Category: DB Notes

Database Articles

  • Table Hints – NoLock vs ReadPast

    When any data in a database is read or modified, the database engine uses special type of mechanism, called locks, to maintain integrity in the database. Locks will be used to make sure the transaction consistency.

    NoLock Table Hint
    – Will allow you to read the uncommited data
    – Only used with SELECT statement
    – Blocking will not occur
    – Will reduce the concurrency and improve the performance at some extent
    – Risk of doing Phantom reads

    Let’s create table for the NOLOCK and READPAST hint demo

    create table tranDemo
(
			id int identity(1,1),
			name varchar(10)
)

insert into tranDemo values ('Jugal')
insert into tranDemo values ('Nehal')

    –Now let’s update the values by specifying the explicit transaction and don’t commit/rollback the transaction

    begin transaction
   update tranDemo
   set name = 'DJ'
   where name = 'Jugal'

    Now open new query window and execute the below query and you will notice query will not return data and will continue running as it is blocked

    select * from trandemo

    Now open new query window and execute the below query to check the blocking, you can see the blocking SPID in the result set

    sp_who2 active

    Now open new query window and execute the below query using NOLOCK hint and it will return data, yet transaction is not committed but it will return the updated value.

    select * from trandemo(NOLOCK)

    READPast Table Hint: Less commonly used table hint than NOLOCK. This hint specifies that the database engine not consider any locked rows or data pages when returning results.
    – Will only read the commited rows which are not locked
    – Blocking will not occur
    – Only used with SELECT statement
    – Will reduce the concurrency and improve the performance at some extent
    – Result set returned by this hint is not perfect as it will not retun the locked rows or pages, so you can not make any decision based on data

    Now open new query window and run the below query it will return only one record (“Nehal”) which is not locked or modified

    select * from trandemo(ReadPast)

  • SQL Server 2008: SQL Mail does not work with the 64-bit version of SQL Server (Microsoft SQL Server, Error: 17938)

    Recently I got email from one of the blog reader that he is getting below error while setting up SQL MAIL as legacy feature in SQL Server 2008.

    Error:
    SQL Mail does not work with the 64-bit version of SQL Server (Microsoft SQL Server, Error: 17938)

    Solution:
    SQL Mail is not supported on 64-bit versions of SQL Server. SQL Mail stored procedures cannot be installed on 64-bit versions. If you want to still use it you have to go with the 32-bit edition or start using database mail, which is more easy.

    Follow below article link to start converting your SQLMAIL to DB Mail.
    http://msdn.microsoft.com/en-us/library/ms187891.aspx

  • How to make Lite Speed Central database and Local repository in sync?

    If you have configured the LiteSpeed in your environment using the Lite Speed Central repository option, there will be a scenario when Lite Speed Central repository misses some data. To make the data available in central repository you can use the LiteSpeed local database and execute the below procedure to ensure that cenral repository comes in sync with the local repository.

    exec dbo.xp_replicate_activity_statistics

  • Permission required for SP_UpdateStats

    To update the statistics on the database you should either be DBO of that database or SysAdmin on SQL instance.
    SPUpdateStats

    If you want to grant the least permission instead of server level permission you make the user DBO of the database. You can do it from database properties windows as below.

    DBO

    You can also make the user DBO using below command.

    USE [My_Policy]
    GO
    EXEC dbo.sp_changedbowner @loginame = N’jugal’, @map = false
    GO