What PostgreSQL command-line utility backs up PostgreSQL database settings

By default, Xpress Insight uses a PostgreSQL 9.5 database server included with the distribution.

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4.1. Getting started with PostgreSQL
4.2. Installing PostgreSQL
4.3. Creating PostgreSQL users
4.4. Configuring PostgreSQL
4.5. Configuring TLS encryption on a PostgreSQL server
4.6. Backing up PostgreSQL data
4.6.1. Backing up PostgreSQL data with an SQL dump
4.6.2. Backing up PostgreSQL data with a file system level backup
4.6.3. Backing up PostgreSQL data by continuous archiving
4.7. Migrating to a RHEL 9 version of PostgreSQL
4.7.1. Fast upgrade using the pg_upgrade utility
4.7.2. Dump and restore upgrade
What is PostgreSQL command line tools?
What are the different approaches to backing up PostgreSQL data?
Which command is used to the list of databases in PostgreSQL?
What is the difference between pg_dump and Pg_dumpall?

In addition to understanding the notes in this section, for enterprise-class deployments of Xpress Insight, system administrators should be proficient in the use of the PostgreSQL command line tools for the backup and restoration of PostgreSQL database. Additionally, an enterprise-class backup system should include a backup rotation policy. Such further guidance is outside the scope of this document.

Note The following instructions assume that PostgreSQL user authentication is disabled. If enabled, the following steps will need to be modified to incorporate the password of the PostgreSQL user account - this can be achieved by setting the PGPASSWORD environment variable.

Extensive options for PostgreSQL backup are described in the online documentation for the database: https://www.postgresql.org/docs/9.5/static/backup-dump.html.

The following steps describe the actions necessary to implement backup and restore using the pg_dump and pg_restore utilitity tools provided. FICO is unable to provide support for alternative methods of backing up and restoring the PostgreSQL database.

The PostgreSQL server is an open source robust and highly-extensible database server based on the SQL language. This part describes 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. Getting started with PostgreSQL

The PostgreSQL server provides an object-relational database system, which allows you to manage extensive datasets and a high number of concurrent users. For these reasons, the 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 building applications. It allows users to extend a database with users' own data types, custom functions, or code from different programming languages without the need to recompile the database.

This part describes:

How to install PostgreSQL in Installing PostgreSQL.
Users, roles, and privileges in Creating PostgreSQL users.
How to adjust PostgreSQL configuration in Configuring PostgreSQL.
How to enable Transport Layer Security (TLS) support on the PostgreSQL server in Configuring TLS encryption on a PostgreSQL server.
How to back up your databases in Backing up PostgreSQL data.
How to migrate to the RHEL 9 version of PostgreSQL 13 in Migrating to a RHEL 9 version of PostgreSQL. One of the prerequisites of migration is performing a data backup.

4.2. Installing PostgreSQL

RHEL 9.0 provides PostgreSQL 13 as the initial version of this Application Stream, which can be installed easily as an RPM package. Additional PostgreSQL versions will be provided as modules with a shorter life cycle in the future minor releases of RHEL 9.

To install PostgreSQL, use the following procedure.

Procedure

Install the PostgreSQL server packages:
```
# dnf install postgresql-server
```
The postgres superuser is created automatically.
Initialize the database cluster:
```
# postgresql-setup --initdb
```
Red Hat recommends storing the data in the default /var/lib/pgsql/data directory.
Start the postgresql service:
```
# systemctl start postgresql.service
```
Enable the postgresql service to start at boot:
```
# systemctl enable postgresql.service
```

4.3. 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.

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.

Install the PosgreSQL server:
```
# dnf install postgresql-server
```

Initialize the database cluster:

# postgresql-setup --initdb
* Initializing database in '/var/lib/pgsql/data'
* Initialized, logs are in /var/lib/pgsql/initdb_postgresql.log

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

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

Start the postgresql service:
```
# systemctl start postgresql.service
```
Log in as the system user named postgres:
```
# su - postgres
```

Start the PostgreSQL interactive terminal:

$ psql
psql (13.7)
Type "help" for help.

postgres=#

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".

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.
Log out of the interactive terminal by using the \q meta command:
```
postgres=# \q
```
Log out of the postgres user session:
```
$ logout
```
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=>
```

Create a database named mydatabase:

postgres=> CREATE DATABASE mydatabase;
CREATE DATABASE
postgres=>

Log out of the session:
```
postgres=# \q
```

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=>

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".

4.4. 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

Edit the respective configuration file, for example, /var/lib/pgsql/data/postgresql.conf.
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

4.5. 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.

Procedure

Install the OpenSSL library:
```
# dnf install openssl
```

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.

Copy your signed certificate and your private key to the required locations on the database server:
```
# cp server.{key,crt} /var/lib/pgsql/data/.
```
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}
```
Restrict the permissions for your private key so that it is readable only by the owner:
```
# chmod 0400 /var/lib/pgsql/data/server.key
```
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
```
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
```
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.
Make the changes effective by restarting the postgresql service:
```
# systemctl restart postgresql.service
```

Verification

To manually verify that the connection is encrypted:

Connect to the PostgreSQL database as the mydbuser user, specify the host name 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.

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 
#include 
#include 

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.

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.

Install the PosgreSQL server:
```
# dnf install postgresql-server
```

Initialize the database cluster:

# postgresql-setup --initdb
* Initializing database in '/var/lib/pgsql/data'
* Initialized, logs are in /var/lib/pgsql/initdb_postgresql.log

Install the OpenSSL library:
```
# dnf install openssl
```

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.

Copy your signed certificate and your private key to the required locations on the database server:
```
# cp server.{key,crt} /var/lib/pgsql/data/.
```
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}
```
Restrict the permissions for your private key so that it is readable only by the owner:
```
# chmod 0400 /var/lib/pgsql/data/server.key
```
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
```
Configure PostgreSQL to use SSL/TLS. In the /var/lib/pgsql/data/postgresql.conf file, change the following line:
```
#ssl = off
```
to:
```
ssl=on
```
Start the postgresql service:
```
# systemctl start postgresql.service
```
Log in as the system user named postgres:
```
# su - postgres
```

Start the PostgreSQL interactive terminal as the postgres user:

$ psql -U postgres
psql (13.7)
Type "help" for help.

postgres=#

Create a user named mydbuser and set a password for mydbuser:

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

Create a database named mydatabase:

postgres=# CREATE DATABASE mydatabase;
CREATE DATABASE
postgres=#

Grant all permissions to the mydbuser user:

postgres=# GRANT ALL PRIVILEGES ON DATABASE mydatabase TO mydbuser;
GRANT
postgres=#

Log out of the interactive terminal:
```
postgres=# \q
```
Log out of the postgres user session:
```
$ logout
```
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
```
Make the changes effective by restarting the postgresql service:
```
# systemctl restart postgresql.service
```

Connect to the PostgreSQL database as the mydbuser user, specify the host name 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=>

4.6. Backing up PostgreSQL data

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

4.6.1. 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.

4.6.1.1. 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.

4.6.1.2. 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.

4.6.1.3. 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.

4.6.1.4. 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

Create a new database:
```
$ createdb dbname
```
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.
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
```

4.6.1.5. 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

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.
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.

4.6.1.6. 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
```

4.6.1.7. 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.

4.6.1.8. Additional resources

PostgreSQL Documentation - SQL dump

4.6.2. 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.

4.6.2.1. 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.

4.6.2.2. Performing file system level backing up

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

Procedure

Choose the location of a database cluster and initialize this cluster:
```
# postgresql-setup --initdb
```
Stop the postgresql service:
```
# systemctl stop postgresql.service
```
Use any method to create a file system backup, for example a tar archive:
```
$ tar -cf backup.tar /var/lib/pgsql/data
```
Start the postgresql service:
```
# systemctl start postgresql.service
```

4.6.3. Backing up PostgreSQL data by continuous archiving

4.6.3.1. 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.

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:

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

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

4.6.3.2. 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.

4.6.3.3. 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

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.
Restart the postgresql service to enable the changes:
```
# systemctl restart postgresql.service
```
Test your archive command and ensure it does not overwrite an existing file and that it returns a non-zero exit status if it fails.
To protect your data, ensure that the segment files are archived into a directory that does not have group or world read access.

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.

4.6.3.4. Making a base backup

You can create a base backup in several ways. This section describes the simplest way of performing a base backup 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.

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

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

4.6.3.5. Restoring the database using a continuous archive backup

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

Procedure

Stop the server:
```
# systemctl stop postgresql.service
```
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.
Remove all existing files and subdirectories under the cluster data directory and under the root directories of any tablespaces you are using.
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.
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.
Copy any unarchived WAL segment files that you saved in step 2 into pg_wal/.
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"'
```
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.
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.

4.6.3.6. Additional resources

Continuous archiving method

4.7. 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, and PostgreSQL 13. In RHEL 9, PostgreSQL 13 is available.

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

Fast upgrade using the pg_upgrade utility
Dump and restore upgrade

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, as well as for all skipped PostgreSQL versions between the one you are migrating from and the target version.

4.7.1. Fast upgrade using the pg_upgrade utility

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

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. See Migrating to a RHEL 9 version of PostgreSQL for more information.
Use the dump and restore upgrade directly between any RHEL 8 version of PostgreSQL and PostgreSQL 13 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

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.
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 plug-ins are installed in the /usr/lib64/pgsql/ directory.
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
```
Copy the database files from the source location to the /var/lib/pgsql/data/ directory on the RHEL 9 system.
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.
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.
Start the new PostgreSQL server:
```
# systemctl start postgresql.service
```
Run the analyze_new_cluster.sh script located in the PostgreSQL home directory:
```
su postgres -c '~/analyze_new_cluster.sh'
```
If you want the new PostgreSQL server to be automatically started on boot, run:
```
# systemctl enable postgresql.service
```

4.7.2. 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 the RHEL 9 version of PostgreSQL 13.

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 Postgreql 10 to the RHEL 9 version of PostgreSQL 13.

Procedure

On your RHEL 8 system, start the PostgreSQL 10 server:
```
# systemctl start postgresql.service
```
On the RHEL 8 system, dump all databases contents into the pgdump_file.sql file:
```
su - postgres -c "pg_dumpall > ~/pgdump_file.sql"
```
Ensure that the databases were dumped correctly:
```
su - postgres -c 'less "$HOME/pgdump_file.sql"'
```
As a result, the path to the dumped sql file is displayed: /var/lib/pgsql/pgdump_file.sql.
On the RHEL 9 system, install the postgresql-server package:
```
# dnf install postgresql-server
```
Optionally, if you used any PostgreSQL server modules on RHEL 8, install them also on the RHEL 9 system. If you need to compile a third-party PostgreSQL server module, build it against the postgresql-devel package.
On the RHEL 9 system, initialize the data directory for the new PostgreSQL server:
```
# postgresql-setup --initdb
```
On the RHEL 9 system, copy the pgdump_file.sql into the PostgreSQL home directory, and check that the file was copied correctly:
```
su - postgres -c 'test -e "$HOME/pgdump_file.sql" && echo exists'
```
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
On the RHEL 9 system, start the new PostgreSQL server:
```
# systemctl start postgresql.service
```
On the RHEL 9 system, import data from the dumped sql file:
```
su - postgres -c 'psql -f ~/pgdump_file.sql postgres'
```

What is PostgreSQL command line tools?

psql is a terminal-based front-end to PostgreSQL. It enables you to type in queries interactively, issue them to PostgreSQL, and see the query results. Alternatively, input can be from a file or from command line arguments.

What are the different approaches to backing up PostgreSQL data?

There are three fundamentally different approaches to backing up PostgreSQL data: SQL dump. File system level backup. Continuous archiving.

Which command is used to the list of databases in PostgreSQL?

To view all of the defined databases on the server you can use the \list meta-command or its shortcut \l .

What is the difference between pg_dump and Pg_dumpall?

One caveat: pg_dump does not dump roles or other database objects including tablespaces, only a single database. To take backups on your entire PostgreSQL cluster, pg_dumpall is the better choice. pg_dumpall can handle the entire cluster, backing up information on roles, tablespaces, users, permissions, etc…

Pg_dump postgres example Pg_restore

What PostgreSQL command-line utility backs up PostgreSQL database settings

4.1. Getting started with PostgreSQL

4.2. Installing PostgreSQL

4.3. Creating PostgreSQL users

4.4. Configuring PostgreSQL

4.5. Configuring TLS encryption on a PostgreSQL server

4.6. Backing up PostgreSQL data

4.6.1. Backing up PostgreSQL data with an SQL dump

4.6.1.1. Advantages and disadvantages of an SQL dump

4.6.1.2. Performing an SQL dump using pg_dump

4.6.1.3. Performing an SQL dump using pg_dumpall

4.6.1.4. Restoring a database dumped using pg_dump

4.6.1.5. Restoring databases dumped using pg_dumpall

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

4.6.1.7. Handling SQL errors during restore

4.6.1.8. Additional resources

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

4.6.2.1. Advantages and limitations of file system backing up

4.6.2.2. Performing file system level backing up

4.6.3. Backing up PostgreSQL data by continuous archiving

4.6.3.1. Introduction to continuous archiving

4.6.3.2. Advantages and disadvantages of continuous archiving

4.6.3.3. Setting up WAL archiving

4.6.3.4. Making a base backup

4.6.3.5. Restoring the database using a continuous archive backup

4.6.3.6. Additional resources

4.7. Migrating to a RHEL 9 version of PostgreSQL

4.7.1. Fast upgrade using the pg_upgrade utility

4.7.2. Dump and restore upgrade

What is PostgreSQL command line tools?

What are the different approaches to backing up PostgreSQL data?

Which command is used to the list of databases in PostgreSQL?

What is the difference between pg_dump and Pg_dumpall?

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