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Storage

Storage:

MSDN Article

EXTENT made up of 8 8k pages. (64k) main adv is to save time when adding new records as new storage isnt sought for every single row - its already been assigned as an extent.

PAGE Number of pages in an extent is fixed at 8. a row can NOT be split between pages - which gives rows a max limit. - unless using a variable length col and the data is then stored "off page", on another page. This means rows can be over 8k if they use variable length types.

There are a number of different kinds of page: Data Index BLOB etc...

ROW Number of rows per page is variable as: can be up to 8k max 1024 cols

DATALENGTH(colname) - gives the number of bytes the data in resultset cell takes up. If varchar for example it only gives used bytes but does NOT include the 2 or so bytes taken to store the length of the data stored. E.g.

SELECT DATALENGTH(colA)

A clustered index and a table cannot be on different filegroups



Estimation article: http://msdn.microsoft.com/en-us/library/aa933068(SQL.80).aspx