Improve message and documentation around moved data (#19453)

* Improve message and documentation around moved data

In Airflow 2.2.2 we introduced a fix in #18953 where the corrupted
data was moved to a separate table. However some of our users
(rightly) might not have the context. We've never had anything
like that before, so the users who treat Airflow DB as
black-boxes might get confused on what the error means and what
they should do in this case.

You can see it in #19440 converted into discussion #19444 and #19421
indicate that the message is a bit unclear for users. This PR attempts to
improve that it adds `upgrading` section to our documentation and have the
message link to it so that rather than asking questions in the issues,
users can find context and answers what they should do in our docs.

It also guides the users who treat Airflow DB as "black-box" on how they
can use their tools and airflow db shell to fix the problem.

(cherry picked from commit de43fb3)

airflow/www/templates/airflow/dags.html

@@ -56,7 +56,8 @@
       Airflow found incompatible data in the <code>{{ original_table_name }}</code> table in the
       metadatabase, and has moved them to <code>{{ moved_table_name }}</code> during the database migration
       to upgrade. Please inspect the moved data to decide whether you need to keep them, and manually drop
-      the <code>{{ moved_table_name }}</code> table to dismiss this warning.
+      the <code>{{ moved_table_name }}</code> table to dismiss this warning. Read more about it
+      in <a href={{ get_docs_url("installing/upgrading.html") }}><b>Upgrading</b></a>.
     {% endcall %}
   {% endfor %}
   {{ super() }}

docs/apache-airflow/installation/index.rst

@@ -32,6 +32,7 @@ Installation
     Installing from sources <installing-from-sources>
     Installing from PyPI <installing-from-pypi>
     Setting up the database <setting-up-the-database>
+    Upgrading <upgrading>
 
 This page describes installations options that you might use when considering how to install Airflow.
 Airflow consists of many components, often distributed among many physical or virtual machines, therefore

docs/apache-airflow/installation/setting-up-the-database.rst

@@ -30,3 +30,6 @@ not running while the upgrade is being executed.
 
 In some deployments, such as :doc:`helm-chart:index`, both initializing and running the database migration
 is executed automatically when Airflow is upgraded.
+
+Sometimes, after the upgrade, you are also supposed to do some post-migration actions.
+See :doc:`/installation/upgrading` for more details about upgrading and doing post-migration actions.

docs/apache-airflow/installation/upgrading.rst

@@ -0,0 +1,94 @@
+ .. Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+ ..   http://www.apache.org/licenses/LICENSE-2.0
+
+ .. Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+
+Upgrading Airflow to a newer version
+------------------------------------
+
+Why you need to upgrade
+=======================
+
+Newer Airflow versions can contain Database migrations so it is recommended that you run
+``airflow db upgrade`` to Upgrade your Database with the schema changes in the Airflow version
+you are upgrading to.
+
+When you need to upgrade
+========================
+
+If you have a custom deployment based on virtualenv or Docker Containers, you usually need to run
+the DB upgrade manually as part of the upgrade process.
+
+In some cases the upgrade happens automatically - it depends if in your deployment, the upgrade is
+built-in as post-install action. For example when you are using :doc:`helm-chart:index` with
+post-upgrade hooks enabled, the database upgrade happens automatically right after the new software
+is installed. Similarly all Airflow-As-A-Service solutions perform the upgrade automatically for you,
+when you choose to upgrade airflow via their UI.
+
+How to upgrade
+==============
+
+In order to manually upgrade the database you should run the ``airflow db upgrade`` command in your
+environment. It can be run either in your virtual environment or in the containers that give
+you access to Airflow ``CLI`` :doc:`/usage-cli` and the database.
+
+Migration best practices
+========================
+
+Depending on the size of your database and the actual migration it might take quite some time to migrate it,
+so if you have long history and big database, it is recommended to make a copy of the database first and
+perform a test migration to assess how long the migration will take. Typically "Major" upgrades might take
+longer as adding new features require sometimes restructuring of the database.
+
+Post-upgrade warnings
+=====================
+
+Typically you just need to successfully run ``airflow db upgrade`` command and this is all. However in
+some cases, the migration might find some old, stale and probably wrong data in your database and moves it
+aside to a separate table. In this case you might get warning in your webserver UI about the data found.
+
+Typical message that you might see:
+
+  Airflow found incompatible data in the <original table> table in the
+  metadatabase, and has moved them to <new table> during the database migration to upgrade.
+  Please inspect the moved data to decide whether you need to keep them,
+  and manually drop the <new table> table to dismiss this warning.
+
+When you see such message, it means that some of your data was corrupted and you should inspect it
+to determine whether you would like to keep or delete some of that data. Most likely the data was corrupted
+and left-over from some bugs and can be safely deleted - because this data would not be anyhow visible
+and useful in Airflow. However if you have particular need for auditing or historical reasons you might
+choose to store it somewhere. Unless you have specific reasons to keep the data most likely deleting it
+is your best option.
+
+There are various ways you can inspect and delete the data - if you have direct access to the
+database using your own tools (often graphical tools showing the database objects), you can drop such
+table or rename it or move it to another database using those tools. If you don't have such tools you
+can use the ``airflow db shell`` command - this will drop you in the db shell tool for your database and you
+will be able to both inspect and delete the table.
+
+Please replace ``<table>`` in the examples with the actual table name as printed in the warning message.
+
+Inspecting a table:
+
+.. code-block:: sql
+
+   SELECT * FROM <table>;
+
+Deleting a table:
+
+.. code-block:: sql
+
+   DROP TABLE <table>;

docs/spelling_wordlist.txt

@@ -956,6 +956,7 @@ mediawiki
 memberOf
 mesos
 metaclass
+metadatabase
 metarouter
 metastore
 mget