Lesson 2: Internal Structures, Vardecimal, & Row Compression
SQL University Compression Week: Lesson 1 About Compression
“So Balls,” you say, “What is Page Compression and how does it compare to Row Compression?”
Great question Dear Reader! Page Compression is a Super Set of Compression that includes Row Compression. We discussed the data types that Row Compress yesterday, the data types for Page Compression are….. All of them! Page Compression, while still only occurring on IN_ROW_Data Pages (More on that later), Compresses at a binary level. For the additional levels of Page Compression other than Row we need to take a deeper dive again!
STARTING OUT WITH SOME MORE INTERNALS
Compression is a very internal technology. I hope you’re not sick of learning about how things work on the inside because this is some of the most fun stuff to me, I realize how sad that will sound to my soon to be teenager. To get a really good understanding of anything it helps to know what you are doing internally and Compression is no different. SQL Server is made up of Records and Pages. There are many types of Pages but the three types that we care about are all called Allocation Units. These 3 types of Allocation Units break down into IN_ROW_Data, ROW_OVERFLOW_Data, and LOB_Data.
Out of those three types of pages only data in IN_ROW_Data Compresses. So what kind of Data Types are IN_ROW_Data Data Types? It is actually easier to list those that are not IN_ROW_Data as the list is much shorter. You can Compress everything but varchar(max), varbinary(max), nvarchar(max), XML, Text, Image, NTEXT, CLR Data Types (Spatial (that changes in SQL 2012) and Geography), and Filestream. Compression is also not compatible with Spares columns. Anything else and you can Page Compress.
You’ll remember our Mall Map of internal structures from yesterday’s lesson. Well today we are moving up a level and we are tackling Pages.
So today you are here on the Pages portion of our Mall Map for the subject at hand Page Compression. Page Compression is made up of 3 different components. Row Compression, Column Prefix Compression, and Page Dictionary Compression. If they are all applied to a page that is the order in which SQL Server will attempt to apply them. We discussed Row Compression at length yesterday. So when you apply Page Compression our Record format changes to the Compressed Record Structure and Row Compression will occur removing all unneeded extra bytes.
Now I don’t know about you Dear Reader but reading binary is not a forte of mine. So in order to explain this I’m going to use some screen shots on one of my presentation decks covering Compression. For these next couple images I want to make sure you understand that this is not actually what a Page looks like, but right now as Louis Davidson(@DrSQL | Blog) would say, your drinking my flavor of Kool-aid. Next you will have Column Prefix Compression.
We'll start by going down each column and taking the common values for the column and populating the anchor tag at the top of the page. Looking at the first column the most common pattern is Brad, between the first 2 columns and Br with the last column. Column Prefix Compression will take the longest value with the longest matching pattern and move that record to the Anchor Tag and replace it with 4 bits representing a special Null that points towards the top of the page. It will also replace the first [4] Characters in Brad22 leaving [4]22 and leaving [2]et31 out of Bret31.
Now let's fill out the rest of our columns. Daniel is our longest value with a matching pattern, we'll move that to the Anchor Tag and leave Dan21 as [3]21 and David33 as [2]vid33. For our last column we'll take value Many31 and move that to our header and leave [2]et31 from Maet31 and [2]ny31 from Many31.
Now we have COMPLETED Row Compression and Column Prefix Compression. The last portion of Page Compression is Page Dictionary Compression. For Page Dictionary Compression we will look for common patterns across the entire page. When we find them we will move them into a multi dimensional zero based array in the Dictionary portion of the Page.
We will start with [2]et31. As you look at the page you can see two of these values. This common value will be placed in the Dictionary portion of the page and a 0 will be entered in it's place. The next value that we will find on the page is [3]21 we will move that value into our Dictionary portion of the page and replace it with a 1 entry for both columns.
Now as you can see our page looks very different from where we started. This is where our CPU over head starts to come into play. Not only do we have the Compressed record format, but we to uncompress our pages when they are handed from the Storage Engine to the Relational Engine.
DEMO
Here is a full and complete demo. For more scripts go to my Resources Page and download any of my presentations on Compression. They all contain the scripts I use as demos. For this one we will create a database, a table, we will add some data, then we will apply compression looking at the space saved. We will be using sp_estimate_data_compression_savings. One BIG WARNING that I like to give is that sp_estimate_data_compression_savings works by taking 5% of your actual physical table or index and copying it into your TempDB applying Compression, and then estimating the space savings against the total size of your table.
So MAKE SURE YOU HAVE ENOUGH TEMPDB SPACE before you use this. Do not use it on your 100 GB table if you do not have 5 GB of free space in Tempdb. For this demo our table is relatively small and you should only need MB worth of free space vs. GB. But before you use this on something at your job make sure your not going to adversely effect your server and test this in a lower life cycle first.
USE demoCompression
GO
/*
Set our Statistics ON
*/
SET STATISTICS IO ON
SET STATISTICS TIME ON
/*
Create a Heap Table
*/
CREATE TABLE heap2(myID int IDENTITY(1,1), myChar CHAR(8000));
GO
/*
Insert Some Records
We are going to Fill up a lot more
of the Char(8000) this time
aa= 2 * 1000 = 2000
1234= 4 * 100 = 400
bb= 2 * 1000 = 2000
mydataandOtherStuff1234 = 23 * 347 = 7981
*/
DECLARE @i INT
SET @i=0
BEGIN TRAN
WHILE (@i<15000)
BEGIN
IF (@i<=1000)
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES(REPLICATE('aa', 1000));
SET @i = @i +1
END
ELSE IF ((@i>1000) AND (@i<=2000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES((REPLICATE('aa', 1000) + REPLICATE('1234', 100)));
SET @i = @i +1
END
ELSE IF ((@i>2000) AND (@i<=3000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES((REPLICATE('aa', 1000)+ REPLICATE('bb', 1000)));
SET @i = @i +1
END
ELSE IF ((@i>3000) AND (@i<=4000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES(REPLICATE('aa', 1000));
SET @i = @i +1
END
ELSE IF ((@i>4000) AND (@i<=5000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES((REPLICATE('aa', 1000) + REPLICATE('1234', 100)));
SET @i = @i +1
END
ELSE IF ((@i>5000) AND (@i<=6000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES((REPLICATE('aa', 1000)+ REPLICATE('bb', 1000)));
SET @i = @i +1
END
ELSE IF ((@i>6000) AND (@i<=7000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES(REPLICATE('aa', 1000));
SET @i = @i +1
END
ELSE IF ((@i>7000) AND (@i<=8000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES((REPLICATE('aa', 1000) + REPLICATE('1234', 100)));
SET @i = @i +1
END
ELSE IF ((@i>8000) AND (@i<=9000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES((REPLICATE('aa', 1000)+ REPLICATE('bb', 1000)));
SET @i = @i +1
END
ELSE IF ((@i>9000) AND (@i<=10000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES(REPLICATE('aa', 1000));
SET @i = @i +1
END
ELSE IF ((@i>10000) AND (@i<=11000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES((REPLICATE('aa', 1000) + REPLICATE('1234', 100)));
SET @i = @i +1
END
ELSE IF ((@i>11000) AND (@i<=12000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES((REPLICATE('aa', 1000)+ REPLICATE('bb', 1000)));
SET @i = @i +1
END
ELSE IF ((@i>12000) AND (@i<=13000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES(REPLICATE('aa', 1000));
SET @i = @i +1
END
ELSE IF ((@i>13000) AND (@i<=14000))
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES((REPLICATE('aa', 1000) + REPLICATE('1234', 100)));
SET @i = @i +1
END
ELSE
BEGIN
INSERT INTO dbo.heap2(mychar)
VALUES(REPLICATE('mydataandOtherStuff1234', 347));
SET @i = @i +1
END
END
COMMIT TRAN
/*
Quick Select of the data
*/
SELECT * FROM dbo.heap2
/*
How big is our Table
*/
sp_spaceused 'dbo.heap2';
GO
/*
Estimate our space savings
*/
sp_estimate_data_compression_savings 'dbo', 'heap2', NULL, NULL, ROW;
GO
/*
We reduced by around 1/3
Can we do better with Page Compression?
*/
sp_estimate_data_compression_savings 'dbo', 'heap2', NULL, NULL, PAGE;
GO
/*
How big is our Table
*/
sp_spaceused 'dbo.heap2';
GO
/*
Rebuild With Compression
*/
ALTER TABLE dbo.heap2
REBUILD WITH(DATA_COMPRESSION=PAGE);
GO
/*
How big is our Table
*/
sp_spaceused 'dbo.heap2';
GO
/*
Do another select
to get the Statistics
*/
SELECT * FROM dbo.heap2
/*
Turn Off Compression
*/
ALTER TABLE dbo.heap2
REBUILD WITH(DATA_COMPRESSION=NONE);
GO
Alright Dear Reader! Your homework do the demo and review row compression make sure you know the difference between the two!
Thanks,
Brad
