Content deleted Content added
Tag: Reverted |
KuyaMoHirowo (talk | contribs) m Reverted edits by 41.35.175.24 (talk) (AV) |
||
Line 107:
For physical storage of a table, its rows are divided into a series of partitions (numbered 1 to n). The partition size is user defined; by default all rows are in a single partition. A table is split into multiple partitions in order to spread a database over a [[cluster computing|computer cluster]]. Rows in each partition are stored in either [[B-tree]] or [[heap (data structure)|heap]] structure. If the table has an associated, clustered [[index (database)|index]] to allow fast retrieval of rows, the rows are stored in-order according to their index values, with a B-tree providing the index. The data is in the leaf node of the leaves, and other nodes storing the index values for the leaf data reachable from the respective nodes. If the index is non-clustered, the rows are not sorted according to the index keys. An indexed [[view (database)|view]] has the same storage structure as an indexed table. A table without a clustered index is stored in an unordered heap structure. However, the table may have non-clustered indices to allow fast retrieval of rows. In some situations the heap structure has performance advantages over the clustered structure. Both heaps and B-trees can span multiple allocation units.<ref name="table" />
SQL Server [[data buffer|buffer]]s pages in RAM to minimize disk I/O. Any 8 KB page can be buffered in-memory, and the set of all pages currently buffered is called the buffer cache. The amount of memory available to SQL Server decides how many pages will be cached in memory. The buffer cache is managed by the ''Buffer Manager''. Either reading from or writing to any page copies it to the buffer cache. Subsequent reads or writes are redirected to the in-memory copy, rather than the on-disc version. The page is updated on the disc by the Buffer Manager only if the in-memory cache has not been referenced for some time. While writing pages back to disc, [[asynchronous I/O]] is used whereby the [[I/O operation]] is done in a background thread so that other operations do not have to wait for the I/O operation to complete. Each page is written along with its [[checksum]] when it is written. When reading the page back, its checksum is computed again and matched with the stored version to ensure the page has not been damaged or tampered with in the meantime.<ref name="Buffer Management" />
| |||