[Java] Scorecard algorithm

Some time ago I took an online coding test which left me with an unfinished exercise; I played too long with the other one and had no time to finish this :(

However it seemed quite easy and interesting so I decided to finish it later, here is the description of the exercise as I remember it and my solution:

Bill plays some kind of game where the score is tracked on a scorecard; in addition to simple scoring moves, he can attempt (and possibly fail) more rewarding moves that will influence the score calculation.

Bill starts with a score of 0 and cannot earn negative points.

Bill can alter his total score by:

• scoring a point (integer) - it will be added to the total score
• doubling a previous score ("X") - the previous score will be doubled before adding it to the total score. If there is no previous score, it counts as 0
• summing two previous scores ("+") - the two precedent scores will be summed together before adding the result to the total score. If there are no predecessors, it counts as 0; if there is only one predecessor, it will be added again to the total score
• failing a move ("Z") - the previous score will be ignored from the total score and its value will also not be considered for subsequent special moves. If there is no previous score, this has no effects

[Oracle] Aurora module exception when compiling Java code on the DB

Sometimes it is possible to receive an exception when compiling Java code or running tools that generate Java code on the Oracle DB with this signature:

ORA-29516: Aurora assertion failure: Assertion failure at some_source:some_line_number

It is usually easy to prevent this error by disabling the JIT compilation on the DB:

alter session set java_jit_enabled = false;

Facebook and Airbnb login bug, now you are someone else

Well, everyone nowadays is running around trying to spot and fix bugs for money and I just stumble upon someone else's information for free.

This is an unexpected Christmas gift given it happened close to this huge fail from VISA, which makes it possible to guess full credit card details in a frighteningly fast amount of time. So read that article first, and then image what could someone do if they had all your personal info, including partial credit card data.

[Oracle] Initialise DB statistics to improve performance

A couple days ago we were analysing a customer issue concerning DB performance on simple and already optimised queries (indexes + hints).

Somehow, the customer had bad performance even after calculating statistics for tables and indexes. Turns out that everything was good except for the fact that Oracle can't imagine future statistics without some help; if a table is only filled after some time that the applications run, it is important to gather statistics again after the first initialisation.

It is also possible however to provide fake statistics immediately after the objects are created or modified so that the DB already has an idea of how the objects will look like and choose different query execution plans than the ones it would use for freshly created objects.

That's what the DBMS_STATS package and its EXPORT_ and IMPORT_ procedures are for.

[Oracle] List uploaded Java resources

After extending functionality on an Oracle DB with Java resources, it is possible to list the available ones and their status with a query on the user_objects table:

SELECT

  object_name

 ,object_type

 ,status

 ,timestamp

FROM

  user_objects

WHERE

  (    object_name NOT LIKE 'SYS_%' 

   AND object_name NOT LIKE 'CREATE$%' 

   AND object_name NOT LIKE 'JAVA$%' 

   AND object_name NOT LIKE 'LOADLOB%'

  ) 

AND object_type LIKE 'JAVA%'

ORDER BY

  object_type

 ,object_name

;

[Oracle] SQL SELECT cast to table

In Oracle it is possible to cast one type to another simply by using the CAST keyword.

This also applies to collections, meaning that the result of a SELECT can be casted to varray or nested table; in this case it is necessary to add the MULTISET keyword and make sure that all elements in the collection have a valid match.

Eg suppose you have a type:

CREATE TYPE int_list AS TABLE OF NUMBER(9);

You can run a query that selects some integers and cast the result set to int_list:

SELECT
  CAST(
    MULTISET(
      SELECT t.int_value
      FROM myTable t
    )
   AS int_list)
FROM dual;

[PL/SQL] existing state of package has been invalidated error

After compiling Oracle packages, it is possible to get a "existing state of packages has been discarded" or "existing state of package has been invalidated" error.

The cause is usually a global variable or constant declared in the package or package body that is re-initialized after the compilation action. The error is thrown to avoid having programs read the wrong state (variable/constant) of the package.

The easiest way to solve this issue is to simply disconnect the session and reconnect.