PostgreSQL is an advanced SQL database server, available on a wide range of platforms. One of the clearest benefits of PostgreSQL is that it is open source, meaning that you have a very permissive license to install, use, and distribute PostgreSQL, without paying anyone any fees or royalties. On top of that, PostgreSQL is known as a database that stays up for long periods and requires little or no maintenance, in most cases. Overall, PostgreSQL provides a very low total cost of ownership.

PostgreSQL is also known for its huge range of advanced features, developed over the course of more than 30 years of continuous development and enhancement. Originally developed by the Database Research Group at the University of California, Berkeley, PostgreSQL is now developed and maintained by a huge army of developers and contributors. Many of these contributors have full-time jobs related to PostgreSQL, working as designers, developers, database administrators, and trainers. Some, but not many, of these contributors work for companies that specialize in support for PostgreSQL. No single company owns PostgreSQL, nor are you required (or even encouraged) to register your usage.

PostgreSQL has the following main features:

• Excellent SQL standards compliance, up to SQL: 2016
• Client-server architecture
• It has a highly concurrent design, where readers and writers don't block each other
• It is highly configurable and extensible for many types of applications
• It has excellent scalability and performance, with extensive tuning features
• It offers support for many kinds of data models, such as relational, post-relational (arrays, nested relations via record types), document (JSON and XML), and key/value

The PostgreSQL project focuses on the following objectives:

• Robust, high-quality software with maintainable, well-commented code
• Low-maintenance administration for both embedded and enterprise use
• Standards-compliant SQL, interoperability, and compatibility
• Performance, security, and high availability

What surprises many people is that PostgreSQL's feature set is more similar to Oracle or SQL Server than it is to MySQL. The only connection between MySQL and PostgreSQL is that these two projects are open source; apart from that, the features and philosophies are almost totally different.

One of the key features of Oracle, since Oracle 7, has been snapshot isolation, where readers don't block writers and writers don't block readers. You may be surprised to learn that PostgreSQL was the first database to be designed with this feature, and it offers a complete implementation. In PostgreSQL, this feature is called Multiversion Concurrency Control (MVCC), and we will discuss this in more detail later in the book.

PostgreSQL is a general-purpose database management system. You define the database that you would like to manage with it. PostgreSQL offers you many ways in which to work. You can either use a normalized database model, augmented with features such as arrays and record subtypes, or use a fully dynamic schema with the help of JSONB and an extension named hstore. PostgreSQL also allows you to create your own server-side functions in any of a dozen different languages.

PostgreSQL is highly extensible, so you can add your own data types, operators, index types, and functional languages. You can even override different parts of the system, using plugins to alter the execution of commands, or add a new query optimizer.

All of these features offer a huge range of implementation options to software architects. There are many ways out of trouble when building applications and maintaining them over long periods of time. Regrettably, we simply don't have space in this book for all the cool features for developers; this book is about administration, maintenance, and backup.

In the early days, when PostgreSQL was still a research database, the focus was solely on the cool new features. Over the last 20 years, enormous amounts of code have been rewritten and improved, giving us one of the largest and most stable software servers available for operational use.

Who is using PostgreSQL? Prominent users include Apple, BASF, Genentech, Heroku, IMDB, Skype, McAfee, NTT, the UK Met Office, and the US National Weather Service. Early in 2010, PostgreSQL received well in excess of 1,000,000 downloads per year, according to data submitted to the European Commission, which concluded that PostgreSQL is considered by many database users to be a credible alternative.

We need to mention one last thing: when PostgreSQL was first developed, it was named Postgres, and therefore, many aspects of the project still refer to the word Postgres; for example, the default database is named postgres, and the software is frequently installed using the Postgres user ID. As a result, people shorten the name PostgreSQL to simply Postgres, and, in many cases, use the two names interchangeably.

PostgreSQL is pronounced as post-grez-q-l. Postgres is pronounced as post-grez.

Some people get confused and refer to it as Postgre, which is hard to say and likely to confuse people. Two names are enough, so don't use a third name!

The following sections explain the key areas in more detail.

PostgreSQL is robust, high-quality software, supported by testing for both features and concurrency. By default, the database provides strong disk-write guarantees, and developers take the risk of data loss very seriously in everything they do. Options to trade robustness for performance exist, though they are not enabled by default.

All actions on the database are performed within transactions, protected by a transaction log that will perform automatic crash recovery in case of software failure.

Databases may optionally be created with data block checksums to help diagnose hardware faults. Multiple backup mechanisms exist, with full and detailed Point-in-time recovery (PITR), in case you need a detailed recovery. A variety of diagnostic tools are available as well.

Database replication is supported natively. Synchronous replication can provide greater than 5 nines (99.999%) availability and data protection, if properly configured and managed, or even higher with appropriate redundancy.

Functions may execute with the permissions of the definer, while views may be defined with security barriers to ensure that security is enforced ahead of other processing.

All aspects of PostgreSQL are assessed by an active security team, while known exploits are categorized and reported at http://www.postgresql.org/support/security/.

Clear, full, and accurate documentation exists as a result of a development process where documentation changes are required. Hundreds of small changes occur with each release, which smooth off any rough edges of usage, supplied directly by knowledgeable users.

PostgreSQL works on small and large systems in the same way, and across operating systems.

Client access and drivers exist for every language and environment, so there is no restriction on what type of development environment is chosen now, or in the future.

The SQL standard is followed very closely; there is no weird behavior, such as silent truncation of data.

Text data is supported via a single data type that allows the storage of anything from 1 byte to 1 gigabyte. This storage is optimized in multiple ways, so 1 byte is stored efficiently, and much larger values are automatically managed and compressed.

PostgreSQL has the clear policy of minimizing the number of configuration parameters, and with each release, we work out ways to auto-tune the settings.

PostgreSQL is designed to be highly extensible. Database extensions can be easily loaded by using CREATE EXTENSION, which automates version checks, dependencies, and other aspects of configuration.

PostgreSQL supports user-defined data types, operators, indexes, functions, and languages.

Many extensions are available for PostgreSQL, including the PostGIS extension, which provides world-class Geographical Information System (GIS) features.

PostgreSQL 11 can achieve significantly more than 1,000,000 reads per second on a 4-socket server, and it benchmarks at more than 30,000 write transactions per second with full durability, depending upon your hardware. With advanced hardware, even higher levels of performance are possible.

PostgreSQL has an advanced optimizer that considers a variety of join types, utilizing user data statistics to guide its choices. PostgreSQL provides the widest range of index types of any commonly available database server, fully supporting all data types.

PostgreSQL provides MVCC, which enables readers and writers to avoid blocking each other.

Taken together, the performance features of PostgreSQL allow a mixed workload of transactional systems and complex search and analytical tasks. This is important because it means we don't always need to unload our data from production systems and reload it into analytical data stores just to execute a few ad hoc queries. PostgreSQL's capabilities make it the database of choice for new systems, as well as the correct long-term choice in almost every case.

PostgreSQL 11 scales well on a single node up to four CPU sockets. PostgreSQL efficiently runs up to hundreds of active sessions, and up to thousands of connected sessions when using a session pool. Further scalability is achieved in each annual release.

PostgreSQL provides multi-node read scalability using the Hot Standby feature. Multi-node write scalability is under active development. The starting point for this is Bi-Directional Replication (discussed in Chapter 12, Replication and Upgrades).

PostgreSQL follows the SQL standard very closely. SQL itself does not force any particular type of model to be used, so PostgreSQL can easily be used for many types of models at the same time, in the same database.

With PostgreSQL acting as a relational database, we can utilize any level of denormalization, from the full third normal form (3NF), to the more normalized star schema models. PostgreSQL extends the relational model to provide arrays, row types, and range types.

A document-centric database is also possible using PostgreSQL's text, XML, and binary JSON (JSONB) data types, supported by indexes optimized for documents and by full text search capabilities.

Key/value stores are supported using the hstore extension.

When MySQL was taken over by a commercial database vendor some years back, it was agreed in the EU monopoly investigation that followed that PostgreSQL was a viable competitor. That's certainly been true, with the PostgreSQL user base expanding consistently for more than a decade.

Various polls have indicated that PostgreSQL is the favorite database for building new, enterprise-class applications. The PostgreSQL feature set attracts serious users who have serious applications. Financial services companies may be PostgreSQL's largest user group, though governments, telecommunication companies, and many other segments are strong users as well. This popularity extends across the world; Japan, Ecuador, Argentina, and Russia have very large user groups, as do the US, Europe, and Australasia.

Amazon Web Services' chief technology officer, Dr. Werner Vogels, described PostgreSQL as An amazing database, going on to say that PostgreSQL has become the preferred open source relational database for many enterprise developers and start-ups, powering leading geospatial and mobile applications. AWS have more recently revealed that PostgreSQL is their fastest growing service.

Many people have commented that strong commercial support is what enterprises need before they can invest in open source technology. Strong support is available worldwide from a number of companies.

The authors (Gianni and Simon) work for 2nd quadrant, which provides commercial support for open source PostgreSQL, offering 24/7 support in English and Spanish with bug-fix resolution times.

Many other companies provide strong and knowledgeable support to specific geographic regions, vertical markets, and specialized technology stacks.

PostgreSQL is also available as a hosted or cloud solution from a variety of companies, since it runs very well in cloud environments.

A full list of companies is kept up to date at http://www.postgresql.org/support/professional_support/.

PostgreSQL was originally developed as a research project at the University of California, Berkeley, in the late 1980s and early 1990s. Further work was carried out by volunteers until the late 1990s. Then, the first professional developer became involved. Over time, more and more companies and research groups became involved, supporting many professional contributors. Further funding for research and development was provided by the NSF. The project also received funding from the EU FP7 Programme, in the form of the 4CaaST project for cloud computing, and the AXLE project for scalable data analytics. AXLE deserves a special mention because it was a three-year project aimed at enhancing PostgreSQL's business-intelligence capabilities, specifically for very large databases. The project covered security, privacy, integration with data mining, and visualization tools and interfaces for new hardware.

Further details about the AXLE project are available at http://www.axleproject.eu. Other funding for PostgreSQL development comes from users who directly sponsor features and companies that sell products and services based around PostgreSQL.

PostgreSQL is 100% open source software and is freely available to use, alter, or redistribute in any way you choose. Its license is an approved open source license, very similar to the Berkeley Software Distribution (BSD) license, though only just different enough that it is now known as The PostgreSQL License (TPL).

PostgreSQL is already being used by many different application packages, so you may find it already installed on your servers. Many Linux distributions include PostgreSQL as part of the basic installation, or include it with the installation disk.

One thing to be wary of is that the included version of PostgreSQL may not be the latest release. It would typically be the latest major release that was available when that operating system release was published. There is usually no good reason to stick to that level – there is no increased stability implied there—and later production versions are just as well supported by the various Linux distributions as the earlier versions.

If you don't have a copy yet, or you don't have the latest version, you can download the source code or binary packages for a wide variety of operating systems from http://www.postgresql.org/download/.

Installation details vary significantly from platform to platform, and there aren't any special tricks or recipes to mention. Just follow the installation guide, and away you go! We've consciously avoided describing the installation processes here to make sure we don't garble or override the information published to assist you.

If you would like to receive email updates of the latest news, you can subscribe to the PostgreSQL announce mailing list, which contains updates from all the vendors that support PostgreSQL. You'll get a few emails each month about new releases of core PostgreSQL, related software, conferences, and user group information. It's worth keeping in touch with these developments.

Many people ask questions such as, How can this be free?, Are you sure I don't have to pay someone? or, Who gives this stuff away for nothing?

Open source applications such as PostgreSQL work on a community basis, where many contributors perform tasks that make the whole process work. For many of these people, their involvement is professional, rather a hobby, and they can do this because there is generally great value for both the contributors and their employers alike.

You might not believe it. You don't have to, because it just works!

Remember that PostgreSQL is more than just the core software. There is a huge range of websites that offer add-ons, extensions, and tools for PostgreSQL. You'll also find an army of bloggers who describe useful tricks and discoveries that will help you in your work.

Besides these, a range of professional companies can offer you help when you need it.

How do we access PostgreSQL?

Connecting to the database is the first experience of PostgreSQL for most people, so we want to make it a good one. Let's do it now, and fix any problems we have along the way. Remember that a connection needs to be made secure, so there may be some hoops for us to jump through to ensure that the data we wish to access is secure.

Before we can execute commands against the database, we need to connect to the database server, to give us a session.

Sessions are designed to be long-lived, so you connect once, perform many requests, and eventually disconnect. There is a small overhead during connection. It may become noticeable if you connect and disconnect repeatedly, so you may wish to investigate the use of connection pools. Connection pools allow pre-connected sessions to be quickly served to you when you wish to reconnect.

First, cache your database. If you don't know where it is, you'll probably have difficulty accessing it. There may be more than one database, and you'll need to know the right one to access, and also have the authority to connect to it.

You need to specify the following parameters to connect to PostgreSQL:

• Host or host address
• Port
• Database name
• User
• Password (or other means of authentication, if any)

To connect, there must be a PostgreSQL server running on host, listening to port number port. On that server, a database named dbname and a user named user must also exist. The host must explicitly allow connections from your client (explained in the Enabling access for network/remote users recipe), and you must also pass authentication using the method the server specifies; for example, specifying a password won't work if the server has requested a different form of authentication.

Almost all PostgreSQL interfaces use the libpq interface library. When using libpq, most of the connection parameter handling is identical, so we can discuss that just once.

If you don't specify the preceding parameters, PostgreSQL looks for values set through environment variables, which are as follows:

• PGHOST or PGHOSTADDR
• PGPORT (set this to 5432 if it is not set already)
• PGDATABASE
• PGUSER
• PGPASSWORD (this is definitely not recommended)

If you somehow specify the first four parameters, but not the password, PostgreSQL looks for a password file, discussed in the Avoiding hardcoding your password recipe.

Some PostgreSQL interfaces use the client-server protocol directly, so the ways in which the defaults are handled may differ. The information we need to supply won't vary significantly, so check the exact syntax for that interface.

Connection details can also be specified using a Uniform Resource Identifier (URI) format, as follows:

psql postgresql://myuser:mypasswd@myhost:5432/mydb

This specifies that we will connect the psql client application to the PostgreSQL server at the myhost host, on the 5432 port, with the mydb database name, myuser user, and mypasswd password.

PostgreSQL is a client-server database. The system it runs on is known as the host. We can access the PostgreSQL server remotely, through the network. However, we must specify host, which is a hostname, or hostaddr, which is an IP address. We can specify a host as localhost if we wish to make a TCP/IP connection to the same system. It is often better to use a Unix socket connection, which is attempted if the host begins with a slash (/) and the name is presumed to be a directory name (the default is /tmp).

On any system, there can be more than one database server. Each database server listens to exactly one well-known network port, which cannot be shared between servers on the same system. The default port number for PostgreSQL is 5432, which has been registered with the Internet Assigned Numbers Authority (IANA) and is uniquely assigned to PostgreSQL (you can see it used in the /etc/services file on most *nix servers). The port number can be used to uniquely identify a specific database server, if many exist. IANA (http://www.iana.org) is the organization that coordinates the allocation of available numbers for various internet protocols.

A database server is also sometimes known as a database cluster, because the PostgreSQL server allows you to define one or more databases on each server. Each connection request must identify exactly one database, identified by its dbname. When you connect, you will only be able to see only the database objects created within that database.

A database user is used to identify the connection. By default, there is no limit on the number of connections for a particular user. In the Enabling access for network/remote users
recipe, we will cover how to restrict that. In the more recent versions of PostgreSQL, users are referred to as login roles, though many clues remind us of the earlier nomenclature, and that still makes sense in many ways. A login role is a role that has been assigned the CONNECT privilege.

Each connection will typically be authenticated in some way. This is defined at the server level: client authentication will not be optional at connection time if the administrator has configured the server to require it.

Once you've connected, each connection can have one active transaction at a time and one fully active statement at any time.

The server will have a defined limit on the number of connections it can serve, so a connection request can be refused if the server is oversubscribed.

If you are already connected to a database server with psql and you want to confirm that you've connected to the right place and in the right way, you can execute some, or all, of the following commands. Here is the command that shows the current_database:

SELECT current_database();

The following command shows the current_user ID:

SELECT current_user;

The next command shows the IP address and port of the current connection, unless you are using Unix sockets, in which case both values are NULL:

SELECT inet_server_addr(), inet_server_port();

A user's password is not accessible using general SQL, for obvious reasons.

You may also need the following:

SELECT version();

From PostgreSQL version 9.1 onward, you can also use the new psql meta-command, \conninfo. This displays most of the preceding information in a single line:

postgres=# \conninfo
You are connected to database postgres, as user postgres, via socket in /var/run/postgresql, at port 5432.

There are many other snippets of information required to understand connections. Some of them are mentioned in this chapter, and others are discussed in Chapter 6, Security. For further details, refer to the PostgreSQL server documentation.

PostgreSQL comes in a variety of distributions. In many of these, you will note that remote access is initially disabled as a security measure.

By default, PostgreSQL gives access to clients who connect using Unix sockets, provided that the database user is the same as the system's username. Here, we'll show you how to enable other connections.

The steps are as follows:

1. Add or edit this line in your postgresql.conf file:

listen_addresses = '*'

2. Add the following line as the first line of pg_hba.conf to allow access to all databases for all users with an encrypted password:

# TYPE   DATABASE   USER        CIDR-ADDRESS   METHOD 
host      all        all          0.0.0.0/0       md5

3. After changing listen_addresses, we restart the PostgreSQL server, as explained in the Updating the parameter file recipe in Chapter 3, Configuration

The listen_addresses parameter specifies which IP addresses to listen to. This allows you to flexibly enable and disable listening on interfaces of multiple network cards (NICs) or virtual networks on the same system. In most cases, we want to accept connections on all NICs, so we use *, meaning all IP addresses.

The pg_hba.conf file contains a set of host-based authentication rules. Each rule is considered in sequence, until one rule fires or the attempt is specifically rejected with a reject method.

The preceding rule means that a remote connection that specifies any user or database on any IP address will be asked to authenticate using an MD5-encrypted password. The following are the parameters required for MD5-encrypted passwords:

• Type: For this, host means a remote connection.
• Database: For this, all means for all databases. Other names match exactly, except when prefixed with a plus (+) symbol, in which case we mean a group role rather than a single user. You can also specify a comma-separated list of users, or use the @ symbol to include a file with a list of users. You can even specify sameuser, so that the rule matches when you specify the same name for the user and database.
• User: For this, all means for all users. Other names match exactly, except when prefixed with a plus (+) symbol, in which case we mean a group role rather than a single user. You can also specify a comma-separated list of users, or use the @ symbol to include a file with a list of users.
• CIDR-ADDRESS: This consists of two parts: an IP address and a subnet mask. The subnet mask is specified as the number of leading bits of the IP address that make up the mask. Thus, /0 means 0 bits of the IP address, so that all IP addresses will be matched. For example, 192.168.0.0/24 would mean matching of the first 24 bits, so any IP address of the form 192.168.0.x would match. You can also use samenet or samehost.
• Method: For this, md5 means that PostgreSQL will ask the client to provide a password encrypted with MD5. Another common setting is trust, which effectively means no authentication. Other authentication methods include GSSAPI, SSPI, LDAP, RADIUS, and PAM. PostgreSQL connections can also be made using SSL, in which case client SSL certificates provide authentication. See the Using SSL certificates to authenticate the client recipe in Chapter 6, Security, for more details.

Don't use the password setting, as this sends the password in plain text. This is not a real security issue if your connection is encrypted with SSL, and there are normally no downsides with MD5 anyway, and you have extra security for non-SSL connections.

In earlier versions of PostgreSQL, access through the network was enabled by adding the -i command-line switch when you started the server. This is still a valid option, but now it means the following:

listen_addresses = '*'

So, if you're reading some notes about how to set things up and this is mentioned, be warned that those notes are probably long out of date. They are not necessarily wrong, but it's worth looking further to see whether anything else has changed.

Look at installer and/or operating system-specific documentation to find the standard location of the files.

Graphical administration tools are often requested by system administrators. PostgreSQL has a range of tool options. In this book, we'll cover pgAdmin4 and OmniDB, which offers access to PostgreSQL and other databases.

Both of these tools are client applications that send and receive SQL to PostgreSQL, displaying the results for you. The admin client can access many databases servers, allowing you to manage a fleet of servers. Both tools work in standalone app mode and within web browsers.

When you start pgAdmin, you will be prompted to register a new server. 

Give your server a name on the General tab, and then click Connection and fill in the five basic connection parameters, as well as the other information. You should uncheck the Save password? box:

If you have many database servers, you can group them together. I suggest keeping any replicated servers together in the same server group. Give each server a sensible name.

Once you've added a server, you can connect to it and display information about it.

The default screen is the Dashboard, which presents a few interesting graphs based on the data it polls from the server. That's not very useful, so click on the Statistics tab.

You will then get access to the main browser screen, with the object tree view on the left and statistics on the right, as shown in the following screenshot:

pgAdmin easily displays much of the data that is available from PostgreSQL. The information is context-sensitive, allowing you to navigate and see everything quickly and easily. The information is not dynamically updated; this will occur only when you click to refresh, so bear this in mind when using the application.

pgAdmin also provides Grant Wizard. This is useful for DBAs for review and immediate maintenance:

The pgAdmin query tool allows you to have multiple active sessions. The query tool has a good-looking visual Explain feature, which displays the EXPLAIN plan for your query:

pgAdmin provides a wide range of features, many of which are provided by other tools as well. This gives us the opportunity to choose which of those tools we want. For many reasons, it is best to use the right tool for the right job, and that is always a matter of expertise, experience, and personal taste.

pgAdmin submits SQL to the PostgreSQL server, and displays the results quickly and easily. As a browser, it is fantastic. For performing small DBA tasks, it is ideal. As you might've guessed from these comments, I don't recommend pgAdmin for every task.

Scripting is an important technique for DBAs. You keep a copy of the task executed, and you can edit and resubmit if problems occur. It's also easy to put all the tasks in a script into a single transaction, which isn't possible using the current GUI tools. pgAdmin provides pgScript, which only works with pgAdmin, so it is more difficult to port. For scripting, I strongly recommend the psql utility, which has many additional features that you'll increasingly appreciate over time.

Although I recommend psql as a scripting tool, many people find it convenient as a query tool. Some people may find this strange, and assume it is a choice for experts only. Two great features of psql are the online help for SQL and the tab completion feature, which allows you to build up SQL quickly without having to remember the syntax. See the Using the psql query and scripting tool recipe for more information.

pgAdmin also provides pgAgent, which is a task scheduler. Again, more portable schedulers are available, and you may wish to use those instead. Schedulers aren't covered in this book.

A quick warning! When you create an object in pgAdmin, the object will be created with a mixed case name if you use capitals anywhere in the object name. If I ask for a table named MyTable, the only way to access that table is by referring to it in double quotes as MyTable. See the Handling objects with quoted names recipe in Chapter 5, Tables and Data:

OmniDB is designed to access PostgreSQL, MySQL, MariaDB, and Oracle in one interface, though it makes sure it provides full features for the PostgreSQL database.

OmniDB is developing quickly, with monthly feature releases, so I recommend that you check out the latest information at https://omnidb.org/.

OmniDB provides a very responsive interface and is designed with full security in mind. It can be used as a desktop application and it can also be served using a web server, to be accessed by the web browser of your choice.

OmniDB has the standard tree-view browsing interface, with multi-tab access for each database server you access. It's easy to be connected to multiple PostgreSQL, MySQL, and Oracle database servers at the same time:

OmniDB has a SQL editor that has code completion and debugging. The EXPLAIN ANALYZE output is colored to highlight the areas of the plan that take the most time:

Administrators in OmniDB can manage users graphically. The interface gives you the ability to add, edit, and remove users, along with the ability to make someone a superuser. These users can then create connections to PostgreSQL, MySQL, MariaDB, and Oracle—all managed through a unified web page. Connections can also be made via SSH tunnels:

In order to ease the process of developing code in PL/pgSQL, OmniDB provides a powerful, full-featured debugger. The debugger works as an inner tab of the SQL Editor and provides insights into parameters, variables, result, messages, and statistics in five tabs:

Another useful feature in OmniDB is the monitoring dashboard. The dashboard gives you real-time statistics of important metrics you might want to monitor, such as system Memory Usage, CPU Usage, and Locks:

OmniDB has been designed to be a flexible and an extensible tool. Though it comes with several default charts, you can use Python and JSON to write new ones or use the existing ones as templates to enhance and expand. OmniDB provides a plugin API, allowing users to write and distribute their own plugins for expanded capabilities.

You may also be interested in commercial tools of various kinds for PostgreSQL. A full listing is given in the PostgreSQL software catalog at http://www.postgresql.org/download/products/1.

psql is the query tool supplied as a part of the core distribution of PostgreSQL, so it is available in all environments, and works similarly in all of them. This makes it an ideal choice for developing portable applications and techniques.

psql provides features for use as both an interactive query tool and as a scripting tool.

From here on, we will assume that the psql command is enough to allow you access to the PostgreSQL server. This assumes that all your connection parameters are defaults, which may not be true.

Written in full, the connection parameters would be either of these options:

psql -h myhost -p 5432 -d mydb -U myuser 
psql postgresql://myuser@myhost:5432/mydb

The default value for the port (-p) is 5432. By default, mydb and myuser are both identical to the operating system's username. The default myhost on Windows is localhost, while on Unix, we use the default directory for Unix socket connections. The location of such directories varies across distributions and is set at compile time. However, note that you don't actually need to know its value, because on local connections, both the server and the client are normally compiled together, so they use the same default.

The command that executes a single SQL command and prints the output is the easiest, as shown here:

$ psql -c "SELECT current_time"
     timetz
-----------------
 18:48:32.484+01
(1 row)

The -c command is non-interactive. If we want to execute multiple commands, we can write those commands in a text file and then execute them using the -f option. This command loads a very small and simple set of examples:

$ psql -f examples.sql

It produces the following output when successful:

SET
SET
SET
SET
SET
SET
DROP SCHEMA
CREATE SCHEMA
SET
SET
SET
CREATE TABLE
CREATE TABLE
COPY 5
COPY 3

The examples.sql script is very similar to a dump file produced by PostgreSQL backup tools, so this type of file and the output it produces are very common. When a command is executed successfully, PostgreSQL outputs a command tag equal to the name of that command; this is how the preceding output was produced.

The psql tool can also be used with both the -c and -f modes together; each one can be used multiple times. In this case, it will execute all the commands consecutively:

$ psql -c "SELECT current_time" –f examples.sql -c "SELECT current_time"
     timetz
-----------------
 18:52:15.287+01
(1 row)
   ...output removed for clarity...
     timetz
-----------------
 18:58:23.554+01
(1 row)

The psql tool can also be used in interactive mode, which is the default, so it requires no option:

$ psql
postgres=#

The first interactive command you'll need is the following:

postgres=# help

You can then enter SQL or other commands. The following is the last interactive command you'll need:

postgres=# \quit

Unfortunately, you cannot type quit on its own, nor can you type \exit, or other options. Sorry, just \quit, or \q for short!

In psql, you can enter the following two types of commands:

• psql meta-commands
• SQL

A meta-command is a command for the psql client, whereas SQL is sent to the database server. An example of a meta-command is \q, which tells the client to disconnect. All lines that begin with \ (backslash) as the first nonblank character are presumed to be meta-commands of some kind.

If it isn't a meta-command, it's SQL. We keep reading SQL until we find a semicolon, so we can spread SQL across many lines and format it any way we find convenient.

The help command is the only exception. We provide this for people who are completely lost, which is a good thought; so let's start from there ourselves.

There are two types of help commands, which are as follows:

• \?: This provides help on psql meta-commands
• \h: This provides help on specific SQL commands

Consider the following snippet as an example:

postgres=# \h DELETE
Command: DELETE
Description: delete rows of a table
Syntax:
[ WITH [ RECURSIVE ] with_query [, ...] ]
DELETE FROM [ ONLY ] table [ [ AS ] alias ]
    [ USING usinglist ]
    [ WHERE condition | WHERE CURRENT OF cursor_name ]
    [ RETURNING * | output_expression [ AS output_name ] [,]]

I find this a great way to discover and remember options and syntax. You'll also appreciate having the ability to scroll back through the previous command history.

You'll get a lot of benefits from tab completion, which will fill in the next part of the syntax when you press the Tab key. This also works for object names, so you can type in just the first few letters and then press Tab; all the options will be displayed. Thus, you can type in just enough letters to make the object name unique, and then hit Tab to get the rest of the name.

One-line comments begin with two dashes, as follows:

-- This is a single-line comment

Multiline comments are similar to those in C and Java:

/*
 * Multiline comment
 */

You'll probably agree that psql looks a little daunting at first, with strange backslash commands. I do hope you'll take a few moments to understand the interface and keep digging for more information. The psql tool is one of the most surprising parts of PostgreSQL, and it is incredibly useful for database administration tasks when used alongside other tools.

psql works across releases and works well with older versions. It may not work at all with newer server versions, so use the latest client level of server you are accessing.

Check out some other useful features of psql, which are as follows:

• Information functions
• Output formatting
• Execution timing using the \timing command
• Input/output and editing commands
• Automatic startup files, such as .psqlrc
• Substitutable parameters (variables)
• Access to the OS command line
• Crosstab views
• Conditional execution

If you are using password authentication, then you may wish to change your password from time to time.

The most basic method is to use the psql tool. The \password command will prompt you once for a new password and again to confirm. Connect to the psql tool and type the following:

SET password_encryption = 'scram-sha-256'; 
\password

Enter a new password. This causes psql to send a SQL statement to the PostgreSQL server, which contains an already encrypted password string. An example of the SQL statement sent is as follows:

ALTER USER postgres PASSWORD 'SCRAM-SHA-256$4096:H45+UIZiJUcEXrB9SHlv5Q==$I0mc87UotsrnezRKv9Ijqn/zjWMGPVdy1zHPARAGfVs=:nSjwT9LGDmAsMo+GqbmC2X/9LMgowTQBjUQsl45gZzA=';

Make sure you use SCRAM-SHA-256 encryption, not the older and easily compromised md5 encryption. Whatever you do, don't use postgres as your password. This will make you vulnerable to idle hackers, so make it a little more difficult than that!

Make sure you don't forget your password either. It may prove difficult to maintain your database if you can't access it.

As changing the password is just a SQL statement, any interface can do this. Other tools also allow this, such as the following:

• pgAdmin4
• phpPgAdmin

If you don't use one of the main routes to change the password, you can still do it yourself, using SQL from any interface. Note that you need to encrypt your password, because if you do submit a password in plain text, such as the following, it will be shipped to the server in plain text:

ALTER USER myuser PASSWORD 'secret'

Luckily, the password in this case will still be stored in an encrypted form. But it will also be recorded in plain text in psql's history file, as well as in any server and application logs, depending on the actual log-level settings.

PostgreSQL doesn't enforce a password change cycle, so you may wish to use more advanced authentication mechanisms, such as GSSAPI, SSPI, LDAP, or RADIUS.

We can all agree that hardcoding your password is a bad idea. This recipe shows you how to keep your password in a secure password file.

Not all database users need passwords; some databases use other means of authentication. Don't perform this step unless you know you will be using password authentication, and you know your password.

First, remove the hardcoded password from where you set it previously. Completely remove the password = xxxx text from the connection string in a program. Otherwise, when you test the password file, the hardcoded setting will override the details you are about to place in the file. Keeping the password hardcoded and in the password file is not any better. Using PGPASSWORD is not recommended either, so remove that also.

If you think someone may have seen the password, change your password before placing it in the secure password file.

A password file contains the usual five fields that we require when connecting, as shown here:

host:port:dbname:user:password

Change this to the following:

myhost:5432:postgres:sriggs:moresecure

The password file is located using an environment variable named PGPASSFILE. If PGPASSFILE is not set, a default filename and location must be searched for, as follows:

• On *nix systems, look for ~/.pgpass
• On Windows systems, look for %APPDATA%\postgresql\pgpass.conf, where %APPDATA% is the application data subdirectory in the path (for me, that would be C:\)

Many people name the password file .pgpass, whether or not they are on Windows, so don't get confused if they do this.

The password file can contain multiple lines. Each line is matched against the requested host:port:dbname:user combination until we find a line that matches. Then, we use that password.

Each item can be a literal value or *, a wildcard that matches anything. There is no support for partial matching. With appropriate permissions, a user can potentially connect to any database. Using the wildcard in the dbname and port fields makes sense, but it is less useful in other fields. The following are a few examples of wildcards:

• myhost:5432:*:sriggs:moresecurepw
• myhost:5432:perf:hannu:okpw
• myhost:*:perf:gianni:sicurissimo

This looks like a good improvement if you have a few database servers. If you have many different database servers, you may want to think about using a connection service file instead (see the Using a connection service file recipe), or perhaps even storing details on a Lightweight Directory Access Protocol (LDAP) server.

As the number of connection options grows, you may want to consider using a connection service file.

The connection service file allows you to give a single name to a set of connection parameters. This can be accessed centrally, to avoid the need for individual users to know the host and port of the database, and it is more resistant to future change.

You can set up a system-wide file as well as individual per-user files. The default file paths for these files are /etc/pg_service.conf and ~/.pg_service.conf, respectively.

A system-wide connection file controls service names for all users from a single place, while a per-user file applies only to that particular user. Keep in mind that the per-user file overrides the system-wide file—if a service is defined in both the files, then the definition in the per-user file will prevail.

First, create a file named pg_service.conf with the following content:

[dbservice1] 
host=postgres1 
port=5432 
dbname=postgres

You can then copy it to either /etc/pg_service.conf or another agreed upon central location. You can then set the PGSYSCONFDIR environment variable to that directory location.

Alternatively, you can copy it to ~/.pg_service.conf. If you want to use a different name, set PGSERVICEFILE. Either way, you can then specify a connection string, such as the following:

service=dbservice1 user=sriggs

The service can also be set using an environment variable named PGSERVICE.

This feature applies to libpq connections only, so it does not apply to JDBC.

The connection service file can also be used to specify the user, though that would mean that the username would be shared.

The pg_service.conf and .pgpass files can work together, or you can use just one of the two. Note that the pg_service.conf file is shared, so it is not a suitable place for passwords. The per-user connection service file is not shared, but in any case, it seems best to keep things separate and confine passwords to .pgpass.

This recipe is all about what you should do when things go wrong.

Bear in mind that 90% of problems are just misunderstandings, and you'll quickly be on track again.

Here, we've made a checklist to be followed if a connection attempt fails:

• Check whether the database name and the username are accurate. You may be requesting a service on one system when the database you require is on another system. Recheck your credentials; ensure that you haven't mixed things up and that you are not using the database name as the username, or vice versa. If you receive too many connections, then you may need to disconnect another session before you can connect, or wait for the administrator to re-enable the connections.
• Check for explicit rejections. If you receive the pg_hba.conf rejects connection for host... error message, it means your connection attempt has been explicitly rejected by the database administrator for that server. You will not be able to connect from the current client system using those credentials. There is little point in attempting to contact the administrator, as you are violating an explicit security policy with what you are attempting to do.
• Check for implicit rejections. If the error message you receive is no pg_hba.conf entry for..., it means there is no explicit rule that matches your credentials. This is likely an oversight on the part of the administrator and is common in very complex networks. Contact the administrator and request a ruling on whether your connection should be allowed (hopefully) or explicitly rejected in the future.
• Check whether the connection works with psql. If you're trying to connect to PostgreSQL from anything other than the psql command-line utility, switch to that now. If you can make psql connect successfully, but cannot make your main connection work correctly, the problem may be in the local interface you are using.
• PostgreSQL 9.3 and later versions ship the pg_isready utility, which checks the status of a database server, either local or remote, by establishing a minimal connection. Only the hostname and port are mandatory, which is great if you don't know the database name, username, or password. The following outcomes are possible:
  • The server is running and accepting connections.
  • The server is running but not accepting connections (because it is starting up, shutting down, or in recovery).
  • A connection attempt was made, but it failed.
  • No connection attempt was made because of a client problem (invalid parameters, out of memory).
  • Check whether the server is up. If a server is shut down, you cannot connect. The typical problem here is simply mixing up the server to which you are connecting. You need to specify the hostname and port, so it's possible that you are mixing up those details.
  • Check whether the server is up and accepting new connections. A server that is shutting down will not accept new connections, apart from superusers. Also, a standby server may not have the hot_standby parameter enabled, preventing you from connecting.
  • Check whether the server is listening correctly, and check the port to which the server is actually listening. Confirm that the incoming request is arriving on the interface listed in the listen_addresses parameter. Check whether it is set to * for remote connections and localhost for local connections.
  • Check whether the database name and username exist. It's possible that the database or user no longer exists.
  • Check the connection request; that is, check whether the connection request was successful and was somehow dropped following the connection. You can confirm this by looking at the server log when the following parameters are enabled:

log_connections = on
log_disconnections = on

• • Check for other reasons for disconnection. If you are connecting to a standby server, it is possible that you have been disconnected because of Hot Standby conflicts. See Chapter 12, Replication and Upgrades, for more information.

Client authentication and security are the rapidly changing areas in subsequent major PostgreSQL releases. You will also find differences between maintenance release levels. The PostgreSQL documents on this topic can be viewed at http://www.postgresql.org/docs/current/interactive/client-authentication.html.

Always check which release level you are using before consulting the manual or asking for support. Many problems are caused simply by confusing the capabilities between release levels.