MS SQL Server Data (Table) Storage

What does MS SQL Server store?

MS SQL Server has data files and (transactional) log files.

How is the log file organized?

Log files contain a series of log records, not divided by pages.

How is the data file organized?

Data files are grouped by extent and each extent by pages. Every page belongs to an extent.

What is page and extent?

Each page is 8 KB (128 pages/MB) and 8 contiguous pages make an extent of 64KB (16 extent/MB).

Disk I/O is performed at page level. That means, the minimum SQL server would read/write at one go is a page ~ 8KB.

MS SQL Server can also read a whole extent at one go instead of reading a single page when it makes sense. Look out for partition alignment so that reading one extent does not end up touching two disk blocks, affecting performance.

What are the different types of pages and what do they store?

Each page starts with a 96-byte header that stores information about the page including page number, page type etc.

Page types include Data, Index, Text/Image, Page Free Space (PFS, information about page allocation/free pages), Global Allocation Map (GAM, extent allocation information), Shared Global Allocation Map (SGAM, mixed extent with at least one unused page information), Bulk Changed Map (information about extents that got changed by a bulk logged operation since last BACKUP LOG statement) Differential Changed Map (information about extents that got changed since last BACKUP DATABASE statement), Index allocation map (IAM, page/extent allocation for an object, say a heap of a table).

So basically, pages store data and metadata for the MS SQL Server to make efficient use of them for various I/O (related) operations.

What are the different types of extents?

There are two types of extents: uniform and mixed extent. When a table is created instead of allocating a whole extent (8 pages), a single page (a page from a mixed extent type) is allocated. This goes on until the table crosses 8-page size. Onwards, every time a whole extent (uniform extent type) is allocated.

IAM pages (come from a mixed extent) keep track of these allocations.

Where does my table data go?

First of all, a table can have partitions. Let’s say we have not partitioned a table. Then we have just one partition. By the way, partition resides in a single file group.

Table can have a clustered index or not. If clustered index then the data organization type is B-tree else Heap (no logical order of data). It can be mentioned here that nonclustered index has same b-tree structure with the difference that it does not have data pages. Rather, it has index pages containing index rows, each having a row locator pointing to a data row in the clustered index or heap. Heap or b-tree of a partition can have up to 3 allocations units.

What is an allocation unit?

Allocation unit is a collection of pages within a heap or B-tree. We have three different types of allocation units based on the kind of data they would store. They are:

  1. IN_ROW_DATA: for data (data page) or index (index page) rows, can save all but LOB.
  1. LOB_DATA: for text, ntext, image, xml, varchar (max), nvarchar (max), varbinary (max) data types. Uses Text/Image pages.
  1. ROW_OVERFLOW_DATA: for variable length data stored in varchar, nvarchar, varbinary, or sql_variant columns that exceed the 8060 byte row size limit. Uses Text/Image pages.

How are the data rows saved in a data page?

We are talking about data pages (used by both heap and b-tree to store data rows and not index rows) in IN_ROW_DATA allocation unit. They are saved serially starting from the end of 96-byte header, not necessarily in the logical order as dictated by a clustered index, if any. It is the row offset table that maintains the logical order.

What is row offset table?

For each data row there is a 2-byte entry, saving the offset. It tells from where the data row starts. The entry for the first data row (logical, not physical) is there at the very end of the page. Suppose the data page contains 20 rows. Then there will be 20 entries and these entries make a row offset table. The first (0th) entry would say where the first logical data row would start. The physical position of this might be somewhere in the middle of the page and not necessarily at the beginning meaning just after the header.

What is the maximum data size in a data page?

Maximum data and overhead size in a data page is 8060 bytes.

How to save a row bigger than 8060 bytes?

A table can contain maximum 8060 bytes per row. Unless it has a variable type column defined. It would then use a 24-byte pointer in data page to point a Text/Image page in ROW_OVERFLOW_DATA allocation unit where they would move the data, starting with the largest width column. Data of a single variable column must not exceed 8000 bytes but combined width can exceed 8060 bytes. However, if a row is less than 8060 bytes and there is no space in the page then page split – not a row-overflow will occur.

A variable column value must be completely in-row or completely out-row. Multiple such columns can spread over several pages in ROW_OVERFLOW_DATA allocation unit. Such Text/Image pages are managed by a chain of IAM pages. Note that, the index key of a clustered index cannot contain varchar columns that have existing data in the ROW_OVERFLOW_DATA allocation unit.

How to save single column data bigger than 8000 bytes?

LOB column can store up to 2 GB data. Whether at attempt to save the data in-row would take place depends on the settings. If saved out-row, a 16-byte pointer from data page for each LOB column would point to the initial Text/Image page in LOB_DATA allocation unit. A page in LOB_DATA can store multiple columns from multiple rows but for a single table. Text/Image pages storing LOB data are managed by a chain of IAM pages.


MS SQL Server Nvarchar Issues

Do you need to store non-English characters, like Chinese characters?

Then you need UNICODE characters. nvarchar, ntext, nchar are your options. n is for ISO synonym national. Uses UNICODE UCS-2 character set, 2 bytes for a character, double than that of char/varchar/text. This is a price you need to pay for multi-ligual support.

nchar or nvarchar/ntext?

If you know the data length is always going to be around a fixed size, say, 20 characters then nchar, else nvarchar/ntext.

nvarchar ($fixed size$) or nvarchar (max)/ntext?

If you know your data is always going to be somewhere between a fixed range, say, 1 to 100 characters and would never exceed 100 then nvarchar (100). It makes sense, looks good. Use fixed length nvarchar as long the size you know would be less than 8000 bytes. More than 8K means the data would be saved out-row. Fixed length nvarchar can’t help.

If your data size can be NULL to 2 GB and anything in between then you have no option but nvarchar (max)/ntext.

nvarchar (max) or ntext?

Go for nvarchar (max), not ntext. ntext will not be supported in future and nvarchar (max) better in performance than ntext.

Is nvarchar (max) bad for performance?

[name] AS tablename, 
FROM sys.tables

If large_value_types_out_of_row for the table that contains your nvarchar (max) column is 0 (=OFF) then MS SQL Server would try to save the data in-row, given that the row fits in 8K page size. If the row does not fit in 8K then the nvarchar (max) data would go out-row, one or more extra pages depending on the nvarchar (max) column data size.

If MS SQL Server has to read extra pages for a row, performance would suffer. But what to do? The data is big, right?

If large_value_types_out_of_row is 1 (=ON), then no matter how small is your nvarchar (max) column data, it will always be saved out-row.

In-row or out-row?

If the select statements you use for the table excludes the nvarchar (max) columns then use out-row. Because when you will be reading the non-nvarchar (max) columns, you are not reading the other pages used to store long nvarchar (max). Not only that, since all those big data are not in the same page, your single page can now fit more rows with the small data. You will be doing good here.

But if you have to read the whole row (all columns) at one go, then in-row good.

How to change in-row to out-row and vice versa?

You can change anytime.

sp_tableoption N'MyTableName', 'large value types out of row', 'ON'

However, doing so won’t immediately change the existing data layout. New inserted data would follow the new way. Existing data, if updated would be saved in the new layout. Well, you can always read the existing data, change it and then update it again with the old value. That way, you save all your data in the new layout.

What about ntext?

Well, we already decided not to use next. But since we are discussing this let’s see the respective issues.

This is how you can know the existing layout.

[name] AS tablename, 
FROM sys.tables

text_in_row_limit = 0 says, the option is not set. If non-zero, say, 256 then that is the number of bytes that will be saved in-row.

This is how to change it.

sp_tableoption N'MyTableName', 'text in row', ON

text_in_row_limit would be 256.

sp_tableoption N'MyTableName', 'text in row', '1000'

If you want that limit to be 1000 bytes. Yes, setting itself won’t change the existing data layout just like nvarchar.

How full are the pages?

Remember, you pages should be full as much as possible. Otherwise you will be reading a page but get very small data. You will be doing bad.

FROM sys.dm_db_index_physical_stats(DB_ID(N'MyDatabaseName'), 
                                    NULL, NULL, 'DETAILED')

But ignore small tables having less than 1000 pages (page_count).

Are the rows fitting in 8K?

OBJECT_NAME(object_id) as TableName, 
OBJECT_NAME(index_id) as IndexName, 
FROM sys.dm_db_index_physical_stats(DB_ID(N'MyDatabaseName'), 
                                    NULL, NULL, 'DETAILED')

It is better to design tables in way so that data of n (n >= 1) rows fit in 8K, a page.

Is there any other limitation for nvarchar (max)?

You cannot create an index on nvarchar (max) column.

Also avoid using nvarchar (max) column in where clause.

What are the issues for migrating varchar to nvarchar?

Your migrated data is going to be double. Some rows might no longer fit in-row. They may over flow to out-row. Performance might be affected.