Denali Paging–Key seek lookups
Posted
by Dave Ballantyne
on SQL Blogcasts
See other posts from SQL Blogcasts
or by Dave Ballantyne
Published on Fri, 19 Nov 2010 13:02:10 GMT
Indexed on
2010/12/06
16:58 UTC
Read the original article
Hit count: 650
In my previous post “Denali Paging – is it win.win ?” I demonstrated the use of using the Paging functionality within Denali. On reflection, I think i may of been a little unfair and should of continued always planned to continue my investigations to the next step.
In Pauls article, he uses a combination of ctes to first scan the ordered keys which is then filtered using TOP and rownumber and then uses those keys to seek the data. So what happens if we replace the scanning portion of the code with the denali paging functionality.
Heres the original procedure, we are going to replace the functionality of the Keys and SelectedKeys ctes :
CREATE PROCEDURE dbo.FetchPageKeySeek
@PageSize BIGINT,
@PageNumber BIGINT
AS
BEGIN
-- Key-Seek algorithm
WITH Keys
AS (
-- Step 1 : Number the rows from the non-clustered index
-- Maximum number of rows = @PageNumber * @PageSize
SELECT TOP (@PageNumber * @PageSize)
rn = ROW_NUMBER() OVER (ORDER BY P1.post_id ASC),
P1.post_id
FROM dbo.Post P1
ORDER BY
P1.post_id ASC
),
SelectedKeys
AS (
-- Step 2 : Get the primary keys for the rows on the page we want
-- Maximum number of rows from this stage = @PageSize
SELECT TOP (@PageSize)
SK.rn,
SK.post_id
FROM Keys SK
WHERE SK.rn > ((@PageNumber - 1) * @PageSize)
ORDER BY
SK.post_id ASC
)
SELECT -- Step 3 : Retrieve the off-index data
-- We will only have @PageSize rows by this stage
SK.rn,
P2.post_id,
P2.thread_id,
P2.member_id,
P2.create_dt,
P2.title,
P2.body
FROM SelectedKeys SK
JOIN dbo.Post P2
ON P2.post_id = SK.post_id
ORDER BY
SK.post_id ASC;
END;
and here is the replacement procedure using paging:
CREATE PROCEDURE dbo.FetchOffsetPageKeySeek
@PageSize BIGINT,
@PageNumber BIGINT
AS
BEGIN
-- Key-Seek algorithm
WITH SelectedKeys
AS (
SELECT post_id
FROM dbo.Post P1
ORDER BY post_id ASC
OFFSET @PageSize * (@PageNumber-1) ROWS
FETCH NEXT @PageSize ROWS ONLY
)
SELECT P2.post_id,
P2.thread_id,
P2.member_id,
P2.create_dt,
P2.title,
P2.body
FROM SelectedKeys SK
JOIN dbo.Post P2
ON P2.post_id = SK.post_id
ORDER BY
SK.post_id ASC;
END;
Notice how all i have done is replace the functionality with the Keys and SelectedKeys CTEs with the paging functionality.
So , what is the comparative performance now ?.
Exactly the same amount of IO and memory usage , but its now pretty obvious that in terms of CPU and overall duration we are onto a winner.
© SQL Blogcasts or respective owner
