psql -U sampleuser -h localhost
A line starting with # represents a comment. Same for everything to the right of a #. If you accidentally type it or copy and paste it in, don’t worry. Nothing will happen.
## This worked to connect to Postgres on DigitalOcean
# -U is the username (it will appear in the \l command)
# -h is the name of the machine where the server is running.
# -p is the port where the database listens to connections. Default is 5432.
# -d is the name of the database to connect to. I think DO generated this for me, or maybe PostgreSQL.
# Password when asked is csizllepewdypieiib
psql -U doadmin -h production-sfo-test1-do-user-4866002-0.db.ondigitalocean.com -p 25060 -d mydb
# Open a database in a remote location.
psql -U sampleuser -h production-sfo-test1-do-user-4866002-0.db.ondigitalocean.com -p 21334Using psql
You’ll use psql (aka the PostgreSQL interactive terminal) most of all because it’s used to create databases and tables, show information about tables, and even to enter information (records) into the database.
Quitting pqsql
Before we learn anything else, here’s how to quit psql and return to the operating system prompt. You type backslash, the letter q, and then you press the Enter or return key.
# Press enter after typing \q
# Remember this is backslash, not forward slash
postgres=# \q
This takes you back out to the operating system prompt.
Opening a connection locally
A common case during development is opening a connection to a local database (one on your own machine). Run psql with -U (for user name) followed by the name of the database, postgres in this example:
# Log into Postgres as the user named postgres
$ psql -U postgres
Opening a connection remotely
To connect your remote PostgreSQL instance from your local machine, use psql at your operating system command line. Here’s a typical connection.
# -U is the username (it will appear in the \l command)
# -h is the name of the machine where the server is running.
# -p is the port where the database listens to connections. Default is 5432.
# -d is the name of the database to connect to. I think DO generated this for me, or maybe PostgreSQL.
$ psql -U doadmin -h production-sfo-test1-do-user-4866002-0.db.ondigitalocean.com -p 25060 -d defaultdb
Here you’d enter the password. In case someone is peering over your shoulder, the characters are hidden. After you’ve entered your information properly you’ll get this message (truncated for clarity):
Looking at the psql prompt
A few things appear, then the psql prompt is displayed. The name of the current database appears before the prompt.
psql (11.1, server 11.0)
Type "help" for help.
postgres=#
At this point you’re expected to type commands and parameters into the command line.
psql vs SQL commands
psql has two different kinds of commands. Those starting with a backslash are for psql itself, as illustrated by the use of \q to quit.
Those starting with valid SQL are of course interactive SQL used to create and modify PostgreSQL databases.
Warning: SQL commands end with a semicolon
One gotcha is that almost all SQL commands you enter into psql must end in a semicolon.
- For example,suppose you want to remove a table named
sample_property_5. You’d enter this command:
postgres=# DROP TABLE "sample_property_5";
It’s easy to forget. If you do forget the semicolon, you’ll see this perplexing prompt. Note that a [ has been inserted before the username portion of the prompt, and another prompt appears below it:
[postgres=# DROP TABLE "sample_property_5"
postgres=#
When you do, just remember to finish it off with that semicolon:
[postgres=# DROP TABLE "sample_property_5"
postgres=# ;
Scrolling through the command history
- Use the up and down arrow keys to move backwards and forwards through the command history.
Getting information about databases
These aren’t SQL commands so just press Enter after them. Remember that:
- When there’s more output than fits the screen, it pauses. Press space to continue
- If you want to halt the output, press
q.
\h Help
# Get help. Note it's a backslash, not a forward slash.
postgres=# \h
You’ll get a long list of commands, then output is paused:
Available help:
ABORT CREATE USER
...
ALTER AGGREGATE CREATE USER MAPPING
ALTER PROCEDURE DROP INDEX
:
- Press space to continue, or
qto stop the output.
You can get help on a particular item by listing it after the \h command.
- For example, to get help on
DROP TABLE:
postgres=# \h drop tableYou’ll get help on just that item:
Command: DROP TABLE
Description: remove a table
Syntax:
DROP TABLE [ IF EXISTS ] name [, ...] [ CASCADE | RESTRICT ]\l List databases
What most people think of as a database (say, a list of customers) is actually a table. A database is a set of tables, information about those tables, information about users and their permissions, and much more. Some of these databases (and the tables within) are updated automatically by PostgreSQL as you use them.
To get a list of all databases:
postgres=# \l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access privileges
-----------+----------+----------+-------------+-------------+-----------------------
visitor | tom | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
markets | tom | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
postgres | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
template0 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 | =c/postgres +
| | | | | postgres=CTc/postgres
template1 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 | =c/postgres +
| | | | | postgres=CTc/postgres
tom | tom | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
You can get info on a single database by following the \l prompt with its name.
- For example, to view information about the
template0database:
postgres=# \l template0The output would be:
postgres=# \l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access privileges
-----------+----------+----------+-------------+-------------+-----------------------
template0 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 | =c/postgres +
| | | | | postgres=CTc/postgres\l+ List databases with size, tablespace, and description
To get additional information on the space consumed by database tables and comments describing those tables, use \l+:
postgres=# \l+\x Expand/narrow table lists
Use \x (X for eXpanded listing) to control whether table listings use a wide or narrow format.
| Command | Effect |
|---|---|
\x off | Show table listings in wide format |
\x on | Show table listings in narrow format |
\x | Reverse the previous state |
\x auto | Use terminal to determine format |
Example: Here’s an expanded listing:
/* List all databases. */
postgres=# \l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access privileges
-----------+----------+----------+-------------+-------------+-----------------------
foo | tom | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
foobarino | tom | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
postgres | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
template0 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 | =c/postgres +
| | | | | postgres=CTc/postgres
template1 | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 | =c/postgres +
| | | | | postgres=CTc/postgres
tom | tom | UTF8 | en_US.UTF-8 | en_US.UTF-8 |
(6 rows)
Use \x on for narrower listings:
/* Turn on narrow listings. */
postgres=# \x on
postgres=# \l
-[ RECORD 1 ]-----+----------------------
Name | foo
Owner | tom
Encoding | UTF8
Collate | en_US.UTF-8
Ctype | en_US.UTF-8
Access privileges |
-[ RECORD 2 ]-----+----------------------
Name | foobarino
Owner | tom
Encoding | UTF8
Collate | en_US.UTF-8
Ctype | en_US.UTF-8
Access privileges |
-[ RECORD 3 ]-----+----------------------
Name | postgres
Owner | postgres
Encoding | UTF8
Collate | en_US.UTF-8
Ctype | en_US.UTF-8
Access privileges |
\c Connect to a database
To see what’s inside a database, connect to it using \c followed by the database name. The prompt changes to match the name of the database you’re connecting to. (The one named postgres is always interesting.) Here we’re connecting to the one named markets:
postgres=# \c markets
psql (11.1, server 11.0)
You are now connected to database "markets" as user "tom".
markets=# \dt Display tables
- Use
\dtto list all the tables (technically, relations) in the database:
markets=# \dt
List of relations
Schema | Name | Type | Owner
--------+------------------------------+-------+----------
public | addresspool | table | tom
public | adlookup | table | tom
public | bidactivitysummary | table | tom
public | bidactivitydetail | table | tom
public | customerpaymentsummary | table | tom
...
- If you choose a database such as
postgresthere could be many tables. Remember you can pause output by pressing space or halt it by pressingq.
\d and \d+ Display columns (field names) of a table
To view the schema of a table, use \d followed by the name of the table.
- To view the schema of a table named
customerpaymentsummary, enter
markets=# \d customerpaymentsummary
Table "public.customerpaymentsummary"
Column | Type | Collation | Nullable | Default
------------------------------+-----------------------------+-----------+----------+--------
usersysid | integer | | not null |
paymentattemptsall | integer | | |
paymentattemptsmailin | integer | | |
paymentattemptspaypal | integer | | |
paymentattemptscreditcard | integer | | |
paymentacceptedoutagecredit | integer | | |
totalmoneyin | numeric(12,2) | | |
updatewhen1 | timestamp without time zone | | |
updatewhen2 | timestamp without time zone | | |
To view more detailed information on a table, use \d+:
markets=# \d customerpaymentsummary
Table "public.customerpaymentsummary"
Column | Type | Collation | Nullable | Default | Storage | Stats target |
------------------------------+-----------------------------+-----------+----------+---------+---------+---------------
usersysid | integer | | not null | | plain | |
paymentattemptsall | integer | | | | plain | |
paymentattemptsmailin | integer | | | | plain | |
paymentattemptspaypal | integer | | | | plain | |
paymentattemptscreditcard | integer | | | | plain | |
paymentacceptedoutagecredit | integer | | | | plain | |
totalmoneyin | numeric(12,2) | | | | main | |
updatewhen1 | timestamp without time zone | | | | plain | |
updatewhen2 | timestamp without time zone | | | | plain | |
Indexes:\du Display user roles
- To view all users and their roles, use
\du:
postgres=# \du
List of roles
Role name | Attributes | Member of
------------+------------------------------------------------------------+-----------
smanager | Superuser | {}
postgres | Superuser, Create role, Create DB, Replication, Bypass RLS | {}
tom | Superuser, Create role, Create DB | {}- To view the role of a specific user, pass it after the
\ducommand. For example, to see the onlytomrole:
postgres=# \du tom
List of roles
Role name | Attributes | Member of
------------+------------------------------------------------------------+-----------
tom | Superuser, Create role, Create DB | {}Creating a database
Before you add tables, you need to create a database to contain those tables. That’s not done with psql, but instead it’s done with createdb (a separate external command; see the PostgreSQL createdb documentation) at the operating system command line:
# Replace markets with your database name
$ createdb markets
On success, there is no visual feedback. Thanks, PostgreSQL.
Adding tables and records
Creating a table (CREATE TABLE)
To add a table schema to the database:
postgres=# create table if not exists product (
id SERIAL,
name VARCHAR(100) NOT NULL,
sku CHAR(8)
);
And psql responds with:
CREATE TABLE
For more see CREATE TABLE in the PostgreSQL official docs.
Adding a record (INSERT INTO)
- Here’s how to add a record, populating every field:
# The id field is an automatically assigned
# when you use DEFAULT. The serial primary key means
# that number will be increased by at least
# 1 and assigned to that same field when
# a new record is created.
# Using DEFAULT is a placeholder.
# In its place PostgreSQL automatically generates a unique integer for it.
postgres=# INSERT INTO product VALUES(DEFAULT, 'Apple, Fuji', '4131');
PostgreSQL responds with:
INSERT 0 1
- Try it again and you get a simliar response.
postgres=# INSERT INTO product VALUES(DEFAULT, 'Banana', '4011');
INSERT 0 1
Adding (inserting) several records at once
- You can enter a list of records using this syntax:
postgres=# INSERT INTO product VALUES
(DEFAULT, 'Carrots', 4562),
(DEFAULT, 'Durian', 5228)
;
Adding only specific (columns) fields from a record
You can add records but specify only selected fields (also known as columns). MySQL will use common sense default values for the rest.
In this example, only the name field will be populated. The sku column is left blank, and the id column is incremented and inserted.
Two records are added:
postgres=# INSERT INTO product (name) VALUES
('Endive'),
('Figs')
;
PostgreSQL responds with the number of records inserted:
INSERT 0 2For more on INSERT, see INSERT in the PostgreSQL official docs
Doing a simple query–get a list of records (SELECT)
Probably the most common thing you’ll do with a table is to obtain information about it with the SELECT statement. It’s a huge topic
- Let’s list all the records in the
producttable:
postgres=# SELECT * FROM product;
The response:
postgres=# select * from product;
id | name | sku
----+-------------+----------
1 | Apple, Fuji | 4131
2 | Banana | 4011
(2 rows)Note
If your table has mixed case objects such as column names or indexes, you’ll need to enclose them in double quotes. For example, If a column name were Product instead of product your query would need to look like this:
SELECT * FROM "product";
For more on SELECT, see the SELECT in the PostgreSQL official docs.
Maintenance and operations issues
Timing
\t Timing SQL operations
Use \t to show timing for all SQL operations performed.
| Command | Effect |
|---|---|
\timing off | Disable timing of SQL operations |
\timing on | Show timing after all SQL operations |
\timing | Toggle (reverse) the setting |
Example of \t Timing command
tom=# insert into todo values ('Retry on Android before app submission,'8.x and earlier');
INSERT 0 1
tom=# \timing on
Timing is on.
tom=# insert into todo values ('Correct footer bug','Mobile version only');
INSERT 0 1
Time: 1.067 ms
tom=# insert into todo values ('Retry on Android before app submission', '8.x and earlier');
INSERT 0 1
Time: 23.312 ms
tom=# \timing
Timing is off.
Watch
The \watch command repeats the previous command at the specified interval. To use it, enter the SQL command you want repeated, then use \watch followed by the number of seconds you want for the interval between repeats, for rexample, \watch 1 to repeat it every second.
Example of the \Watch command
Here’s an example of using \watch to see if any records have been inserted within the last 5 seconds.
tom=# select count(*);
count
--------
726
(726 rows)
tom=# \watch 5
Mon Nov 16 13:50:36 2020 (every 2s)
count
--------
726
(726 rows)
Mon Nov 16 13:50:38 2020 (every 2s)
count
--------
726
(726 rows)
Mon Nov 16 13:50:40 2020 (every 2s)
count
--------
726
(726 rows)
Locate the pg_hba.conf file
Postgres configuration is stored in a file named pg_hba.conf somewhere in the file system, but that location varies widely. The way to find it is to use show hba_file like this:
show hba_file;
See below for hot reloading this file while Postgres is running.
Reload the configuration file while Postgres is running
If you make changes to the pg_hba.conf Postgres configuration sometimes you need to restart. But you may just choose to reload the pg_hba.conf configuration file like this:
SELECT pg_reload_conf();
Reference
- https://tomcam.github.io/postgres
- PostgreSQL offical docs: Server Administration
psql, a.k.a the PostgreSQL interactive terminalcreatedbin the PostgreSQL offical docsCREATE TABLEin the PostgreSQL official docsINSERTin the PostgreSQL official docs