Monday, June 30, 2008

FAQ : Explain a scenario which supports vertical partitioning of a table in SQL Server.

The scenario may be different according to the SQL Server Version you have (SQL 7.0, SQL 2000, SQL 2005). I will be covering SQL Server 2000 and 2005

SQL Server 2000.
In SQL Server (in 2000 and 2005) the IN_ROW_DATA or the row size of a table can have only max 8060 bytes. If you have a table which contains 4 columns of varchar (3000) then though you can create the table but if the data being inserted is more than 8060 bytes then the insert will fail. So what we generally do is, we vertically partition the table to two or more table as per the requirement and keep ONE to ONE relation between all the tables. So this is a valid reason to partition your table vertically. When you do vertical partitioning, try to keep most commonly used small size column in single table.

SQL Server 2005
In SQL Server 2005, the above mentioned problem of SQL Server 2000 is not there because of the storage architecture change called ROW_OVERFLOW_DATA. Ie. In SQL Server 2005 you can have a row which exceed 8060 bytes provided the columns are variable types(Varchar,nvarchar). What Database engine internally does is, it keep all the variable datatypes (varchar, nvarchar )columns data in ROW_OVERFLOW_DATA page. Precisely, the Row size limitation is only applicable to fixed size columns like CHAR, NCHAR. So SQL Server 2000 scenario of partitioning table because of the row size exceeds 8060 bytes is not valid in SQL Server 2005.
But there is a valid reason to do vertical partition of the table in SQL Server 2005. If you are using ONLINE INDEX feature of SQL Server 2005 then you cannot use LOB data as a part of index at the leaf level. And you want to use the LOB column in Index because of the performance benefit it provides. In that case best method is to partition the table vertically in such a way that, keep the small ,mostly used columns in single table (like product detailed description may not be asked frequently by the user) and the columns those are referred in less frequency in another table. Since you do not have LOB data in the table you can use ONLINE INDEX feature in the table.


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