PSQL

Byadmin

Jul 5, 2022

Magic words:

psql -U postgres

Some interesting flags (to see all, use -h or –help depending on your psql version):

-E: will describe the underlaying queries of the \ commands (cool for learning!)
-l: psql will list all databases and then exit (useful if the user you connect with doesn't has a default database, like at AWS RDS)

Most \d commands support additional param of schema.name__ and accept wildcards like .

\?: Show help (list of available commands with an explanation)
\q: Quit/Exit
\c __database__: Connect to a database
\d __table__: Show table definition (columns, etc.) including triggers
\d+ __table__: More detailed table definition including description and physical disk size
\l: List databases
\dy: List events
\df: List functions
\di: List indexes
\dn: List schemas
\dt *.*: List tables from all schemas (if *.* is omitted will only show SEARCH_PATH ones)
\dT+: List all data types
\dv: List views
\dx: List all extensions installed
\df+ __function__ : Show function SQL code.
\x: Pretty-format query results instead of the not-so-useful ASCII tables
\copy (SELECT * FROM __table_name__) TO 'file_path_and_name.csv' WITH CSV: Export a table as CSV
\des+: List all foreign servers
\dE[S+]: List all foreign tables
\! __bash_command__: execute __bash_command__ (e.g. \! ls)

User Related:

\du: List users
\du __username__: List a username if present.
create role __test1__: Create a role with an existing username.
create role __test2__ noinherit login password __passsword__;: Create a role with username and password.
set role __test__;: Change role for current session to __test__.
grant __test2__ to __test1__;: Allow __test1__ to set its role as __test2__.
\deu+: List all user mapping on server

Configuration

Service management commands:

sudo service postgresql stop
sudo service postgresql start
sudo service postgresql restart

Changing verbosity & querying Postgres log:
1) First edit the config file, set a decent verbosity, save and restart postgres:

sudo vim /etc/postgresql/9.3/main/postgresql.conf

Uncomment/Change inside:

log_min_messages = debug5
log_min_error_statement = debug5
log_min_duration_statement = -1

sudo service postgresql restart

Now you will get tons of details of every statement, error, and even background tasks like VACUUMs

tail -f /var/log/postgresql/postgresql-9.3-main.log

How to add user who executed a PG statement to log (editing postgresql.conf):

log_line_prefix = ‘%t %u %d %a ‘

Check Extensions enabled in postgres: SELECT * FROM pg_extension;

Show available extensions: SELECT * FROM pg_available_extension_versions;

Create command

There are many CREATE choices, like CREATE DATABASE database_name, CREATE TABLE table_name … Parameters differ but can be checked at the official documentation.
Handy queries

SELECT * FROM pg_proc WHERE proname='__procedurename__': List procedure/function
SELECT * FROM pg_views WHERE viewname='__viewname__';: List view (including the definition)
SELECT pg_size_pretty(pg_total_relation_size('__table_name__'));: Show DB table space in use
SELECT pg_size_pretty(pg_database_size('__database_name__'));: Show DB space in use
show statement_timeout;: Show current user's statement timeout
SELECT * FROM pg_indexes WHERE tablename='__table_name__' AND schemaname='__schema_name__';: Show table indexes
Get all indexes from all tables of a schema:

SELECT
t.relname AS table_name,
i.relname AS index_name,
a.attname AS column_name
FROM
pg_class t,
pg_class i,
pg_index ix,
pg_attribute a,
pg_namespace n
WHERE
t.oid = ix.indrelid
AND i.oid = ix.indexrelid
AND a.attrelid = t.oid
AND a.attnum = ANY(ix.indkey)
AND t.relnamespace = n.oid
AND n.nspname = ‘kartones’
ORDER BY
t.relname,
i.relname

Execution data:
    Queries being executed at a certain DB:

SELECT datname, application_name, pid, backend_start, query_start, state_change, state, query
FROM pg_stat_activity
WHERE datname=’database_name‘;

Get all queries from all dbs waiting for data (might be hung):

SELECT * FROM pg_stat_activity WHERE waiting=’t’

Currently running queries with process pid:

SELECT
pg_stat_get_backend_pid(s.backendid) AS procpid,
pg_stat_get_backend_activity(s.backendid) AS current_query
FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s;

Get Connections by Database: SELECT datname, numbackends FROM pg_stat_database;

Casting:

CAST (column AS type) or column::type
'__table_name__'::regclass::oid: Get oid having a table name

Query analysis:

EXPLAIN __query__: see the query plan for the given query
EXPLAIN ANALYZE __query__: see and execute the query plan for the given query
ANALYZE [__table__]: collect statistics

Generating random data (source):

INSERT INTO some_table (a_float_value) SELECT random() * 100000 FROM generate_series(1, 1000000) i;

Get sizes of tables, indexes and full DBs:

select current_database() as database,
pg_size_pretty(total_database_size) as total_database_size,
schema_name,
table_name,
pg_size_pretty(total_table_size) as total_table_size,
pg_size_pretty(table_size) as table_size,
pg_size_pretty(index_size) as index_size
from ( select table_name,
table_schema as schema_name,
pg_database_size(current_database()) as total_database_size,
pg_total_relation_size(table_name) as total_table_size,
pg_relation_size(table_name) as table_size,
pg_indexes_size(table_name) as index_size
from information_schema.tables
where table_schema=current_schema() and table_name like ‘table_%’
order by total_table_size
) as sizes;

COPY command: Import/export from CSV to tables:

COPY table_name [ ( column_name [, …] ) ]
FROM { ‘filename’ | STDIN }
[ [ WITH ] ( option [, …] ) ]

COPY { table_name [ ( column_name [, …] ) ] | ( query ) }
TO { ‘filename’ | STDOUT }
[ [ WITH ] ( option [, …] ) ]

List all grants for a specific user

SELECT table_catalog, table_schema, table_name, privilege_type
FROM information_schema.table_privileges
WHERE grantee = ‘user_to_check’ ORDER BY table_name;

List all assigned user roles

SELECT
r.rolname,
r.rolsuper,
r.rolinherit,
r.rolcreaterole,
r.rolcreatedb,
r.rolcanlogin,
r.rolconnlimit,
r.rolvaliduntil,
ARRAY(SELECT b.rolname
FROM pg_catalog.pg_auth_members m
JOIN pg_catalog.pg_roles b ON (m.roleid = b.oid)
WHERE m.member = r.oid) as memberof,
r.rolreplication
FROM pg_catalog.pg_roles r
ORDER BY 1;

Check permissions in a table:

SELECT grantee, privilege_type
FROM information_schema.role_table_grants
WHERE table_name=’name-of-the-table’;

Kill all Connections:

SELECT pg_terminate_backend(pg_stat_activity.pid)
FROM pg_stat_activity
WHERE datname = current_database() AND pid <> pg_backend_pid();

Keyboard shortcuts

CTRL + R: reverse-i-search

Tools

ptop and pg_top: top for PG. Available on the APT repository from apt.postgresql.org.
pg_activity: Command line tool for PostgreSQL server activity monitoring.
Unix-like reverse search in psql:

$ echo “bind “^R” em-inc-search-prev” > $HOME/.editrc
$ source $HOME/.editrc

Show IP of the DB Instance: SELECT inet_server_addr();
File to save PostgreSQL credentials and permissions (format: hostname:port:database:username:password): chmod 600 ~/.pgpass
Collect statistics of a database (useful to improve speed after a Database Upgrade as previous query plans are deleted): ANALYZE VERBOSE;
To obtain the CREATE TABLE query of a table, any visual GUI like pgAdmin allows to easily, but else you can use pg_dump, e.g.: pg_dump -t '<schema>.<table>' --schema-only <database> (source)

Resources & Documentation

Postgres Weekly newsletter: The best way IMHO to keep up to date with PG news
100 psql Tips: Name says all, lots of useful tips!
PostgreSQL Exercises: An awesome resource to learn to learn SQL, teaching you with simple examples in a great visual way. Highly recommended.
A Performance Cheat Sheet for PostgreSQL: Great explanations of EXPLAIN, EXPLAIN ANALYZE, VACUUM, configuration parameters and more. Quite interesting if you need to tune-up a postgres setup.
annotated.conf: Annotations of all 269 postgresql.conf settings for PostgreSQL 10.
psql -c "\l+" -H -q postgres > out.html: Generate a html report of your databases (source: Daniel Westermann)

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