.. -*- coding: utf-8; -*-

==========

 Postgres

==========

.. contents::

   :local:

Installing on Debian

====================

Install and create new user and database::

  $ sudo apt-get install postgresql postgresql-client

  $ sudo su - postgres

  % psql

  postgres=# CREATE USER myuser WITH PASSWORD 'mypass';

  postgres=# CREATE DATABASE mydb OWNER myuser;

  postgres=# \q

Connect as user ``mypguser`` to new database::

  $ su - mypguser

  $ psql mypgdatabase

In order to create local host superuser::

  $ sudo su - postgres

  $ createuser --superuser USER

  $ exit

  $ sudo -u USER psql

..

https://wiki.debian.org/PostgreSql

  Debian wiki instructions.

Check version

=============

By SQL query::

  SELECT version();

  SHOW server_version;

  SHOW server_version_num;

From executable::

  $ postgres -V

Check locations

===============

::

  SHOW hba_file;

  SHOW config_file;

Check settings

==============

::

   SELECT name, setting FROM pg_settings;

List databases, schemas and tables

==================================

Default database is ``postgres``.

To list databases and database locales::

  $ psql -U pgadmin -l

or::

  => SELECT datname FROM pg_database WHERE datistemplate = false;

  => \l

To switch databases::

  => \connect NAME

Schemas::

To list all tables in the current database::

or in all schemas::

  => \dt *.*

List views::

  \dv

List users::

  \du

  \du+

List schemas::

  \dn

List indexes::

  \di

List functions::

  \df

List triggers::

  \dy

List misc info::

  select current_database();

  select current_schema();

  select current_user;

  select session_user;

  select current_date;

  select version();

https://www.postgresql.org/docs/current/functions-info.html

  System Information Functions.

Set default schema.

===================

::

  set search_path to NAME;

  set schema 'NAME';

Database, table, index size

===========================

Database size::

  SELECT pg_database_size('geekdb');  -- in bytes

  SELECT pg_size_pretty(pg_database_size('dbname'));

List of databases sizes::

  \l+

List tables sizes::

  \d+

Table total size (with indexes)::

  SELECT pg_size_pretty(pg_total_relation_size('schemaname.tablename'));

Sole table size (without indexes and other)::

  SELECT pg_size_pretty(pg_relation_size('schemaname.tablename'));

Largest table in the PostgreSQL database::

  SELECT relname, relpages FROM pg_class ORDER BY relpages DESC;

MVCC and transaction id

=======================

As being MVCC Postgres doesn't delete or update existing rows. Row visibility is controled by

transaction id (monotonous incremented number): ``xmin`` - first transaction that can see row,

``xmax`` - last transaction that can see row:::

  SELECT xmin, xmax, * FROM foo WHERE id < 5 ORDER BY id;

Using psql client

=================

Using password file ``~/.pgpass``::

  # comment

  hostname:port:database:username:password

  hostname:port:*:username:password

  hostname:*:*:username:password

Connect by::

  $ psql -U $USER -h $HOST  $SCHEMA

How to view execution plan::

  EXPLAIN query;

  EXPLAIN ANALYZE query;

Collect statistics::

  ANALYZE table;

How to redirect the output of query to a file::

  \o output_file

  SELECT * FROM pg_class;

Switch database::

  \connect DBNAME

  \c DBNAME

Load external SQL file::

  \i my.sql

Export data

===========

Export from ``psql``::

  \copy (SELECT * FROM foo) TO '/tmp/my.csv' WITH CSV

Import data

===========

Import CSV file::

  \copy tbl_name from 'my.csv' csv;

  \copy tbl_name from 'my.csv' delimiter ':' csv;

  $ psql -U $USER -h $HOST $DB -c "\\copy tbl_name from 'my.csv' csv"

JDBC driver

===========

https://jdbc.postgresql.org/about/about.html

  The current version of the driver should be compatible with PostgreSQL 8.2 and higher, and Java 6

  (JDBC 4.0), Java 7 (JDBC 4.1), Java 8 (JDBC 4.2) and Java 9.

https://jdbc.postgresql.org/download.html

  Java/JDBC/Postgres compatibility table.