django db_index code example

Example 1: on_delete options django

There are seven possible actions to take when such event occurs:

CASCADE: When the referenced object is deleted, also delete the objects that have references to it 
  (when you remove a blog post for instance, you might want to delete comments as well). 
  SQL equivalent: CASCADE.
      
PROTECT: Forbid the deletion of the referenced object. 
  To delete it you will have to delete all objects that reference it manually. 
  SQL equivalent: RESTRICT.
    
RESTRICT: (introduced in Django 3.1) Similar behavior as PROTECT that matches SQL's RESTRICT more accurately. (See django documentation example)
SET_NULL: Set the reference to NULL (requires the field to be nullable). 
  For instance, when you delete a User, you might want to keep the comments he posted on blog posts, 
  but say it was posted by an anonymous (or deleted) user. 
  SQL equivalent: SET NULL.
    
SET_DEFAULT: Set the default value. SQL equivalent: SET DEFAULT.
    
SET(...): Set a given value. This one is not part of the SQL standard and is entirely handled by Django.
  
DO_NOTHING: Probably a very bad idea since this would create integrity issues in your database 
  (referencing an object that actually doesn't exist). SQL equivalent: NO ACTION.

Example 2: db_index django

This is not really django specific; more to do with databases. You add indexes on columns when you want to speed up searches on that column.

Typically, only the primary key is indexed by the database. This means look ups using the primary key are optimized.

If you do a lot of lookups on a secondary column, consider adding an index to that column to speed things up.

Keep in mind, like most problems of scale, these only apply if you have a statistically large number of rows (10,000 is not large).

Additionally, every time you do an insert, indexes need to be updated. So be careful on which column you add indexes.

As always, you can only optimize what you can measure - so use the EXPLAIN statement and your database logs (especially any slow query logs) to find out where indexes can be useful.