Microsoft Access–Reseed Auto Number

I was developing a small database using Microsoft Access 2010. In that database, I use Auto Number on some of the tables. During the development of the database, I’ve entered a lot of test data into those tables that have auto number.

Then it comes the time to clean up the test data, so that the database can be used for the live data. Deleting the data from the tables is pretty straightforward. However, when I delete the data it does not reset any of the auto number columns that I have. In SQL Server, we could use the “Truncate” command to delete the data in the table and reset the identity column (the auto number) in the table. Or use DBCC CHECKIDENT to reseed the identity column. However, we could not use those in Microsoft Access.

So to reseed auto number in Microsoft Access, you can do the following:

1. Delete the data from the table
2. Run the “Compact and Repair Database Tools”. It can be found under the “Database Tools” on Microsoft Access ribbon menu.
   

High Virtual Log Files (VLF) Count

Virtual log files are basically segments of the database log file. Every time the transaction log grow in size, the number of virtual log files would also grow. If you have a database with transaction log that keep growing in small increment each time, you can end up with a lot of virtual log files, and that can be a bad thing.

In SQL Server 2012, if you have a database with more than 10,000 virtual log file, you will get a warning message on the SQL Server logs. To demonstrate this:

First I would create a database called VLFTest. I set the log file for the database to have an initial size of 512 KB (for this size 2 VLF will be created).

USE [master];

IF DB_ID('VLFTest') IS NOT NULL
BEGIN
    DROP DATABASE VLFTest;
END;
GO

CREATE DATABASE VLFTest
ON (
    NAME = 'VLFTest_Data',
    FILENAME = 'C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\DATA\VLFTest_Data.mdf')
LOG ON (
    NAME = 'VLFTest_Log',
    FILENAME = 'C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\DATA\VLFTest_Log.ldf',
    SIZE = 512 KB,
    MAXSIZE = 10240 MB,
    FILEGROWTH = 512 KB);
GO

Once the database is created, I can check the number of VLF created by running the following query:

DBCC LOGINFO ('VLFTest');

The query should return 2 rows, which would indicate that there are two VLFs created.

The next thing I would do is to grow the transaction log size by 512 KB increment for 5,000 times. Since each time we increment the transaction log by 512 KB, 2 new VLFs would be created. Thus, doing it for 5,000 times would cause it to generate 10,000 new VLFs. So at the end I would end up with 10,002 VLFs. I can do this by using the following script:

DECLARE @CurrentSize AS INT,
        @LogFileQuery AS NVARCHAR(1000);
    
--- Get current log file size.
PRINT 'Current log file size is ' + CAST(@CurrentSize AS VARCHAR(1000)) + ' KB';
SELECT @CurrentSize = size*8 FROM sys.master_files WHERE database_id = DB_ID('VLFTest') AND name = 'VLFTest_Log'

--- Try to grow the transaction log by 512 KB. This should result in two additional VLF.
PRINT 'Growing the transaction log to ' + CAST((@CurrentSize + 1) AS VARCHAR(1000)) + ' KB';
SET @LogFileQuery = 'ALTER DATABASE VLFTest MODIFY FILE (NAME = VLFTest_Log, SIZE = ' + CAST((@CurrentSize + 512) AS VARCHAR(1000)) + ' KB);';
EXEC (@LogFileQuery);
GO 5000

Now if I run the following query:

DBCC LOGINFO('VLFTest');

I would get back 10,002 rows back, which means that I have 10,002 VLFs on VLFTest database’s transaction log.

Now, the warning in SQL Server log unfortunately will not be trigger when the VLF count goes beyond 10,000. I found out that it will be trigger under the following conditions:

• When the SQL Server is restarted
• When the VLF database is set online (after it is set offline)
• When we restore the database

So for a quick test, I try to set the database offline and then back online by using the following query:

USE master;
GO
ALTER DATABASE VLFTest SET OFFLINE;
GO
ALTER DATABASE VLFTest SET ONLINE;
GO

After VLFTest database is back online, when I go to SQL Server log, I would get the following message:

Database VLFTest has more than 1000 virtual log files which is excessive. Too many virtual log files can cause long startup and backup times. Consider shrinking the log and using a different growth increment to reduce the number of virtual log files.

Why Having Too Much of VLFs Count Can Be Bad Thing?

Having an excessive VLFs count can be a bad thing to your database. For one, when I tried to make the database back online after turning it offline, it took longer than expected. When I run the following query:

USE master;
ALTER DATABASE VLFTest SET OFFLINE;
GO
SET STATISTICS TIME ON;
ALTER DATABASE VLFTest SET ONLINE;
GO

After running it 5 times, on average it took about 23 seconds for the database to be online. Now compare this with the following:

USE master;
GO
IF DB_ID('VLFTest2') IS NOT NULL
BEGIN
    DROP DATABASE VLFTest2;
END;
GO

CREATE DATABASE VLFTest2
ON (
    NAME = 'VLFTest2_Data',
    FILENAME = 'C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\DATA\VLFTest2_Data.mdf')
LOG ON (
    NAME = 'VLFTest2_Log',
    FILENAME = 'C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\DATA\VLFTest2_Log.ldf',
    SIZE = 2560512 KB,
    MAXSIZE = 10240 MB,
    FILEGROWTH = 1024 KB);
GO

It is basically the same database, but instead of growing the transaction log file from 512 KB to 2,560,512 KB by 512 KB increment (and in the process created 10,002 VLFs), I immediately create the transaction log with size 2,560,512 KB. By doing this, I only have 16 VLFs. Now when I try to run the following query:

USE master;
ALTER DATABASE VLFTest2 SET OFFLINE;
GO
SET STATISTICS TIME ON;
ALTER DATABASE VLFTest2 SET ONLINE;
GO

After running it 5 times, on average it took about 0.2 seconds to bring VLFTest2 database online. That’s quite a huge difference, 23 seconds vs. 0.2 seconds (about 2,200% difference).

I also noticed big difference in the time that it took to restore VLFTest1 database (with 10,002 VLFs) compared to the VLFTest2 database (with 16 VLFs). It took about 1 minutes and 35 seconds to restore VLFTest1 database, but it only took about 6 seconds to restore VLFTest2.

In addition to the above, having an excessive number of VLFs in your database can also have an impact on the database transaction. A while ago, Linchi Shea wrote a blog post regarding this. It can be found here.

Take Away

Having a high count of VLFs can hurt your database. To avoid it, we need to carefully plan our database size, especially in this case, the transaction log size. We also need to make sure that the transaction file growth size is not too small. This is to ensure that we don’t end up with many auto growth on the transaction log file, which cause the VLFs to grow.

Further Reading

You might also want to read Kimberly Tripp’s blog post regarding VLF. It can be found here.

Utilizing smtp4dev with SQL Server Database Mail for Testing

SQL Server has database mail, a solution which would allow us to send e-mail from our database server. It allows us to send either e-mail in html or text format to users. It’s great because we can use it to send query results to users (or ourselves). We can also configure SQL Server Agent to use database mail to send alerts to operator.

Database mail utilizes SMTP server to send the e-mails. In some cases, we might not have a readily available SMTP server in our environment (like in my home lab environment) or we might be block from using the SMTP server from our development environment for security reason. In those situations, I’ve been using smtp4dev. It is a neat little utility that would allow you to have a dummy SMTP server on your local computer. It is lightweight and also no installation is needed. When we run the utility, it would sit on the system tray and listen to port 25 (default SMTP port). It will receive the e-mail that get send to its way, without sending the e-mail to its final destination (the e-mail recipients). It is pretty handy when we need to run some tests.

Setting up database mail to work with smtp4dev is pretty simple.

• Download smtp4dev from http://smtp4dev.codeplex.com.
• Extract the file and save it on your system (assuming that it is your development system and it has the SQL Server).
• Run smtp4dev. Notes: If you have Windows Firewall turned on, you might get a “Windows Security Alert” that ask you if you want to allow smtp4dev to communicate with either the private network, public network or both. Since normally I run smtp4dev on my development system with also has SQL Server that I want to have database mail turn on, I would just hit the cancel button.
• If database mail in SQL Server has not been enable, we can enable it by using the following SQL script:

exec sp_configure 'show advanced', 1;
go
reconfigure;
go
exec sp_configure 'Database Mail XPs', 1;
go
reconfigure;
go

• We then need to set up database mail profile, account and then associate the profile with the account in SQL Server. To do that, we can use the following script (based on the “Database Mail Simple Configuration Template”):

DECLARE @profile_name sysname,
        @account_name sysname,
        @SMTP_servername sysname,
        @email_address NVARCHAR(128),
        @display_name NVARCHAR(128),
        @error_display NVARCHAR(500);

-- Profile name. Replace with the name for your profile
        SET @profile_name = 'SMTP4Dev';

-- Account information. Replace with the information for your account.

        SET @account_name = 'SMTP4Dev Account';
        SET @SMTP_servername = 'localhost';            --- Since the smtp4dev is on local system. 
        SET @email_address = 'john.doe@email.com';
        SET @display_name = 'John Doe';

-- Verify the specified account and profile do not already exist.
IF EXISTS (SELECT * FROM msdb.dbo.sysmail_profile WHERE name = @profile_name)
BEGIN
    SET @error_display = 'The specified Database Mail profile (' + @profile_name + ') already exists.';
    RAISERROR(@error_display, 16, 1);
    GOTO done;
END;

IF EXISTS (SELECT * FROM msdb.dbo.sysmail_account WHERE name = @account_name )
BEGIN
    SET @error_display = 'The specified Database Mail account (' + @account_name + ') already exists.';
    RAISERROR(@error_display, 16, 1) ;
    GOTO done;
END;

-- Start a transaction before adding the account and the profile
BEGIN TRANSACTION ;

DECLARE @rv INT;

-- Add the account
EXECUTE @rv=msdb.dbo.sysmail_add_account_sp
    @account_name = @account_name,
    @email_address = @email_address,
    @display_name = @display_name,
    @mailserver_name = @SMTP_servername;

IF @rv<>0
BEGIN
    SET @error_display = 'Failed to create the specified Database Mail account (' + @account_name + ').';
    RAISERROR(@error_display, 16, 1) ;
    GOTO done;
END

-- Add the profile
EXECUTE @rv=msdb.dbo.sysmail_add_profile_sp
    @profile_name = @profile_name ;

IF @rv<>0
BEGIN
    SET @error_display = 'Failed to create the specified Database Mail profile (' + @profile_name + ').';
    RAISERROR(@error_display, 16, 1);
    ROLLBACK TRANSACTION;
    GOTO done;
END;

-- Associate the account with the profile.
EXECUTE @rv=msdb.dbo.sysmail_add_profileaccount_sp
    @profile_name = @profile_name,
    @account_name = @account_name,
    @sequence_number = 1 ;

IF @rv<>0
BEGIN
    SET @error_display = 'Failed to associate the speficied profile with the specified account (' + @account_name + ').';
    RAISERROR(@error_display, 16, 1) ;
    ROLLBACK TRANSACTION;
    GOTO done;
END;

COMMIT TRANSACTION;

done:

GO

• Now the moment of truth, we can test to see if it works by trying to send an e-mail using sp_send_dbmail

exec msdb..sp_send_dbmail @profile_name = 'SMTP4Dev',
    @recipients = 'jane.doe@email.com',
    @subject = 'Test E-mail',
    @body = 'This is a test email.';

If everything works as plan hopefully on your tray notification, you would see the following message:

And if you open smtp4dev, you should see the following:

To view the e-mail, you can click on the “View” or “Inspect” button.

Some caveats that I’ve noticed:

• I have Microsoft Office 2013 installed on my system, which includes Outlook 2013. For some reason, when I click the “View” button, I would get an error message “The attempted operation failed. An object could not be found.” and it just would not open the e-mail (if I try to open it using Outlook 2013). It is ok if I use Windows Live Mail 2012. I would normally use the “Inspect” button instead.
• If you exit out smtp4dev, the e-mails that you have on the Messages tab will get deleted.

Cleaning Up

If you want to remove the database mail from your SQL Server, first you want to remove the profile association with account, the profile and the account. You can do that by using the following script:

DECLARE @profile_name sysname,
        @account_name sysname,
        @error_display nvarchar(500);

-- Profile name. Replace with the name for your profile
        SET @profile_name = 'SMTP4Dev';

-- Account information. Replace with the information for your account.
        SET @account_name = 'SMTP4Dev Account';

IF NOT EXISTS (SELECT * FROM msdb.dbo.sysmail_profile WHERE name = @profile_name)
BEGIN
    SET @error_display = 'The specified Database Mail profile (' + @profile_name + ') does not exists.';
    RAISERROR(@error_display, 16, 1);
    GOTO done;
END

IF NOT EXISTS (SELECT * FROM msdb.dbo.sysmail_account WHERE name = @account_name)
BEGIN
    SET @error_display = 'The specified Database Mail account (' + @account_name + ') does not exists.';
    RAISERROR(@error_display, 16, 1);
    GOTO done;
END

-- Start a transaction before deleting the profile account, profile and account.
BEGIN TRANSACTION ;

DECLARE @rv INT;

EXEC @rv=msdb.dbo.sysmail_delete_profileaccount_sp 
    @profile_name = @profile_name, 
    @account_name = @account_name;

IF @rv<>0
BEGIN
    SET @error_display = 'Failed to delete the profile with the specified account (' + @account_name + ').';
    RAISERROR(@error_display, 16, 1) ;
    ROLLBACK TRANSACTION;
    GOTO done;
END;

EXEC @rv=msdb.dbo.sysmail_delete_profile_sp
    @profile_name = @profile_name;

IF @rv<>0
BEGIN
    SET @error_display = 'Failed to delete the database mail profile (' + @profile_name + ').';
    RAISERROR(@error_display, 16, 1) ;
    ROLLBACK TRANSACTION;
    GOTO done;
END;

EXEC @rv=msdb.dbo.sysmail_delete_account_sp
    @account_name = @account_name;
IF @rv<>0
BEGIN
    SET @error_display = 'Failed to delete the database mail account (' + @account_name + ').';
    RAISERROR(@error_display, 16, 1) ;
    ROLLBACK TRANSACTION;
    GOTO done;
END;

COMMIT TRANSACTION;

done:

GO

Then after that, you can disabled the database mail by using the following script:

exec sp_configure 'Database Mail XPs', 0;
go
reconfigure;
go
exec sp_configure 'show advanced', 0
go
reconfigure;
go

Unique Filtered Index

Consider a scenario where you need to create a table in which you need to store contact information, and for one of the columns, we need to store passport number for each contact, if one is available. Assuming that passport number is unique for each contact, but not all have passport, how can we ensure the uniqueness for the passport number enter while allowing null values for those contacts who don't have passport number?

Unique constraint/index would come to mind. However, the problem of using unique constraint/index in this scenario is while unique constraint/index would ensure that the data enter is unique, a column with unique constraint/index can only allow one null value in that column.

A quick demo for this using unique index (but the result should be similar if we use unique constraint as well):

IF (OBJECT_ID('dbo.ContactInfo') IS NOT NULL)
BEGIN
    DROP TABLE dbo.ContactInfo
END

CREATE TABLE dbo.ContactInfo (
    ContactID INT PRIMARY KEY IDENTITY (1,1),
    ContactName VARCHAR(100) NOT NULL,
    PassportNumber VARCHAR(25) NULL);

CREATE UNIQUE INDEX  UI_PassportNumber
ON dbo.ContactInfo(PassportNumber)

Now let's try to test to make sure that we can't enter duplicate passport number:

INSERT INTO dbo.ContactInfo (ContactName, PassportNumber) VALUES ('John Doe', 'B1234567890');
INSERT INTO dbo.ContactInfo (ContactName, PassportNumber) VALUES ('Jane Doe', 'B1234567890');

We would get the following messages:

(1 row(s) affected)
Msg 2601, Level 14, State 1, Line 2
Cannot insert duplicate key row in object 'dbo.ContactInfo' with unique index 'UI_PassportNumber'. The duplicate key value is (B1234567890).
The statement has been terminated.

Which is to be expected, it would insert the information for John Doe just fine, but then it would failed to insert the information for Jane Doe since Jane Doe’s passport number is the same as John Doe’s.

Now let’s test to make sure that we can enter multiple contact info with blank passport number.

INSERT INTO dbo.ContactInfo (ContactName, PassportNumber) VALUES ('Mary Doe', NULL);
INSERT INTO dbo.ContactInfo (ContactName, PassportNumber) VALUES ('Jack Doe', NULL);

We would get the following messages:

(1 row(s) affected)
Msg 2601, Level 14, State 1, Line 2
Cannot insert duplicate key row in object 'dbo.ContactInfo' with unique index 'UI_PassportNumber'. The duplicate key value is (<NULL>).
The statement has been terminated.

It would insert the contact info for Marry Doe just fine, but it would fail when trying to insert the contact info for Jack Doe. This is because the unique constraint can allow only one null value.

Possible Solution

So how we can ensure that our passport column can accept null values while ensure uniqueness for the non-null value? With SQL Server 2008 and above, one possibility is by using the unique filtered index. A quick demo for this one:

IF (OBJECT_ID('dbo.ContactInfo') IS NOT NULL)
BEGIN
    DROP TABLE dbo.ContactInfo
END

CREATE TABLE dbo.ContactInfo (
    ContactID INT PRIMARY KEY IDENTITY (1,1),
    ContactName VARCHAR(100) NOT NULL,
    PassportNumber VARCHAR(25) NULL);

CREATE UNIQUE INDEX  UI_PassportNumber
ON dbo.ContactInfo(PassportNumber)
WHERE PassportNumber IS NOT NULL

Let’s try to see what happen if we try to enter contact info with duplicate passport number:

INSERT INTO dbo.ContactInfo (ContactName, PassportNumber) VALUES ('John Doe', 'B1234567890');
INSERT INTO dbo.ContactInfo (ContactName, PassportNumber) VALUES ('Jane Doe', 'B1234567890');

We would get the following messages:

(1 row(s) affected)
Msg 2601, Level 14, State 1, Line 2
Cannot insert duplicate key row in object 'dbo.ContactInfo' with unique index 'UI_PassportNumber'. The duplicate key value is (B1234567890).
The statement has been terminated.

That works as expected. Only the record for John Doe that got inserted.

Now let’s try to see what happen if we try to enter multiple contact info with blank passport number:

INSERT INTO dbo.ContactInfo (ContactName, PassportNumber) VALUES ('Mary Doe', NULL);
INSERT INTO dbo.ContactInfo (ContactName, PassportNumber) VALUES ('Jack Doe', NULL);

We would get the following messages:

(1 row(s) affected)

(1 row(s) affected)

Both records are being inserted just fine, which what we are expecting.

Conclusion

We can use unique filtered index as a way to enforce uniqueness on filtered data. In the example above, it is to enforce uniqueness on a not null value on the passport number column. There is one caveat. Unlike unique constraint/index where it can be referenced by a foreign key constraint, unique filtered index can’t be referenced by a foreign key constraint.

So if we continue on from the above examples and try to create a new table named PassportVisit with the following definition:

IF (OBJECT_ID('dbo.PassportVisit') IS NOT NULL)
BEGIN
    DROP TABLE dbo.PassportVisit
END

CREATE TABLE dbo.PassportVisit (
    PassportNumber VARCHAR(25) FOREIGN KEY REFERENCES dbo.ContactInfo(PassportNumber),
    DateofVisit DateTime,
    CountryofVisit VARCHAR(50))

So we are trying to reference the passport number column on the passport visit table to the passport number on the contact info table. This will be successful if we have a unique constraint/index. But it will fail if we have unique filtered index. It will fail with the following messages:

Msg 1776, Level 16, State 0, Line 6
There are no primary or candidate keys in the referenced table 'dbo.ContactInfo' that match the referencing column list in the foreign key 'FK__PassportV__Passp__73B0DA39'.
Msg 1750, Level 16, State 0, Line 6
Could not create constraint. See previous errors.