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Table and Index Organization

Tables and indexes are stored as a collection of 8-KB pages. This topic describes the way table and index pages are organized.

The following illustration shows the organization of a table. A table is contained in one or more partitions and each partition contains data rows in either a heap or a clustered index structure. The pages of the heap or clustered index are managed in one or more allocation units, depending on the column types in the data rows.

Table organization with partitions

Table and index pages are contained in one or more partitions. A partition is a user-defined unit of data organization. By default, a table or index has only one partition that contains all the table or index pages. The partition resides in a single filegroup. A table or index with a single partition is equivalent to the organizational structure of tables and indexes in earlier versions of SQL Server.

When a table or index uses multiple partitions, the data is partitioned horizontally so that groups of rows are mapped into individual partitions, based on a specified column. The partitions can be put on one or more filegroups in the database. The table or index is treated as a single logical entity when queries or updates are performed on the data. For more information, see Partitioned Tables and Indexes.

To view the partitions used by a table or index, use the sys.partitions (Transact-SQL) catalog view.

SQL Server tables use one of two methods to organize their data pages within a partition:

  • Clustered tables are tables that have a clustered index.

    The data rows are stored in order based on the clustered index key. The clustered index is implemented as a B-tree index structure that supports fast retrieval of the rows, based on their clustered index key values. The pages in each level of the index, including the data pages in the leaf level, are linked in a doubly-linked list. However, navigation from one level to another is performed by using key values. For more information, see Clustered Index Structures.

  • Heaps are tables that have no clustered index.

    The data rows are not stored in any particular order, and there is no particular order to the sequence of the data pages. The data pages are not linked in a linked list. For more information, see Heap Structures.

Indexed views have the same storage structure as clustered tables.

When a heap or a clustered table has multiple partitions, each partition has a heap or B-tree structure that contains the group of rows for that specific partition. For example, if a clustered table has four partitions, there are four B-trees; one in each partition.

Nonclustered Indexes

Nonclustered indexes have a B-tree index structure similar to the one in clustered indexes. The difference is that nonclustered indexes do not affect the order of the data rows. The leaf level contains index rows. Each index row contains the nonclustered key value, a row locator and any included, or nonkey, columns. The locator points to the data row that has the key value. For more information, see Nonclustered Index Structures.

XML Indexes

One primary and several secondary XML indexes can be created on each xml column in the table. An XML index is a shredded and persisted representation of the XML binary large objects (BLOBs) in the xml data type column. XML indexes are stored as internal tables. To view information about xml indexes, use the sys.xml_indexes or sys.internal_tables catalog views.

For more information about XML indexes, see Indexes on XML Data Type Columns.

An allocation unit is a collection of pages within a heap or B-tree used to manage data based on their page type. The following table lists the types of allocation units used to manage data in tables and indexes.

Allocation unit type

Is used to manage

IN_ROW_DATA

Data or index rows that contain all data, except large object (LOB) data.

Pages are of type Data or Index.

LOB_DATA

Large object data stored in one or more of these data types: text, ntext, image, xml, varchar(max), nvarchar(max), varbinary(max), or CLR user-defined types (CLR UDT).

Pages are of type Text/Image.

ROW_OVERFLOW_DATA

Variable length data stored in varchar, nvarchar, varbinary, or sql_variant columns that exceed the 8,060 byte row size limit.

Pages are of type Text/Image.

For more information about page types, see Understanding Pages and Extents.

A heap or B-tree can have only one allocation unit of each type in a specific partition. To view the table or index allocation unit information, use the sys.allocation_units catalog view.

IN_ROW_DATA Allocation Unit

For every partition used by a table (heap or clustered table), index, or indexed view, there is one IN_ROW_DATA allocation unit that is made up of a collection of data pages. This allocation unit also contains additional collections of pages to implement each nonclustered and XML index defined for the table or view. The page collections in each partition of a table, index, or indexed view are anchored by page pointers in the sys.system_internals_allocation_units system view.

Important noteImportant

The sys.system_internals_allocation_units system view is reserved for MicrosoftSQL Server internal use only. Future compatibility not guaranteed.

Each table, index, and indexed view partition has a row in sys.system_internals_allocation_units uniquely identified by a container ID (container_id). The container ID has a one-to-one mapping to the partition_id in the sys.partitions catalog view that maintains the relationship between the table, index, or the indexed view data stored in a partition and the allocation units used to manage the data within the partition.

The allocation of pages to a table, index, or an indexed view partition is managed by a chain of IAM pages. The column first_iam_page in sys.system_internals_allocation_units points to the first IAM page in the chain of IAM pages managing the space allocated to the table, index, or the indexed view in the IN_ROW_DATA allocation unit.

sys.partitions returns a row for each partition in a table or index.

  • A heap has a row in sys.partitions with index_id = 0.

    The first_iam_page column in sys.system_internals_allocation_units points to the IAM chain for the collection of heap data pages in the specified partition. The server uses the IAM pages to find the pages in the data page collection, because they are not linked.

  • A clustered index on a table or a view has a row in sys.partitions with index_id = 1.

    The root_page column in sys.system_internals_allocation_units points to the top of the clustered index B-tree in the specified partition. The server uses the index B-tree to find the data pages in the partition.

  • Each nonclustered index created for a table or a view has a row in sys.partitions with index_id > 1.

    The root_page column in sys.system_internals_allocation_units points to the top of the nonclustered index B-tree in the specified partition.

  • Each table that has at least one LOB column also has a row in sys.partitions with index_id > 250.

    The first_iam_page column points to the chain of IAM pages that manage the pages in the LOB_DATA allocation unit.

ROW_OVERFLOW_DATA Allocation Unit

For every partition used by a table (heap or clustered table), index, or indexed view, there is one ROW_OVERFLOW_DATA allocation unit. This allocation unit contains zero (0) pages until a data row with variable length columns (varchar, nvarchar, varbinary, or sql_variant) in the IN_ROW_DATA allocation unit exceeds the 8 KB row size limit. When the size limitation is reached, SQL Server moves the column with the largest width from that row to a page in the ROW_OVERFLOW_DATA allocation unit. A 24-byte pointer to this off-row data is maintained on the original page.

Text/Image pages in the ROW_OVERFLOW_DATA allocation unit are managed in the same way pages in the LOB_DATA allocation unit are managed. That is, the Text/Image pages are managed by a chain of IAM pages.

LOB_DATA Allocation Unit

When a table or index has one or more LOB data types, one LOB_DATA allocation unit per partition is allocated to manage the storage of that data. The LOB data types include text, ntext, image, xml, varchar(max), nvarchar(max), varbinary(max), and CLR user-defined types.

The following example returns partition and allocation unit data for two tables: DatabaseLog, a heap with LOB data and no nonclustered indexes, and Currency, a clustered table without LOB data and one nonclustered index. Both tables have a single partition.

USE AdventureWorks;
GO
SELECT o.name AS table_name,p.index_id, i.name AS index_name , au.type_desc AS allocation_type, au.data_pages, partition_number
FROM sys.allocation_units AS au
    JOIN sys.partitions AS p ON au.container_id = p.partition_id
    JOIN sys.objects AS o ON p.object_id = o.object_id
    JOIN sys.indexes AS i ON p.index_id = i.index_id AND i.object_id = p.object_id
WHERE o.name = N'DatabaseLog' OR o.name = N'Currency'
ORDER BY o.name, p.index_id;

Here is the result set. Notice that the DatabaseLog table uses all three allocation unit types, because it contains both data and Text/Image page types. The Currency table does not have LOB data, but does have the allocation unit required to manage data pages. If the Currency table is later modified to include a LOB data type column, a LOB_DATA allocation unit is created to manage that data.

table_name  index_id index_name               allocation_type     data_pages  partition_number 
----------- -------- -----------------------  ---------------     -----------  ------------
Currency    1        PK_Currency_CurrencyCode IN_ROW_DATA         1           1
Currency    3        AK_Currency_Name         IN_ROW_DATA         1           1
DatabaseLog 0        NULL                     IN_ROW_DATA         160         1
DatabaseLog 0        NULL                     ROW_OVERFLOW_DATA   0           1
DatabaseLog 0        NULL                     LOB_DATA            49          1
(5 row(s) affected)

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