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Excel Parameter Validation

without comments

While working on a VBA write-up for some documentation on Excel 2007/2010, I ran into some interesting parameter validation rules for Excel User-Defined Functions (UDFs). I found that optional values are suppressed when you pass a cell reference that points to an empty cell.

Let’s say you develop a simple test function like the following. You may expect that it returns the number passed as a call parameter or the default value of the opt variable, 22, but it doesn’t.

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Function OptionalOverride(Optional opt As Variant = 22)
  ' Return the unfiltered call parameter.
  OptionalOverride = opt
End Function

A call to an OptionalOverride function like this works when the cell reference points to a value, but fails when it points to an empty cell. It returns a value of 0 with a numeric data type, as verified by the built-in TYPE function.

=OptionalOverride(D1)

This behavior means you always need to check for empty cell references and reassigns them a value inside an if-block. That is if you really want the default value to apply in all cases. The function includes the explicit call parameter assignment in the modified function.

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Function OptionalOverride(Optional opt As Variant = 22)
  ' Explicit assignments required when a cell reference points to an empty cell.
  If IsEmpty(opt) Then
    opt = 21
  End If
  ' Return the unfiltered call parameter unless empty then return 21.
  OptionalOverride = opt
End Function

The modified function returns (a) the call parameter value, (b) 21 when the call parameter points to (references) an empty cell, (c) 22 when you exclude the variable from the call parameter list, or (d) any string value found in the call parameter. The return of a string value is clearly not the desired behavior.

You must modify the if-block by checking whether the call parameter is some data type other than a number before assigning a default value. The following demonstrates the final parameter validating function.

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Function OptionalOverride(Optional opt As Variant = 22)
  ' Explicit assignments required when a cell reference points to an empty cell.
  If IsEmpty(opt) Or Not IsNumeric(opt) Then
    opt = 21
  End If
  ' Return the filtered call parameter.
  OptionalOverride = opt
End Function

Hope this helps a few folks trying to avoid that ugly #VALUE! error returning from your UDFs.

Written by maclochlainn

October 30th, 2010 at 11:58 pm

Reset MySQL root Password

with 8 comments

Sometimes the MySQL installation goes great but students forget their root password. It’s almost like the DBA who has the only copy of the root user’s password getting hit by a bus. How do you recover it? It’s not terribly difficult when deployed on the Windows OS (you’ll find a nice article on Linux here). This page takes you to standard documentation for resetting permissions.

There are two ways to do it. The first is quick and easy but risks letting others into the database through the network. The second requires a bit more work but ensures that network is shut while you disable security to reset the root password.

  1. The quick and easy way to disable security and reset the root password.

You add the following parameter to the my.ini configuration file in the [mysqld] block. While you’re editing the configuration file, you should also enter the other two. You’ll uncomment them in subsequent steps because they’re necessary to connect via a localhost OS pipe when you suppress the listener.

[mysqld]
 
# These let you safely reset the lost root password.
skip-grant-tables
#enable-named-pipe
#skip-networking

After you’ve saved these changes in the my.ini file, you should stop and restart the mysql51 service. If you named the Microsoft service something else, you should substitute it for mysql51 in the sample statements. The command-line steps are:

To stop the service:

net stop mysql51

To start the service:

net start mysql51

Now you can sign on as the root (superuser) without a password and change the password. However, you can’t do it through the normal command:

SET PASSWORD FOR 'student'@'%' = password('cangetin');

If you attempt that normal syntax, MySQL raises the following exception:

ERROR 1290 (HY000): The MySQL server IS running WITH the --skip-grant-tables option so it cannot execute this statement

You need to first connect to the mysql database, which holds the data dictionary or catalog. Then, you use a simple UPDATE statement to reset the root password.

-- Connect to the data dictionary.
USE mysql
 
-- Manually update the data dictionary entry.
UPDATE USER
SET    password = password('cangetin')
WHERE  USER = 'root'
AND    host = 'localhost';
  1. The secure way to disable security and reset the root password.

Remove the comment marks before the enable-named-pipe and skip-networking, if you added all three parameters while testing the easy way. Otherwise you should add the following three parameters to the my.ini configuration file in the [mysqld] block. The enable-named-pipe opens an OS pipe through which you can connect to the database. The skip-networking instructs the database not to start the MySQL listener.

[mysqld]
 
# These let you safely reset the lost root password.
skip-grant-tables
enable-named-pipe
skip-networking

After you’ve saved these changes in the my.ini file, you should stop and restart the mysql51 service. The command-line steps are:

To stop the service:

net stop mysql51

To start the service:

net start mysql51

You still can’t reset a password with the SET PASSWORD FOR 'user'@'host' syntax when you’ve disabled reading the database instance’s metadata. The syntax to connect to the database through the OS pipe as the unauthenticated root user is:

mysql -h . -uroot

Unfortunately, once you’ve connected, you can’t reset the password through the normal command because that’s disabled by the skip-grant-tables parameter. Check the example in the quick and easy way above.

With the data dictionary validation disabled, you need to first connect to the mysql database to make this change. The mysql database holds the data dictionary or catalog. You use a simple UPDATE statement to reset the root password once connected to the mysql database.

-- Connect to the data dictionary.
USE mysql
 
-- Manually update the data dictionary entry.
UPDATE USER
SET    password = password('cangetin')
WHERE  USER = 'root'
AND    host = 'localhost';

After you’ve updated the password, remove the previous statement lines from the my.ini file. Then, reboot the server.

Hope this helps a few people.

Written by maclochlainn

October 21st, 2010 at 12:09 am

Posted in MySQL,Windows7

Tagged with ,

Two-stepping Sequences

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Sometimes I’m amazed at things that come up. A student wondered why the sequences were incrementing by two when they’re defined to increment by one. It turns out that they were using Oracle APEX to create SQL statements to build a table, constraints, and a auto-numbering sequence trigger. Before executing the code, they’d copy it into their re-runnable script that created their schema.

Here’s an example of code that was generated by APEX for a table:

-- Create the table.
CREATE TABLE onesy
( onesy_id NUMBER
, onesy_text VARCHAR2(20));
 
-- Add the primary key constraint.
ALTER TABLE onesy ADD CONSTRAINT onesy_seq PRIMARY KEY (onesy_id);
 
-- Add a database trigger.
CREATE OR REPLACE TRIGGER onesy_trg 
BEFORE INSERT ON onesy
FOR EACH ROW
BEGIN
  :NEW.onesy_id := onesy_s1.NEXTVAL;
END;
/

This works in APEX because it doesn’t create forms that call onesy_seq.NEXTVAL but they did create that logic in their forms. The INSERT statement would look like:

INSERT INTO onesy VALUES (onesy_seq.NEXTVAL, 'One');

Therefore, the INSERT statement incremented the trigger by one and the database trigger incremented it by one. The result is that sequences two-step, which isn’t effective or the desired behavior.

After I explained the two-step problem, they asked if they could only call the trigger when the primary key value was null. While they could do that like this:

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-- Add a database trigger.
CREATE OR REPLACE TRIGGER onesy_trg 
BEFORE INSERT ON onesy
FOR EACH ROW
WHEN (NEW.one_id IS NULL)
BEGIN
  :NEW.onesy_id := onesy_s1.NEXTVAL;
END;
/

The problem is that this type of trigger doesn’t stop other possible problems. While it prevents two-stepping the sequence, it doesn’t prevent two other errors.

One possible error that isn’t managed in this scenario is the use of numeric literals beyond the next value of the sequence. It writes the row but eventually the sequence catches up to the higher value and a production insert would fail. It would raise the following exception.

INSERT INTO onesy (onesy_text) VALUES ('Eight')
*
ERROR at line 1:
ORA-00001: UNIQUE CONSTRAINT (STUDENT.PK_ONE) violated

Another possible error can occur when you use a bulk insert operation. Assuming you’re inserting 500 rows at a go, you query the maximum value of the onesy_id column and then create an array of 500 numbers. Then, you perform the bulk INSERT statement. The next call to the trigger would raise another ORA-00001 unique constraint error.

Yes, you could lock the table before you perform the bulk operation. After the bulk operation you would drop and recreate the sequence with a new value equal to the maximum value in the column, and unlock the table. This limits concurrency of operation. You could treat these bulk operations as off-line transactions (batch processing) and it would work nicely.

You could also implement a policy that no bulk operations provide generated column values that link to a sequence. Beyond it’s impracticality to manage, that type of restriction does limit the benefit of bulk operations.

The students wanted a solution. So, here’s my take on a trigger that prevents collision with values above the next sequence value. It assumes that bulk operations will be performed as batch processing where you can disable this trigger.

This trigger disallows numeric literals, logs any attempts to use them, and stops processing when an INSERT statement tries to use anything other than the .NEXTVAL of the sequence. It will only work in an Oracle Database 11g database because the context of using a sequence_name.CURRVAL in a comparison isn’t supported in prior releases. The onesy table is renamed the one table in the example.

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-- Create a sequence for table ONE that starts with 1 and increments by 1.
CREATE SEQUENCE msg_s1;
 
CREATE OR REPLACE TRIGGER one_t1 
BEFORE INSERT ON one
FOR EACH ROW
DECLARE
 
  /* Define an autonomous transaction scope to the trigger. */
  PRAGMA AUTONOMOUS_TRANSACTION;
 
  /* Declare a local exception raised when a .CURRVAL pseudo column for a sequence
     is called before a .NEXTVAL for the same sequence in the same session. */
  no_sequence_in_scope EXCEPTION;
  PRAGMA EXCEPTION_INIT(no_sequence_in_scope,-08002);
 
BEGIN
 
  /* Check if surrogate key is provided and the sequence not out of transaction scope. */
  IF :NEW.one_id IS NOT NULL AND NOT :NEW.one_id = one_s1.CURRVAL THEN
 
    /* Write message when sequence value is a numeric literal not a sequence
       generated value but a one_s1.NEXTVAL was previously called in the session.
       Commit after write or information is lost because it throws an user-defined
       exception. */
    INSERT INTO msg VALUES (msg_s1.NEXTVAL,'ID value less or greater than .NEXTVAL ['||:NEW.one_id||']['||:NEW.one_text||'].');
    COMMIT;
 
    /* Stop processing by throwing exception. */
    RAISE_APPLICATION_ERROR(-20002,'ID provided by calling scope is not next sequence value ['||:NEW.one_id||']['||:NEW.one_text||'].');
 
  ELSIF :NEW.one_id = one_s1.CURRVAL THEN
 
    /* Do nothing, calling scope is correct with a one_s1.NEXTVAL sequence call. */
    NULL;
 
  ELSE
 
    /* Increment sequence and assign a value when one isn't provided, like a NULL value. */
    :NEW.one_id := one_s1.NEXTVAL;
 
  END IF;
 
EXCEPTION
 
  /* Handle a no sequence in scope error. */
  WHEN no_sequence_in_scope THEN
 
    /* Write and commit log message for error. */
    INSERT INTO msg VALUES (msg_s1.NEXTVAL,'ID provided by calling scope is invalid ['||:NEW.one_id||']['||:NEW.one_text||'].');
    COMMIT;
 
    /* Stop processing by throwing an exception. */
    RAISE_APPLICATION_ERROR(-20001,'Not a sequence generated value ['||:NEW.one_id||'].');
 
END;
/

Since anonymous transaction triggers are tricky, it’s important to note that the message writing requires two commits. One before raising the exception when the .CURRVAL is in session scope and another in the exception handler before raising the error. If you forget those COMMIT statements, this is a sample of the error stack:

INSERT INTO one VALUES (one_s1.NEXTVAL,'Six')
                        *
ERROR at line 1:
ORA-06519: active autonomous TRANSACTION detected AND rolled back
ORA-06512: at "STUDENT.ONE_T1", line 31
ORA-04088: error during execution OF TRIGGER 'STUDENT.ONE_T1'

The trigger raises the following type of exceptions for an offending INSERT statement. The first occurs when the sequence is valid in the session scope, like:

DECLARE
*
ERROR at line 1:
ORA-20001: NOT a SEQUENCE generated VALUE [1].
ORA-06512: at "STUDENT.ONE_T1", line 48
ORA-04088: error during execution OF TRIGGER 'STUDENT.ONE_T1'
ORA-06512: at line 15

The second occurs when the sequence isn’t valid in the session scope.

INSERT INTO one VALUES (401,'Nine')
            *
ERROR at line 1:
ORA-20002: ID provided BY calling scope IS NOT NEXT SEQUENCE VALUE [401][Nine].
ORA-06512: at "STUDENT.ONE_T1", line 24
ORA-04088: error during execution OF TRIGGER 'STUDENT.ONE_T1'

A value that’s below the current high-watermark of the sequence raises a unique constraint, like this:

INSERT INTO one VALUES (1,'Eight')
*
ERROR at line 1:
ORA-00001: UNIQUE CONSTRAINT (STUDENT.PK_ONE) violated

The following is a script with all the necessary code components to test the example.

If I’ve fat fingered any typing or made logical errors, please let me know.

Written by maclochlainn

October 5th, 2010 at 10:59 pm