[doc] add blog post about SQL servers

Modified merge of [22a79a4] from searx.

[22a79a4] 22a79a4896

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/blog/index.rst

@@ -13,3 +13,4 @@ Blog
    private-engines
    command-line-engines
    search-indexer-engines
+   sql-engines

docs/blog/sql-engines.rst

@@ -0,0 +1,160 @@
+===========
+SQL engines
+===========
+
+.. sidebar:: further read
+
+   - `SQLite <https://www.sqlite.org/index.html>`_
+   - `PostgreSQL <https://www.postgresql.org>`_
+   - `MySQL <https://www.mysql.com>`_
+
+With the *SQL engines* you can bind SQL databases into SearxNG.  The following
+Relational Database Management System (RDBMS) are supported:
+
+- :ref:`engine sqlite`
+- :ref:`engine postgresql`
+- :ref:`engine mysql_server`
+
+All of the engines above are just commented out in the :origin:`settings.yml
+<searx/settings.yml>`, as you have to set the required attributes for the
+engines, e.g. ``database:`` ...
+
+.. code:: yaml
+
+   - name: ...
+     engine: {sqlite|postgresql|mysql_server}
+     database: ...
+     result_template: {template_name}
+     query_str: ...
+
+By default, the engines use the ``key-value`` template for displaying results /
+see :origin:`oscar <searx/templates/oscar/result_templates/key-value.html>` &
+:origin:`simple <searx/templates/simple/result_templates/key-value.html>`
+themes.  If you are not satisfied with the original result layout, you can use
+your own template, set ``result_template`` attribute to ``{template_name}`` and
+place the templates at::
+
+  searx/templates/{theme_name}/result_templates/{template_name}
+
+As mentioned in previous blog posts, if you do not wish to expose these engines
+on a public instance, you can still add them and limit the access by setting
+``tokens`` as described in section :ref:`private engines`.
+
+Configure the engines
+=====================
+
+The configuration of the new database engines are similar.  You must put a valid
+SQL-SELECT query in ``query_str``.  At the moment you can only bind at most one
+parameter in your query.  By setting the attribute ``limit`` you can define how
+many results you want from the SQL server.  Basically, it is the same as the
+``LIMIT`` keyword in SQL.
+
+Please, do not include ``LIMIT`` or ``OFFSET`` in your SQL query as the engines
+rely on these keywords during paging.  If you want to configure the number of
+returned results use the option ``limit``.
+
+.. _engine sqlite:
+
+SQLite
+------
+
+.. _MediathekView: https://mediathekview.de/
+
+SQLite is a small, fast and reliable SQL database engine.  It does not require
+any extra dependency.  To demonstrate the power of database engines, here is a
+more complex example which reads from a MediathekView_ (DE) movie database.  For
+this example of the SQlite engine download the database:
+
+- https://liste.mediathekview.de/filmliste-v2.db.bz2
+
+and unpack into ``searx/data/filmliste-v2.db``.  To search the database use e.g
+Query to test: ``!mediathekview concert``
+
+.. code:: yaml
+
+   - name: mediathekview
+     engine: sqlite
+     disabled: False
+     categories: general
+     result_template: default.html
+     database: searx/data/filmliste-v2.db
+     query_str:  >-
+       SELECT title || ' (' || time(duration, 'unixepoch') || ')' AS title,
+              COALESCE( NULLIF(url_video_hd,''), NULLIF(url_video_sd,''), url_video) AS url,
+              description AS content
+         FROM film
+        WHERE title LIKE :wildcard OR description LIKE :wildcard
+        ORDER BY duration DESC
+
+
+Extra Dependencies
+------------------
+
+For using :ref:`engine postgresql` or :ref:`engine mysql_server` you need to
+install additional packages in Python's Virtual Environment of your SearxNG
+instance.  To switch into the environment (:ref:`searx-src`) you can use
+:ref:`searx.sh`::
+
+  $ sudo utils/searx.sh shell
+  (searx-pyenv)$ pip install ...
+
+
+.. _engine postgresql:
+
+PostgreSQL
+----------
+
+.. _psycopg2: https://www.psycopg.org/install
+
+.. sidebar:: requirements
+
+   ``pip install`` psycopg2_
+
+PostgreSQL is a powerful and robust open source database.  Before configuring
+the PostgreSQL engine, you must install the dependency ``psychopg2``.  You can
+find an example configuration below:
+
+.. code:: yaml
+
+   - name: my_database
+     engine: postgresql
+     database: my_database
+     username: searx
+     password: password
+     query_str: 'SELECT * from my_table WHERE my_column = %(query)s'
+
+.. _engine mysql_server:
+
+MySQL
+-----
+
+.. _mysql-connector-python: https://pypi.org/project/mysql-connector-python
+
+.. sidebar:: requirements
+
+   ``pip install`` mysql-connector-python_
+
+MySQL is said to be the most popular open source database. Before enabling MySQL
+engine, you must install the package ``mysql-connector-python``.
+
+The authentication plugin is configurable by setting ``auth_plugin`` in the
+attributes.  By default it is set to ``caching_sha2_password``.  This is an
+example configuration for quering a MySQL server:
+
+.. code:: yaml
+
+   - name: my_database
+     engine: mysql_server
+     database: my_database
+     username: searx
+     password: password
+     limit: 5
+     query_str: 'SELECT * from my_table WHERE my_column=%(query)s'
+
+
+Acknowledgement
+===============
+
+This development was sponsored by `Search and Discovery Fund
+<https://nlnet.nl/discovery>`_ of `NLnet Foundation <https://nlnet.nl/>`_ .
+