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SQL Internals Reading Data Records Part 3: Fixed Length Columns

  • 29 June 2012
  • Author: BradleyBall
  • Number of views: 5355

Hello Dear Reader and welcome back to part 3 of our ongoing series on how to Read a Data Record. 

It is Friday just before the weekend, and now we are getting to the part of the record that means a great deal to us. 

Not that the Tag Bytes and Null Bitmap offset weren’t important but now we are finally out of the meta data and into OUR DATA.  The data that we as DBA’s store or that our users store.

So I’ll dispense with the turn of the phrase and dive directly into our data.



First let’s update our chart so we know what part of the Data Record we are tackling.  Once again these images come by way of Paul Randal (@PaulRandal | Blog) and the MCM Video series on Data Structures and the good people from Microsoft.


Our Fixed Length Columns are any data type that always uses the defined amount of space.  They are BIGINT, INT, FLOAT, DECIMAL, MONEY, CHAR, NCHAR, BIT, SMALLMONEY, MONEY, REAL, DATETIME, DATETIME2, DATETIMEOFFSET, and BINARY.  As mentioned these fields will use 100% of the space assigned to them regardless of the length of the data stored within them.  For example if I define a CHAR(500) and we insert ‘Some Product’, which is 12 characters long 1 byte per character means 12 bytes for storage, we will have 488 bytes of left over space.  If we have an Integer it always uses 4 bytes even if the number only needs 1 byte of space.  Before I break down into a Compression blog let’s just go straight into our record.


The scripts we’ve been using from our previous blogs created a fixed length record that was 1504 bytes which is a little much to put on a page, but the second table we created yesterday that had just two fields and took up  9 bytes would work out well.  So here’s the code if you need it.


CREATE TABLE fixedRecord
         myID INT
        ,mychar CHAR(5)
INSERT INTO dbo.fixedRecord(myid, mychar)
VALUES(1, 'X')


Don’t forget we’ll use DBCC IND to get our page number.


DBCC IND(demoInternals, 'fixedRecord', 1)


Then we’ll set Trace Flag 3604 on and use DBCC PAGE to look at the output.


DBCC PAGE('demoInternals', 1, 282, 3)


Here is the output greatly paired down just to what is relevant to us. 

0000000000000000:   10000d00 01000000 58202020 20020000           ........X    ...


The record we inserted was (1,X) our 1 will be highlighted in red and our X in orange.  We need to remember that each section of code grouped together in groups of two hexadecimal digits in 4 byte segments.  So that means everything in red makes up a perfect 4 byte segment because it is and INT data type, our CHAR(5) takes up all of one segment and two digits, a hexadecimal pair, in orange. 


Now that we have them grouped we can start translating them.  The trailing 0’s in 01000000 represent the empty unused space in the Integer field.  Our value 01 translates to 0x01 which equals 1, if you want you can convert it to validate  Our next record will be a little more interesting 58202020 20.  The first part that we are interested in if 58, we’ll use our converter tool to convert that from Hex to binary, 01011000.  Our next step is to find out what the binary value actually translates to in text.  For that we’ll use another conversion tool, just plug in 010110000 and we see that it is binary for the letter X.


“So Balls”,  you say, “What are all the 202020’s for then?”


Dear Reader your keen powers of perception never fail to amaze me!  The 202020 20’s are the white space left over in the CHAR(5) field.  Remember we only need 1 byte worth of space for our letter X, but our CHAR(5) takes up 5 bytes no matter what.  If you take a 20 and place it into our handy Hex converter you get the binary value of 00100000, which when converted to ASCII is a simple space.   So 58 is our X and 202020 20 are the 4 spaces in the record as well.




While we have a choice in using CHAR and VARCHAR data types and whether we waste white space or not, we do not have that option with Integers or Dates and Times, which is a perfect reason to start learning about SQL Server Compression and reclaim some of that wasted space.


Thanks again for stopping by Dear Reader.





Categories: Analysis Services
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