Use case sensitive query for username in user table to decrease execution time.

Setup

1. Create a Moodle application with a PostgreSQL database.
2. Login as an admin.
3. GOTO Site administration->Development tab.
4. Click on 'Make test course'.
5. Size of the course should be Large or Extra Large. This can take some time.
6. Fill in the rest of the required fields with dummy data.
7. Click 'Create course'

Test the efficiency of new SQL query

1. Login to the PostgreSQL database.
2. Execute

   EXPLAIN ANALYZE SELECT * FROM mdl_user WHERE LOWER(username) = LOWER('john') AND deleted <> 1 AND mnethostid = 1;

3. Execute

   EXPLAIN ANALYZE SELECT * FROM mdl_user WHERE username = 'john' AND deleted <> 1 AND mnethostid = 1;

4. Confirm that the first query uses a Seq Scan.
5. Confirm that the second query uses an Index Scan.
6. Confirm that the second query takes less execution time than the first
    

Test that code now executes desired query

1. Ensure that logging is enabled in your database. Instructions for psql.
2. Open moodle application in a web browser
3. If already logged in, log out.
4. Enter login information and click login. Can be any type of user.
5. Open psql log.
6. Use a search function such as `/` in vim and search for the statement below.
7. Verify that the following statement was found in the search.

   SELECT * FROM mdl_user WHERE username = $1 AND deleted <> 1 AND mnethostid = $2

 Test altered code does not affect existing business logic

1. Run existing unit tests in Moodle
2. Tests should be run via Travis CI using the moodle core travis.yml config file.
3. Ensure that all unit tests still pass