There are some major differences between temp tables, table variables and common table expressions (CTEs). Some of the big differences are:
Temp Tables vs. Table Variables
- SQL Server does not place locks on table variables when the table variables are used.
- Temp tables allow for multiple indexes to be created
- Table variables allow a single index the Primary Key to be created when the table variable is declared only. There is an exception to this, that if you can create the index inline, for example by creating a unique constraint inline as shown in the comments. However these indexes (and the table variable in general) will always be assumed to have 1 row in them, no matter how much data is within the table variable.
- Temp tables can be created locally (#TableName) or globally (##TableName)
- Table variables are destroyed as the batch is completed.
- Temp tables can be used throughout multiple batches.
- Temp tables can be used to hold the output of a stored procedure (temp tables will get this functionality in SQL Server 2008).
Table variables and Temp Tables vs. CTEs
- CTEs are used after the command which creates them.
- CTEs can be recursive within a single command (be careful because they can cause an infinite loop).
- Table variables and Temp Tables can be used throughout the batch.
- The command before the CTE must end with a semi-colon (;).
- As Temp tables and table variables are tables you can insert, update and delete the data within the table.
- CTEs can not have any indexes created on them, source tables much have indexes created on them.
If you can think of anything that I’ve missed, feel free to post them in the comments.
Doing a code freeze at this time of year can be crucial to keeping your sanity this time of year. Often times the business unit likes to push out last minute changes before the year end. This often means pushing last minute code (which as everyone knows isn’t always perfect) which can lead to unpleasant little phone calls from the business or the NOC in the middle of holiday festivities.
Do your self and your family a favor, push for a code freeze until after the new year. The developers will thank you, the business with hate you (but really what else is new). I like about a three week code freeze (emergency bug fixes are except, but they have to actually be bug fixes, and they have to be an emergency) starting about December 15 ending about January 3 (depending on when the weekends fall).
It can be tough to get a code freeze implemented at first. But after the first year with the code freeze everyone will want to go it the next year.
We’ll I’ve finely gotten around to installing the SQL 2008 November CTP. While walking through the installer I have seen some excellent changes. Not only is the option to change the paths of the data files no longer hidden, there are more than just two options. There are at least 7 paths that you get to specify while installing. The first one is the Shared component directory. This appears to be the base path where all your binarys will be based off of.
Next you get the Instance root directory. This is where the system database files will be based off of, so make sure that you don’t point this to the C drive is you want the system databases on another drive.
A couple of screens later you get to set 6 install paths. The first is the data root directory. This changes the base path for all the others options. The others are the User database folder, user log database folder, tempdb data folder, tempdb log folder, and the backup directory.
This amount of flexability in the installer is a first, and it’s a welcome change. Look for more posts, tips and articles about SQL Server 2008 in the comming weeks and months up till the release.
If you have looked at pretty much any Microsoft provided T/SQL code you have probably seen an N in front of a string value when setting a variable much like this.
WHERE name = N'master'
What this N does is tell the SQL Server that the data which is being passed in is uni-code and not character data. When using only the Latin character set this is not really needed. However if using characters which are not part of the basic Latin character set then the N is needed so that SQL knows that the data being given it is uni-code data.
The path that Microsoft is going says yes. They are moving all there samples and defaults to using uni-code. In reality uni-code may not be needed in your environment. If you are storing only Latin characters (the US alphabet characters) then you probably don’t need to store uni-code characters. However if you are planning on moving your operations to support customers who do not speak English then you will want to setup your database and application to use the uni-code data types (nvarchar, nchar, ntext).
It doesn’t take much extra to use uni-code data types, however do keep in mind that it does require more disk space to use uni-code data types. For each character saved two bytes of disk space are used instead of one byte. While this may not seam like much space if you will have a large table with a large ntext field this can use a lot of space quickly.
I know that this is normally a technical blog, but with the US Thanksgiving day holiday coming tomorrow I wanted to be sure to point out that time needs to be made out of our busy work schedules to make sure that we spend time with our families. It’s very important that we find a good balance between work and family.
Microsoft announced at TechEd in Barcelona, Spain that the SQL Server 2008 November CTP would be made available this month. Read more here.
Microsoft has just posted the URL to download the new CTP. http://www.microsoft.com/downloads/details.aspx?FamilyId=3BF4C5CA-B905-4EBC-8901-1D4C1D1DA884&displaylang=en
I’ve published a new tip over at SearchSQLServer.com entitled Update SQL Server table statistics for performance kick.
When you setup your data center, something that seams to be getting overlooked these days is the data center environmentals. We all know that we need batteries and a generator for the power to keep the servers online, and that we need AC to keep them cool. But I’ve seen a disturbing number of very large data centers who do not have the AC on the generators. This creates a problem when the power goes out. While it’s great that the servers will stay online the data center will quickly get up into the 100+ degree range (F). This can quickly lead to data integrity issues is the hard drives start to fail.
Just a couple of days ago a client of mine who hosts there servers at a company (who shall remain nameless) and their power when out for about 3 hours. The temperature went up to 120 degrees in the data center, because the AC system which they brag about having wasn’t on the generator. They actually called all their clients asking the clients to power down any systems which aren’t mission critical in order to save heat. Frankly, I was surprised that the machines didn’t start to power them selves down. I guess the HP servers are built a little more robust than I thought (not that I really want to try again).