Posted by Hugo Kornelis on 01/02/07 22:53

On 2 Jan 2007 08:57:21 -0800, Emin wrote:

(snip)
>if I insert a record for a given stock does the database have to
>physicall "move down" all the records that are below that or does it do
>something smarter?

Hi Emin,

Roy answered most of your questions, but was friendly enough to leave
this bit for me to comment on.

Yes, it does something smarter. Rows are *NOT* physically stored in the
order imposed by the clustered index. The clustered index imposes a
*logical* ordering on the data, which is implemented by pointers that
form a logical chain of database pages.

If a row in inserted, SQL Server first navigates the clustered index to
find the correct location (database page). Then, there are two
possibilities:

1. There's still space left on the page. The row is added and the insert
is finished.

2. There's not enough free space on the page. A page split occurs: half
of the data on the page is moved to a newly allocated page and pointers
are updated to reflect the location of the new page in the chain. After
that, the new row is inserted.

If your database has to process *LOTS* of inserts (in the order of
thousands per second or more), it makes sense to define your clustered
index such that new rows are always inserted at the logical end of the
pointer chain. In that case, page splits will never happen.
For lower amounts of inserts, the overhead of a page split is
insignificant.

--
Hugo Kornelis, SQL Server MVP
My SQL Server blog: http://sqlblog.com/blogs/hugo_kornelis

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