Server architect definition8/31/2023 Thus, the function of the row offset table is to help SQL Server locate rows on a page quickly. Each row offset entry stores how far the first byte of the row is from the start of the page. A row offset table starts at the end of the page, and each row offset table contains one entry for each row on the page. They contain a series of log records which do not have a fixed size.ĭata rows are stored on the page serially, starting immediately after the header. Information about extents that have changed since the last BACKUP DATABASE statement per allocation unit. Information about extents modified by bulk operations since the last BACKUP LOG statement per allocation unit. Information about extents used by a table or index per allocation unit. Information about page allocation and free space available on pages. Information about whether extents are allocated. Variable length columns when the data row exceeds 8 KB: varchar, nvarchar, varbinary, and sql_variant. Large object data types: text, ntext, image, nvarchar(max), varchar(max), varbinary(max), and xml data. Page typeĭata rows with all data, except text, ntext, image, nvarchar(max), varchar(max), varbinary(max), and xml data, when text in row is set to ON. The following table shows the page types used in the data files of a SQL Server database. This information includes the page number, page type, the amount of free space on the page, and the allocation unit ID of the object that owns the page. Finally, there are system pages that store various metadata about the organization of the data.Įach page begins with a 96-byte header that is used to store system information about the page. The index pages contain index references about where the data is. Again, SQL Server is no different: most pages contain actual rows of data that were stored by users these are called data pages and text/image pages (for special cases). In a book, most pages contain the data - the main content of the book - and some pages contain metadata about the content (for example, the table of contents and index). Similar to a book, SQL Server writes all data rows on pages, and all data pages are the same size: 8 KB. In a regular book, all content is written on pages. That is, SQL Server reads or writes whole data pages.Įxtents are a collection of eight physically contiguous pages and are used to efficiently manage the pages. Disk I/O operations are performed at the page level. ndf) in a database is logically divided into pages numbered contiguously from 0 to n. The disk space allocated to a data file (.mdf or. The fundamental unit of data storage in SQL Server is the page. Understanding the architecture of pages and extents is important for designing and developing databases that perform efficiently. ![]() This guide describes the data structures that are used to manage pages and extents in all versions of SQL Server. ![]() An extent is a collection of eight physically contiguous pages. The page is the fundamental unit of data storage in SQL Server. Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)
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