Created database structure (markdown)

database-structure.md

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+mindstorming for a database which would be used inside searx in future:
+
+regarding to Issue [#205](https://github.com/asciimoo/searx/issues/205)
+
+### Database-Systems
+
+#### SQLite3
+
+* minimalistic, already available inside Python
+
+#### Peewee
+
+* can use SQLite as well as MySQL and PostgreSQL
+* more flexible for the admin
+
+### Tables
+
+#### server preferences
+
+#### users
+
+if we have a login (at least for admins), we require a user-table
+
+**user**
+
+| db column   | db options       | description                               |
+| ----------- | ---------------- | ----------------------------------------- |
+| id          | AI, PRIMARY      |                                           |
+| username    | INDEX            |                                           |
+| password    |                  | salted and hashed                         |
+| is_admin    |                  | finer right system would be more flexibel |
+
+#### stats
+
+how could we represent the stats in the best way?
+
+it would be cool if we can represent the data inside a timeline, but without too big overhead. [#162](https://github.com/asciimoo/searx/issues/162)
+
+#### https_rewrite
+
+if we are specify only one url_pattern for every possible url, we can use Database search instead of regex-matching. This should improve the speed well, specifically for larger datasets.
+
+I think the most efficient implementation would be a n:m representation. We can cache the rewrite_rules inside python, and use the database to find out what rewrite_rules have to be called.
+
+**https_urls**
+
+| db column   | db options       | description   |
+| ----------- | ---------------- | ------------- |
+| id          | AI, PRIMARY      |               |
+| url_pattern | UNIQE, INDEX     | wildcard is inside database, how to handle this? |
+
+**https_url_to_rewrite**
+
+| db column   | db options       | description   |
+| ----------- | ---------------- | ------------- |
+| id          | AI, PRIMARY      |               |
+| url_id      | INDEX            |               |
+| rewrite_id  |                  |               |
+
+**https_rewrite_rules**
+
+| db column    | db options       | description   |
+| ------------ | ---------------- | ------------- |
+| id           | AI, PRIMARY      |               |
+| rewrite_from |                  |               |
+| rewrite_to   |                  |               |