PostgreSQL configuration consists of the following files:

  • postgresql.conf - is used for setting the database cluster parameters.
  • postgresql.auto.conf - holds basic PostgreSQL settings similarly to postgresql.conf. However, this file is under the server control. It is edited by the ALTER SYSTEM queries, and cannot be edited manually.
  • pg_ident.conf - is used for mapping user identities from external authentication mechanisms into the PostgreSQL user identities.
  • pg_hba.conf - is used for configuring client authentication for PostgreSQL databases.

To change the PostgreSQL configuration, use the following procedure.

Procedure

  1. Edit the respective configuration file, for example, /var/lib/pgsql/data/postgresql.conf.
  2. Restart the postgresql service so that the changes become effective:

    # systemctl restart postgresql.service

Example 4.2. Configuring PostgreSQL database cluster parameters

This example shows basic settings of the database cluster parameters in the /var/lib/pgsql/data/postgresql.conf file.

# This is a comment
log_connections = yes
log_destination = 'syslog'
search_path = '"$user", public'
shared_buffers = 128MB
password_encryption = scram-sha-256

Example 4.3. Setting client authentication in PostgreSQL

This example demonstrates how to set client authentication in the /var/lib/pgsql/data/pg_hba.conf file.

# TYPE    DATABASE       USER        ADDRESS              METHOD
local     all            all                              trust
host      postgres       all         192.168.93.0/24      ident
host      all            all         .example.com         scram-sha-256

4.4. Configuring TLS encryption on a PostgreSQL server

By default, PostgreSQL uses unencrypted connections. For more secure connections, you can enable Transport Layer Security (TLS) support on the PostgreSQL server and configure your clients to establish encrypted connections.

Prerequisites

  • The PostgreSQL server is installed.
  • The database cluster is initialized.
  • If the server runs RHEL 9.2 or later and the FIPS mode is enabled, clients must either support the Extended Master Secret (EMS) extension or use TLS 1.3. TLS 1.2 connections without EMS fail. For more information, see the TLS extension "Extended Master Secret" enforced Knowledgebase article.

Procedure

  1. Install the OpenSSL library:

    # dnf install openssl
  2. Generate a TLS certificate and a key:

    # openssl req -new -x509 -days 365 -nodes -text -out server.crt \
      -keyout server.key -subj "/CN=dbhost.yourdomain.com"

    Replace dbhost.yourdomain.com with your database host and domain name.

  3. Copy your signed certificate and your private key to the required locations on the database server:

    # cp server.{key,crt} /var/lib/pgsql/data/.
  4. Change the owner and group ownership of the signed certificate and your private key to the postgres user:

    # chown postgres:postgres /var/lib/pgsql/data/server.{key,crt}
  5. Restrict the permissions for your private key so that it is readable only by the owner:

    # chmod 0400 /var/lib/pgsql/data/server.key
  6. Set the password hashing algorithm to scram-sha-256 by changing the following line in the /var/lib/pgsql/data/postgresql.conf file:

    #password_encryption = md5              # md5 or scram-sha-256

    to:

    password_encryption = scram-sha-256
  7. Configure PostgreSQL to use SSL/TLS by changing the following line in the /var/lib/pgsql/data/postgresql.conf file:

    #ssl = off

    to:

    ssl=on
  8. Restrict access to all databases to accept only connections from clients using TLS by changing the following line for the IPv4 local connections in the /var/lib/pgsql/data/pg_hba.conf file:

    host		all		all		127.0.0.1/32		ident

    to:

    hostssl 	all		all		127.0.0.1/32		scram-sha-256

    Alternatively, you can restrict access for a single database and a user by adding the following new line:

    hostssl	mydatabase	mydbuser	127.0.0.1/32		scram-sha-256

    Replace mydatabase with the database name and mydbuser with the username.

  9. Make the changes effective by restarting the postgresql service:

    # systemctl restart postgresql.service

Verification

  • To manually verify that the connection is encrypted:

    1. Connect to the PostgreSQL database as the mydbuser user, specify the hostname and the database name:

      $ psql -U mydbuser -h 127.0.0.1 -d mydatabase
      Password for user mydbuser:

      Replace mydatabase with the database name and mydbuser with the username.

    2. Obtain information about the current database connection:

      mydbuser=> \conninfo
      You are connected to database "mydatabase" as user "mydbuser" on host "127.0.0.1" at port "5432".
      SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384, bits: 256, compression: off)
  • You can write a simple application that verifies whether a connection to PostgreSQL is encrypted. This example demonstrates such an application written in C that uses the libpq client library, which is provided by the libpq-devel package:

    #include <stdio.h>
    #include <stdlib.h>
    #include <libpq-fe.h>
    
    int main(int argc, char* argv[])
    {
    //Create connection
    PGconn* connection = PQconnectdb("hostaddr=127.0.0.1 password=mypassword port=5432 dbname=mydatabase user=mydbuser");
    
    if (PQstatus(connection) ==CONNECTION_BAD)
        {
        printf("Connection error\n");
        PQfinish(connection);
        return -1; //Execution of the program will stop here
        }
        printf("Connection ok\n");
        //Verify TLS
        if (PQsslInUse(connection)){
         printf("TLS in use\n");
         printf("%s\n", PQsslAttribute(connection,"protocol"));
        }
        //End connection
        PQfinish(connection);
        printf("Disconnected\n");
        return 0;
    }

    Replace mypassword with the password, mydatabase with the database name, and mydbuser with the username.

    Note

    You must load the pq libraries for compilation by using the -lpq option. For example, to compile the application by using the GCC compiler:

    $ gcc source_file.c -lpq -o myapplication

    where the source_file.c contains the example code above, and myapplication is the name of your application for verifying secured PostgreSQL connection.

Example 4.4. Initializing, creating, and connecting to a PostgreSQL database using TLS encryption

This example demonstrates how to initialize a PostgreSQL database, create a database user and a database, and how to connect to the database using a secured connection.

  1. Install the PosgreSQL server:

    # dnf install postgresql-server
  2. Initialize the database cluster:

    # postgresql-setup --initdb
    * Initializing database in '/var/lib/pgsql/data'
    * Initialized, logs are in /var/lib/pgsql/initdb_postgresql.log
  3. Install the OpenSSL library:

    # dnf install openssl
  4. Generate a TLS certificate and a key:

    # openssl req -new -x509 -days 365 -nodes -text -out server.crt \
      -keyout server.key -subj "/CN=dbhost.yourdomain.com"

    Replace dbhost.yourdomain.com with your database host and domain name.

  5. Copy your signed certificate and your private key to the required locations on the database server:

    # cp server.{key,crt} /var/lib/pgsql/data/.
  6. Change the owner and group ownership of the signed certificate and your private key to the postgres user:

    # chown postgres:postgres /var/lib/pgsql/data/server.{key,crt}
  7. Restrict the permissions for your private key so that it is readable only by the owner:

    # chmod 0400 /var/lib/pgsql/data/server.key
  8. Set the password hashing algorithm to scram-sha-256. In the /var/lib/pgsql/data/postgresql.conf file, change the following line:

    #password_encryption = md5              # md5 or scram-sha-256

    to:

    password_encryption = scram-sha-256
  9. Configure PostgreSQL to use SSL/TLS. In the /var/lib/pgsql/data/postgresql.conf file, change the following line:

    #ssl = off

    to:

    ssl=on
  10. Start the postgresql service:

    # systemctl start postgresql.service
  11. Log in as the system user named postgres:

    # su - postgres
  12. Start the PostgreSQL interactive terminal as the postgres user:

    $ psql -U postgres
    psql (13.7)
    Type "help" for help.
    
    postgres=#
  13. Create a user named mydbuser and set a password for mydbuser:

    postgres=# CREATE USER mydbuser WITH PASSWORD 'mypasswd';
    CREATE ROLE
    postgres=#
  14. Create a database named mydatabase:

    postgres=# CREATE DATABASE mydatabase;
    CREATE DATABASE
    postgres=#
  15. Grant all permissions to the mydbuser user:

    postgres=# GRANT ALL PRIVILEGES ON DATABASE mydatabase TO mydbuser;
    GRANT
    postgres=#
  16. Log out of the interactive terminal:

    postgres=# \q
  17. Log out of the postgres user session:

    $ logout
  18. Restrict access to all databases to accept only connections from clients using TLS by changing the following line for the IPv4 local connections in the /var/lib/pgsql/data/pg_hba.conf file:

    host		all		all		127.0.0.1/32		ident

    to:

    hostssl 	all		all		127.0.0.1/32		scram-sha-256
  19. Make the changes effective by restarting the postgresql service:

    # systemctl restart postgresql.service
  20. Connect to the PostgreSQL database as the mydbuser user, specify the hostname and the database name:

    $ psql -U mydbuser -h 127.0.0.1 -d mydatabase
    Password for user mydbuser:
    psql (13.7)
    SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384, bits: 256, compression: off)
    Type "help" for help.
    
    mydatabase=>

4.5. Backing up PostgreSQL data

To back up PostgreSQL data, use one of the following approaches:

SQL dump
See Backing up with SQL dump.
File system level backup
See File system level backup.
Continuous archiving
See Continuous archiving.

4.5.1. Backing up PostgreSQL data with an SQL dump

The SQL dump method is based on generating a dump file with SQL commands. When a dump is uploaded back to the database server, it recreates the database in the same state as it was at the time of the dump.

The SQL dump is ensured by the following PostgreSQL client applications:

  • pg_dump dumps a single database without cluster-wide information about roles or tablespaces
  • pg_dumpall dumps each database in a given cluster and preserves cluster-wide data, such as role and tablespace definitions.

By default, the pg_dump and pg_dumpall commands write their results into the standard output. To store the dump in a file, redirect the output to an SQL file. The resulting SQL file can be either in a text format or in other formats that allow for parallelism and for more detailed control of object restoration.

You can perform the SQL dump from any remote host that has access to the database.

4.5.1.1. Advantages and disadvantages of an SQL dump

An SQL dump has the following advantages compared to other PostgreSQL backup methods:

  • An SQL dump is the only PostgreSQL backup method that is not server version-specific. The output of the pg_dump utility can be reloaded into later versions of PostgreSQL, which is not possible for file system level backups or continuous archiving.
  • An SQL dump is the only method that works when transferring a database to a different machine architecture, such as going from a 32-bit to a 64-bit server.
  • An SQL dump provides internally consistent dumps. A dump represents a snapshot of the database at the time pg_dump began running.
  • The pg_dump utility does not block other operations on the database when it is running.

A disadvantage of an SQL dump is that it takes more time compared to file system level backup.

4.5.1.2. Performing an SQL dump using pg_dump

To dump a single database without cluster-wide information, use the pg_dump utility.

Prerequisites

  • You must have read access to all tables that you want to dump. To dump the entire database, you must run the commands as the postgres superuser or a user with database administrator privileges.

Procedure

  • Dump a database without cluster-wide information:

    $ pg_dump dbname > dumpfile

To specify which database server pg_dump will contact, use the following command-line options:

  • The -h option to define the host.

    The default host is either the local host or what is specified by the PGHOST environment variable.

  • The -p option to define the port.

    The default port is indicated by the PGPORT environment variable or the compiled-in default.

4.5.1.3. Performing an SQL dump using pg_dumpall

To dump each database in a given database cluster and to preserve cluster-wide data, use the pg_dumpall utility.

Prerequisites

  • You must run the commands as the postgres superuser or a user with database administrator privileges.

Procedure

  • Dump all databases in the database cluster and preserve cluster-wide data:

    $ pg_dumpall > dumpfile

To specify which database server pg_dumpall will contact, use the following command-line options:

  • The -h option to define the host.

    The default host is either the local host or what is specified by the PGHOST environment variable.

  • The -p option to define the port.

    The default port is indicated by the PGPORT environment variable or the compiled-in default.

  • The -l option to define the default database.

    This option enables you to choose a default database different from the postgres database created automatically during initialization.

4.5.1.4. Restoring a database dumped using pg_dump

To restore a database from an SQL dump that you dumped using the pg_dump utility, follow the steps below.

Prerequisites

  • You must run the commands as the postgres superuser or a user with database administrator privileges.

Procedure

  1. Create a new database:

    $ createdb dbname
  2. Verify that all users who own objects or were granted permissions on objects in the dumped database already exist. If such users do not exist, the restore fails to recreate the objects with the original ownership and permissions.
  3. Run the psql utility to restore a text file dump created by the pg_dump utility:

    $ psql dbname < dumpfile

    where dumpfile is the output of the pg_dump command. To restore a non-text file dump, use the pg_restore utility instead:

    $ pg_restore non-plain-text-file

4.5.1.5. Restoring databases dumped using pg_dumpall

To restore data from a database cluster that you dumped using the pg_dumpall utility, follow the steps below.

Prerequisites

  • You must run the commands as the postgres superuser or a user with database administrator privileges.

Procedure

  1. Ensure that all users who own objects or were granted permissions on objects in the dumped databases already exist. If such users do not exist, the restore fails to recreate the objects with the original ownership and permissions.
  2. Run the psql utility to restore a text file dump created by the pg_dumpall utility:

    $ psql < dumpfile

    where dumpfile is the output of the pg_dumpall command.

4.5.1.6. Performing an SQL dump of a database on another server

Dumping a database directly from one server to another is possible because pg_dump and psql can write to and read from pipes.

Procedure

  • To dump a database from one server to another, run:

    $ pg_dump -h host1 dbname | psql -h host2 dbname

4.5.1.7. Handling SQL errors during restore

By default, psql continues to execute if an SQL error occurs, causing the database to restore only partially.

To change the default behavior, use one of the following approaches when restoring a dump.

Prerequisites

  • You must run the commands as the postgres superuser or a user with database administrator privileges.

Procedure

  • Make psql exit with an exit status of 3 if an SQL error occurs by setting the ON_ERROR_STOP variable:

    $ psql --set ON_ERROR_STOP=on dbname < dumpfile
  • Specify that the whole dump is restored as a single transaction so that the restore is either fully completed or canceled.

    • When restoring a text file dump using the psql utility:

      $ psql -1
    • When restoring a non-text file dump using the pg_restore utility:

      $ pg_restore -e

      Note that when using this approach, even a minor error can cancel a restore operation that has already run for many hours.

4.5.1.8. Additional resources

4.5.2. Backing up PostgreSQL data with a file system level backup

To create a file system level backup, copy PostgreSQL database files to another location. For example, you can use any of the following approaches:

  • Create an archive file using the tar utility.
  • Copy the files to a different location using the rsync utility.
  • Create a consistent snapshot of the data directory.

4.5.2.1. Advantages and limitations of file system backing up

File system level backing up has the following advantage compared to other PostgreSQL backup methods:

  • File system level backing up is usually faster than an SQL dump.

File system level backing up has the following limitations compared to other PostgreSQL backup methods:

  • This backing up method is not suitable when you want to upgrade from RHEL 8 to RHEL 9 and migrate your data to the upgraded system. File system level backup is specific to an architecture and a RHEL major version. You can restore your data on your RHEL 8 system if the upgrade is not successful but you cannot restore the data on a RHEL 9 system.
  • The database server must be shut down before backing up and restoring data.
  • Backing up and restoring certain individual files or tables is impossible. Backing up a file system works only for complete backing up and restoring of an entire database cluster.

4.5.2.2. Performing file system level backing up

To perform file system level backing up, use the following procedure.

Procedure

  1. Choose the location of a database cluster and initialize this cluster:

    # postgresql-setup --initdb
  2. Stop the postgresql service:

    # systemctl stop postgresql.service
  3. Use any method to create a file system backup, for example a tar archive:

    $ tar -cf backup.tar /var/lib/pgsql/data
  4. Start the postgresql service:

    # systemctl start postgresql.service

4.5.3. Backing up PostgreSQL data by continuous archiving

4.5.3.1. Introduction to continuous archiving

PostgreSQL records every change made to the database’s data files into a write ahead log (WAL) file that is available in the pg_wal/ subdirectory of the cluster’s data directory. This log is intended primarily for a crash recovery. After a crash, the log entries made since the last checkpoint can be used for restoring the database to a consistency.

The continuous archiving method, also known as an online backup, combines the WAL files with a copy of the database cluster in the form of a base backup performed on a running server or a file system level backup.

If a database recovery is needed, you can restore the database from the copy of the database cluster and then replay log from the backed up WAL files to bring the system to the current state.

With the continuous archiving method, you must keep a continuous sequence of all archived WAL files that extends at minimum back to the start time of your last base backup. Therefore the ideal frequency of base backups depends on:

  • The storage volume available for archived WAL files.
  • The maximum possible duration of data recovery in situations when recovery is necessary. In cases with a long period since the last backup, the system replays more WAL segments, and the recovery therefore takes more time.
Note

You cannot use pg_dump and pg_dumpall SQL dumps as a part of a continuous archiving backup solution. SQL dumps produce logical backups and do not contain enough information to be used by a WAL replay.

To perform a database backup and restore using the continuous archiving method, follow these instructions:

  1. Set up and test your procedure for archiving WAL files - see WAL archiving.
  2. Perform a base backup - see base backup.

To restore your data, follow instructions in Restoring database with continuous archiving.

4.5.3.2. Advantages and disadvantages of continuous archiving

Continuous archiving has the following advantages compared to other PostgreSQL backup methods:

  • With the continuous backup method, it is possible to use a base backup that is not entirely consistent because any internal inconsistency in the backup is corrected by the log replay. Therefore you can perform a base backup on a running PostgreSQL server.
  • A file system snapshot is not needed; tar or a similar archiving utility is sufficient.
  • Continuous backup can be achieved by continuing to archive the WAL files because the sequence of WAL files for the log replay can be indefinitely long. This is particularly valuable for large databases.
  • Continuous backup supports point-in-time recovery. It is not necessary to replay the WAL entries to the end. The replay can be stopped at any point and the database can be restored to its state at any time since the base backup was taken.
  • If the series of WAL files are continuously available to another machine that has been loaded with the same base backup file, it is possible to restore the other machine with a nearly-current copy of the database at any point.

Continuous archiving has the following disadvantages compared to other PostgreSQL backup methods:

  • Continuous backup method supports only restoration of an entire database cluster, not a subset.
  • Continuous backup requires extensive archival storage.

4.5.3.3. Setting up WAL archiving

A running PostgreSQL server produces a sequence of write ahead log (WAL) records. The server physically divides this sequence into WAL segment files, which are given numeric names that reflect their position in the WAL sequence. Without WAL archiving, the segment files are reused and renamed to higher segment numbers.

When archiving WAL data, the contents of each segment file are captured and saved at a new location before the segment file is reused. You have multiple options where to save the content, such as an NFS-mounted directory on another machine, a tape drive, or a CD.

Note that WAL records do not include changes to configuration files.

To enable WAL archiving, use the following procedure.

Procedure

  1. In the /var/lib/pgsql/data/postgresql.conf file:

    1. Set the wal_level configuration parameter to replica or higher.
    2. Set the archive_mode parameter to on.
    3. Specify the shell command in the archive_command configuration parameter. You can use the cp command, another command, or a shell script.
  2. Restart the postgresql service to enable the changes:

    # systemctl restart postgresql.service
  3. Test your archive command and ensure it does not overwrite an existing file and that it returns a nonzero exit status if it fails.
  4. To protect your data, ensure that the segment files are archived into a directory that does not have group or world read access.
Note

The archive command is executed only on completed WAL segments. A server that generates little WAL traffic can have a substantial delay between the completion of a transaction and its safe recording in archive storage. To limit how old unarchived data can be, you can:

  • Set the archive_timeout parameter to force the server to switch to a new WAL segment file with a given frequency.
  • Use the pg_switch_wal parameter to force a segment switch to ensure that a transaction is archived immediately after it finishes.

Example 4.5. Shell command for archiving WAL segments

This example shows a simple shell command you can set in the archive_command configuration parameter.

The following command copies a completed segment file to the required location:

archive_command = 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%f'

where the %p parameter is replaced by the relative path to the file to archive and the %f parameter is replaced by the file name.

This command copies archivable WAL segments to the /mnt/server/archivedir/ directory. After replacing the %p and %f parameters, the executed command looks as follows:

test ! -f /mnt/server/archivedir/00000001000000A900000065 && cp pg_wal/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065

A similar command is generated for each new file that is archived.

Additional resources

4.5.3.4. Making a base backup

You can create a base backup in several ways. The simplest way of performing a base backup is using the pg_basebackup utility on a running PostgreSQL server.

The base backup process creates a backup history file that is stored into the WAL archive area and is named after the first WAL segment file that you need for the base backup.

The backup history file is a small text file containing the starting and ending times, and WAL segments of the backup. If you used the label string to identify the associated dump file, you can use the backup history file to determine which dump file to restore.

Note

Consider keeping several backup sets to be certain that you can recover your data.

To perform a base backup, use the following procedure.

Prerequisites

  • You must run the commands as the postgres superuser, a user with database administrator privileges, or another user with at least REPLICATION permissions.
  • You must keep all the WAL segment files generated during and after the base backup.

Procedure

  1. Use the pg_basebackup utility to perform the base backup.

    • To create a base backup as individual files (plain format):

      $ pg_basebackup -D backup_directory -Fp

      Replace backup_directory with your chosen backup location.

      If you use tablespaces and perform the base backup on the same host as the server, you must also use the --tablespace-mapping option, otherwise the backup will fail upon an attempt to write the backup to the same location.

    • To create a base backup as a tar archive (tar and compressed format):

      $ pg_basebackup -D backup_directory -Ft -z

      Replace backup_directory with your chosen backup location.

      To restore such data, you must manually extract the files in the correct locations.

  2. After the base backup process is complete, safely archive the copy of the database cluster and the WAL segment files used during the backup, which are specified in the backup history file.
  3. Delete WAL segments numerically lower than the WAL segment files used in the base backup because these are older than the base backup and no longer needed for a restore.

To specify which database server pg_basebackup will contact, use the following command-line options:

  • The -h option to define the host.

    The default host is either the local host or a host specified by the PGHOST environment variable.

  • The -p option to define the port.

    The default port is indicated by the PGPORT environment variable or the compiled-in default.

4.5.3.5. Restoring the database using a continuous archive backup

To restore a database using a continuous backup, use the following procedure.

Procedure

  1. Stop the server:

    # systemctl stop postgresql.service
  2. Copy the necessary data to a temporary location.

    Preferably, copy the whole cluster data directory and any tablespaces. Note that this requires enough free space on your system to hold two copies of your existing database.

    If you do not have enough space, save the contents of the cluster’s pg_wal directory, which can contain logs that were not archived before the system went down.

  3. Remove all existing files and subdirectories under the cluster data directory and under the root directories of any tablespaces you are using.
  4. Restore the database files from your base backup.

    Ensure that:

    • The files are restored with the correct ownership (the database system user, not root).
    • The files are restored with the correct permissions.
    • The symbolic links in the pg_tblspc/ subdirectory are restored correctly.
  5. Remove any files present in the pg_wal/ subdirectory.

    These files resulted from the base backup and are therefore obsolete. If you did not archive pg_wal/, recreate it with proper permissions.

  6. Copy any unarchived WAL segment files that you saved in step 2 into pg_wal/.
  7. Create the recovery.conf recovery command file in the cluster data directory and specify the shell command in the restore_command configuration parameter. You can use the cp command, another command, or a shell script. For example:

    restore_command = 'cp /mnt/server/archivedir/%f "%p"'
  8. Start the server:

    # systemctl start postgresql.service

    The server will enter the recovery mode and proceed to read through the archived WAL files that it needs.

    If the recovery is terminated due to an external error, the server can be restarted and it will continue the recovery. When the recovery process is completed, the server renames recovery.conf to recovery.done. This prevents the server from accidental re-entering the recovery mode after it starts normal database operations.

  9. Check the contents of the database to verify that the database has recovered into the required state.

    If the database has not recovered into the required state, return to step 1. If the database has recovered into the required state, allow the users to connect by restoring the client authentication configuration in the pg_hba.conf file.

For more information about restoring using the continuous backup, see PostgreSQL Documentation.

4.5.3.6. Additional resources

4.6. Migrating to a RHEL 9 version of PostgreSQL

Red Hat Enterprise Linux 8 provides PostgreSQL in multiple module streams: PostgreSQL 10 (the default postgresql stream), PostgreSQL 9.6, PostgreSQL 12, PostgreSQL 13, PostgreSQL 15, and PostgreSQL 16.

In RHEL 9, PostgreSQL 13, PostgreSQL 15, and PostgreSQL 16 are available.

On RHEL, you can use two PostgreSQL migration paths for the database files:

The fast upgrade method is quicker than the dump and restore process. However, in certain cases, the fast upgrade does not work, and you can only use the dump and restore process, for example in case of cross-architecture upgrades.

As a prerequisite for migration to a later version of PostgreSQL, back up all your PostgreSQL databases.

Dumping the databases and performing backup of the SQL files is required for the dump and restore process and recommended for the fast upgrade method.

Before migrating to a later version of PostgreSQL, see the upstream compatibility notes for the version of PostgreSQL to which you want to migrate, and for all skipped PostgreSQL versions between the one you are migrating from and the target version.

4.6.1. Notable differences between PostgreSQL 15 and PostgreSQL 16

PostgreSQL 16 introduced the following notable changes.

The postmasters binary is no longer available

PostgreSQL is no longer distributed with the postmaster binary. Users who start the postgresql server by using the provided systemd unit file (the systemctl start postgres command) are not affected by this change. If you previously started the postgresql server directly through the postmaster binary, you must now use the postgres binary instead.

Documentation is no longer packaged

PostgreSQL no longer provides documentation in PDF format within the package. Use the online documentation instead.

4.6.2. Notable differences between PostgreSQL 13 and PostgreSQL 15

PostgreSQL 15 introduced the following backwards incompatible changes.

Default permissions of the public schema

The default permissions of the public schema have been modified in PostgreSQL 15. Newly created users need to grant permission explicitly by using the GRANT ALL ON SCHEMA public TO myuser; command.

The following example works in PostgreSQL 13 and earlier:

postgres=# CREATE USER mydbuser;
postgres=# \c postgres mydbuser
postgres=$ CREATE TABLE mytable (id int);

The following example works in PostgreSQL 15 and later:

postgres=# CREATE USER mydbuser;
postgres=# GRANT ALL ON SCHEMA public TO mydbuser;
postgres=# \c postgres mydbuser
postgres=$ CREATE TABLE mytable (id int);
Note

Ensure that the mydbuser access is configured appropriately in the pg_hba.conf file. See Creating PostgreSQL users for more information.

PQsendQuery() no longer supported in pipeline mode

Since PostgreSQL 15, the libpq PQsendQuery() function is no longer supported in pipeline mode. Modify affected applications to use the PQsendQueryParams() function instead.

4.6.3. Fast upgrade using the pg_upgrade utility

As a system administrator, you can upgrade to the most recent version of PostgreSQL by using the fast upgrade method. To perform a fast upgrade, copy binary data files to the /var/lib/pgsql/data/ directory and use the pg_upgrade utility.

You can use this method for migrating data:

  • From the RHEL 8 version of PostgreSQL 12 to a RHEL version of PostgreSQL 13
  • From a RHEL 8 or 9 version of PostgreSQL 13 to a RHEL version of PostgreSQL 15
  • From a RHEL 8 or 9 version of PostgreSQL 15 to a RHEL version of PostgreSQL 16

The following procedure describes migration from the RHEL 8 version of PostgreSQL 12 to the RHEL 9 version of PostgreSQL 13 using the fast upgrade method. For migration from postgresql streams other than 12, use one of the following approaches:

  • Update your PostgreSQL server to version 12 on RHEL 8 and then use the pg_upgrade utility to perform the fast upgrade to RHEL 9 version of PostgreSQL 13.
  • Use the dump and restore upgrade directly between any RHEL 8 version of PostgreSQL and an equal or later PostgreSQL version in RHEL 9.

Prerequisites

  • Before performing the upgrade, back up all your data stored in the PostgreSQL databases. By default, all data is stored in the /var/lib/pgsql/data/ directory on both the RHEL 8 and RHEL 9 systems.

Procedure

  1. On the RHEL 9 system, install the postgresql-server and postgresql-upgrade packages:

    # dnf install postgresql-server postgresql-upgrade

    Optionally, if you used any PostgreSQL server modules on RHEL 8, install them also on the RHEL 9 system in two versions, compiled both against PostgreSQL 12 (installed as the postgresql-upgrade package) and the target version of PostgreSQL 13 (installed as the postgresql-server package). If you need to compile a third-party PostgreSQL server module, build it both against the postgresql-devel and postgresql-upgrade-devel packages.

  2. Check the following items:

    • Basic configuration: On the RHEL 9 system, check whether your server uses the default /var/lib/pgsql/data directory and the database is correctly initialized and enabled. In addition, the data files must be stored in the same path as mentioned in the /usr/lib/systemd/system/postgresql.service file.
    • PostgreSQL servers: Your system can run multiple PostgreSQL servers. Ensure that the data directories for all these servers are handled independently.
    • PostgreSQL server modules: Ensure that the PostgreSQL server modules that you used on RHEL 8 are installed on your RHEL 9 system as well. Note that plugins are installed in the /usr/lib64/pgsql/ directory.
  3. Ensure that the postgresql service is not running on either of the source and target systems at the time of copying data.

    # systemctl stop postgresql.service
  4. Copy the database files from the source location to the /var/lib/pgsql/data/ directory on the RHEL 9 system.
  5. Perform the upgrade process by running the following command as the PostgreSQL user:

    # postgresql-setup --upgrade

    This launches the pg_upgrade process in the background.

    In case of failure, postgresql-setup provides an informative error message.

  6. Copy the prior configuration from /var/lib/pgsql/data-old to the new cluster.

    Note that the fast upgrade does not reuse the prior configuration in the newer data stack and the configuration is generated from scratch. If you want to combine the old and new configurations manually, use the *.conf files in the data directories.

  7. Start the new PostgreSQL server:

    # systemctl start postgresql.service
  8. Analyze the new database cluster.

    • For PostgreSQL 13:

      su postgres -c '~/analyze_new_cluster.sh'
    • For PostgreSQL 15 or later:

      su postgres -c 'vacuumdb --all --analyze-in-stages'
      Note

      You may need to use ALTER COLLATION name REFRESH VERSION, see the upstream documentation for details.

  9. If you want the new PostgreSQL server to be automatically started on boot, run:

    # systemctl enable postgresql.service

4.6.4. Dump and restore upgrade

When using the dump and restore upgrade, you must dump all databases contents into an SQL file dump file. Note that the dump and restore upgrade is slower than the fast upgrade method and it may require some manual fixing in the generated SQL file.

You can use this method for migrating data from any RHEL 8 version of PostgreSQL to any equal or later version of PostgreSQL in RHEL 9.

On RHEL 8 and RHEL 9 systems, PostgreSQL data is stored in the /var/lib/pgsql/data/ directory by default.

To perform the dump and restore upgrade, change the user to root.

The following procedure describes migration from the RHEL 8 default version of PostgreSQL 10 to the RHEL 9 version of PostgreSQL 13.

Procedure

  1. On your RHEL 8 system, start the PostgreSQL 10 server:

    # systemctl start postgresql.service
  2. On the RHEL 8 system, dump all databases contents into the pgdump_file.sql file:

    su - postgres -c "pg_dumpall > ~/pgdump_file.sql"
  3. Ensure that the databases were dumped correctly:

    su - postgres -c 'less "$HOME/pgdump_file.sql"'

    As a result, the path to the dumped sql file is displayed: /var/lib/pgsql/pgdump_file.sql.

  4. On the RHEL 9 system, install the postgresql-server package:

    # dnf install postgresql-server

    Optionally, if you used any PostgreSQL server modules on RHEL 8, install them also on the RHEL 9 system. If you need to compile a third-party PostgreSQL server module, build it against the postgresql-devel package.

  5. On the RHEL 9 system, initialize the data directory for the new PostgreSQL server:

    # postgresql-setup --initdb
  6. On the RHEL 9 system, copy the pgdump_file.sql into the PostgreSQL home directory, and check that the file was copied correctly:

    su - postgres -c 'test -e "$HOME/pgdump_file.sql" && echo exists'
  7. Copy the configuration files from the RHEL 8 system:

    su - postgres -c 'ls -1 $PGDATA/*.conf'

    The configuration files to be copied are:

    • /var/lib/pgsql/data/pg_hba.conf
    • /var/lib/pgsql/data/pg_ident.conf
    • /var/lib/pgsql/data/postgresql.conf
  8. On the RHEL 9 system, start the new PostgreSQL server:

    # systemctl start postgresql.service
  9. On the RHEL 9 system, import data from the dumped sql file:

    su - postgres -c 'psql -f ~/pgdump_file.sql postgres'
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