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+-- LICENSE AND COPYRIGHT
+--
+-- Copyright (C) 2010 Hubert depesz Lubaczewski
+--
+-- This program is distributed under the (Revised) BSD License:
+-- L<http://www.opensource.org/licenses/bsd-license.php>
+--
+-- Redistribution and use in source and binary forms, with or without
+-- modification, are permitted provided that the following conditions
+-- are met:
+--
+-- * Redistributions of source code must retain the above copyright
+-- notice, this list of conditions and the following disclaimer.
+--
+-- * Redistributions in binary form must reproduce the above copyright
+--   notice, this list of conditions and the following disclaimer in the
+--   documentation and/or other materials provided with the distribution.
+--
+-- * Neither the name of Hubert depesz Lubaczewski's Organization
+--   nor the names of its contributors may be used to endorse or
+--   promote products derived from this software without specific
+--   prior written permission.
+--
+-- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+-- AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+-- IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+-- DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
+-- FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+-- DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+-- SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+-- CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+-- OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+-- OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+--
+-- Code origin: https://gitlab.com/depesz/Versioning/blob/master/install.versioning.sql
+--
+--
+-- # NAME
+--
+-- **Versioning** - simplistic take on tracking and applying changes to databases.
+--
+-- # DESCRIPTION
+--
+-- This project strives to provide simple way to manage changes to
+-- database.
+--
+-- Instead of making changes on development server, then finding
+-- differences between production and development, deciding which ones
+-- should be installed on production, and finding a way to install them -
+-- you start with writing diffs themselves!
+--
+-- # INSTALLATION
+--
+-- To install versioning simply run install.versioning.sql in your database
+-- (all of them: production, stage, test, devel, ...).
+--
+-- # USAGE
+--
+-- In your files with patches to database, put whole logic in single
+-- transaction, and use \_v.\* functions - usually \_v.register_patch() at
+-- least to make sure everything is OK.
+--
+-- For example. Let's assume you have patch files:
+--
+-- ## 0001.sql:
+--
+-- ```
+-- create table users (id serial primary key, username text);
+-- ```
+--
+-- ## 0002.sql:
+--
+-- ```
+-- insert into users (username) values ('depesz');
+-- ```
+-- To change it to use versioning you would change the files, to this
+-- state:
+--
+-- 0000.sql:
+--
+-- ```
+-- BEGIN;
+-- select _v.register_patch('000-base', NULL, NULL);
+-- create table users (id serial primary key, username text);
+-- COMMIT;
+-- ```
+--
+-- ## 0002.sql:
+--
+-- ```
+-- BEGIN;
+-- select _v.register_patch('001-users', ARRAY['000-base'], NULL);
+-- insert into users (username) values ('depesz');
+-- COMMIT;
+-- ```
+--
+-- This will make sure that patch 001-users can only be applied after
+-- 000-base.
+--
+-- # AVAILABLE FUNCTIONS
+--
+-- ## \_v.register_patch( TEXT )
+--
+-- Registers named patch, or dies if it is already registered.
+--
+-- Returns integer which is id of patch in \_v.patches table - only if it
+-- succeeded.
+--
+-- ## \_v.register_patch( TEXT, TEXT[] )
+--
+-- Same as \_v.register_patch( TEXT ), but checks is all given patches (given as
+-- array in second argument) are already registered.
+--
+-- ## \_v.register_patch( TEXT, TEXT[], TEXT[] )
+--
+-- Same as \_v.register_patch( TEXT, TEXT[] ), but also checks if there are no conflicts with preexisting patches.
+--
+-- Third argument is array of names of patches that conflict with current one. So
+-- if any of them is installed - register_patch will error out.
+--
+-- ## \_v.unregister_patch( TEXT )
+--
+-- Removes information about given patch from the versioning data.
+--
+-- It doesn't remove objects that were created by this patch - just removes
+-- metainformation.
+--
+-- ## \_v.assert_user_is_superuser()
+--
+-- Make sure that current patch is being loaded by superuser.
+--
+-- If it's not - it will raise exception, and break transaction.
+--
+-- ## \_v.assert_user_is_not_superuser()
+--
+-- Make sure that current patch is not being loaded by superuser.
+--
+-- If it is - it will raise exception, and break transaction.
+--
+-- ## \_v.assert_user_is_one_of(TEXT, TEXT, ... )
+--
+-- Make sure that current patch is being loaded by one of listed users.
+--
+-- If ```current_user``` is not listed as one of arguments - function will raise
+-- exception and break the transaction.
+
+BEGIN;
+
+-- This file adds versioning support to database it will be loaded to.
+-- It requires that PL/pgSQL is already loaded - will raise exception otherwise.
+-- All versioning "stuff" (tables, functions) is in "_v" schema.
+
+-- All functions are defined as 'RETURNS SETOF INT4' to be able to make them to RETURN literally nothing (0 rows).
+-- >> RETURNS VOID<< IS similar, but it still outputs "empty line" in psql when calling.
+CREATE SCHEMA IF NOT EXISTS _v;
+COMMENT ON SCHEMA _v IS 'Schema for versioning data and functionality.';
+
+CREATE TABLE IF NOT EXISTS _v.patches (
+    patch_name  TEXT        PRIMARY KEY,
+    applied_tsz TIMESTAMPTZ NOT NULL DEFAULT now(),
+    applied_by  TEXT        NOT NULL,
+    requires    TEXT[],
+    conflicts   TEXT[]
+);
+COMMENT ON TABLE _v.patches              IS 'Contains information about what patches are currently applied on database.';
+COMMENT ON COLUMN _v.patches.patch_name  IS 'Name of patch, has to be unique for every patch.';
+COMMENT ON COLUMN _v.patches.applied_tsz IS 'When the patch was applied.';
+COMMENT ON COLUMN _v.patches.applied_by  IS 'Who applied this patch (PostgreSQL username)';
+COMMENT ON COLUMN _v.patches.requires    IS 'List of patches that are required for given patch.';
+COMMENT ON COLUMN _v.patches.conflicts   IS 'List of patches that conflict with given patch.';
+
+CREATE OR REPLACE FUNCTION _v.register_patch( IN in_patch_name TEXT, IN in_requirements TEXT[], in_conflicts TEXT[], OUT versioning INT4 ) RETURNS setof INT4 AS $$
+DECLARE
+    t_text   TEXT;
+    t_text_a TEXT[];
+    i INT4;
+BEGIN
+    -- Thanks to this we know only one patch will be applied at a time
+    LOCK TABLE _v.patches IN EXCLUSIVE MODE;
+
+    SELECT patch_name INTO t_text FROM _v.patches WHERE patch_name = in_patch_name;
+    IF FOUND THEN
+        RAISE EXCEPTION 'Patch % is already applied!', in_patch_name;
+    END IF;
+
+    t_text_a := ARRAY( SELECT patch_name FROM _v.patches WHERE patch_name = any( in_conflicts ) );
+    IF array_upper( t_text_a, 1 ) IS NOT NULL THEN
+        RAISE EXCEPTION 'Versioning patches conflict. Conflicting patche(s) installed: %.', array_to_string( t_text_a, ', ' );
+    END IF;
+
+    IF array_upper( in_requirements, 1 ) IS NOT NULL THEN
+        t_text_a := '{}';
+        FOR i IN array_lower( in_requirements, 1 ) .. array_upper( in_requirements, 1 ) LOOP
+            SELECT patch_name INTO t_text FROM _v.patches WHERE patch_name = in_requirements[i];
+            IF NOT FOUND THEN
+                t_text_a := t_text_a || in_requirements[i];
+            END IF;
+        END LOOP;
+        IF array_upper( t_text_a, 1 ) IS NOT NULL THEN
+            RAISE EXCEPTION 'Missing prerequisite(s): %.', array_to_string( t_text_a, ', ' );
+        END IF;
+    END IF;
+
+    INSERT INTO _v.patches (patch_name, applied_tsz, applied_by, requires, conflicts ) VALUES ( in_patch_name, now(), current_user, coalesce( in_requirements, '{}' ), coalesce( in_conflicts, '{}' ) );
+    RETURN;
+END;
+$$ language plpgsql;
+COMMENT ON FUNCTION _v.register_patch( TEXT, TEXT[], TEXT[] ) IS 'Function to register patches in database. Raises exception if there are conflicts, prerequisites are not installed or the migration has already been installed.';
+
+CREATE OR REPLACE FUNCTION _v.register_patch( TEXT, TEXT[] ) RETURNS setof INT4 AS $$
+    SELECT _v.register_patch( $1, $2, NULL );
+$$ language sql;
+COMMENT ON FUNCTION _v.register_patch( TEXT, TEXT[] ) IS 'Wrapper to allow registration of patches without conflicts.';
+CREATE OR REPLACE FUNCTION _v.register_patch( TEXT ) RETURNS setof INT4 AS $$
+    SELECT _v.register_patch( $1, NULL, NULL );
+$$ language sql;
+COMMENT ON FUNCTION _v.register_patch( TEXT ) IS 'Wrapper to allow registration of patches without requirements and conflicts.';
+
+CREATE OR REPLACE FUNCTION _v.unregister_patch( IN in_patch_name TEXT, OUT versioning INT4 ) RETURNS setof INT4 AS $$
+DECLARE
+    i        INT4;
+    t_text_a TEXT[];
+BEGIN
+    -- Thanks to this we know only one patch will be applied at a time
+    LOCK TABLE _v.patches IN EXCLUSIVE MODE;
+
+    t_text_a := ARRAY( SELECT patch_name FROM _v.patches WHERE in_patch_name = ANY( requires ) );
+    IF array_upper( t_text_a, 1 ) IS NOT NULL THEN
+        RAISE EXCEPTION 'Cannot uninstall %, as it is required by: %.', in_patch_name, array_to_string( t_text_a, ', ' );
+    END IF;
+
+    DELETE FROM _v.patches WHERE patch_name = in_patch_name;
+    GET DIAGNOSTICS i = ROW_COUNT;
+    IF i < 1 THEN
+        RAISE EXCEPTION 'Patch % is not installed, so it can''t be uninstalled!', in_patch_name;
+    END IF;
+
+    RETURN;
+END;
+$$ language plpgsql;
+COMMENT ON FUNCTION _v.unregister_patch( TEXT ) IS 'Function to unregister patches in database. Dies if the patch is not registered, or if unregistering it would break dependencies.';
+
+CREATE OR REPLACE FUNCTION _v.assert_patch_is_applied( IN in_patch_name TEXT ) RETURNS TEXT as $$
+DECLARE
+    t_text TEXT;
+BEGIN
+    SELECT patch_name INTO t_text FROM _v.patches WHERE patch_name = in_patch_name;
+    IF NOT FOUND THEN
+        RAISE EXCEPTION 'Patch % is not applied!', in_patch_name;
+    END IF;
+    RETURN format('Patch %s is applied.', in_patch_name);
+END;
+$$ language plpgsql;
+COMMENT ON FUNCTION _v.assert_patch_is_applied( TEXT ) IS 'Function that can be used to make sure that patch has been applied.';
+
+CREATE OR REPLACE FUNCTION _v.assert_user_is_superuser() RETURNS TEXT as $$
+DECLARE
+    v_super bool;
+BEGIN
+    SELECT usesuper INTO v_super FROM pg_user WHERE usename = current_user;
+    IF v_super THEN
+        RETURN 'assert_user_is_superuser: OK';
+    END IF;
+    RAISE EXCEPTION 'Current user is not superuser - cannot continue.';
+END;
+$$ language plpgsql;
+COMMENT ON FUNCTION _v.assert_user_is_superuser() IS 'Function that can be used to make sure that patch is being applied using superuser account.';
+
+CREATE OR REPLACE FUNCTION _v.assert_user_is_not_superuser() RETURNS TEXT as $$
+DECLARE
+    v_super bool;
+BEGIN
+    SELECT usesuper INTO v_super FROM pg_user WHERE usename = current_user;
+    IF v_super THEN
+        RAISE EXCEPTION 'Current user is superuser - cannot continue.';
+    END IF;
+    RETURN 'assert_user_is_not_superuser: OK';
+END;
+$$ language plpgsql;
+COMMENT ON FUNCTION _v.assert_user_is_not_superuser() IS 'Function that can be used to make sure that patch is being applied using normal (not superuser) account.';
+
+CREATE OR REPLACE FUNCTION _v.assert_user_is_one_of(VARIADIC p_acceptable_users TEXT[] ) RETURNS TEXT as $$
+DECLARE
+BEGIN
+    IF current_user = any( p_acceptable_users ) THEN
+        RETURN 'assert_user_is_one_of: OK';
+    END IF;
+    RAISE EXCEPTION 'User is not one of: % - cannot continue.', p_acceptable_users;
+END;
+$$ language plpgsql;
+COMMENT ON FUNCTION _v.assert_user_is_one_of(TEXT[]) IS 'Function that can be used to make sure that patch is being applied by one of defined users.';
+
+COMMIT;