Index Allocation Maps

Question

I am a developer reading the book "Querying Microsoft SQL Server 2012." I have come across the following diagram:

Why are the pages in the first two rows of the diagram (row 1 starting: 1:174 and row 2 starting: 1:41) not part of an extent? The documentation says: "all pages are stored in extents":
How does the number scheme work e.g. what does 1:174 mean?
Directly above the diagram, the book says: "IAM pages for an object are organized as a doubly linked list;" The diagram does not show any links between the pages.

Thanks @ypercube, sorry, edited on mobile, didn't even notice the missing tags. — Aaron Bertrand, Feb 13 '16 at 16:20

Aaron Bertrand · Accepted Answer · 2016-02-13 16:57:46Z

5

They are part of a mixed extent, the diagram just didn't draw a grey box behind them, probably more to keep it from being too busy as much as anything.
Data File : Page
Again, I think this is just meant to simplify and not make the diagram too busy, as well as not give any sort of suggestion that pages will necessarily be linked in a certain order. Surely you can picture the arrows that would represent a doubly-linked list?

For most of this, you're much better off asking the author of the book and creator of the diagram, than a community of peers.

answered Feb 13 '16 at 16:25

Aaron Bertrand

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Thanks. What is a data file? As far as I am concerned there is only one data file I.e. The .mdf. Is there one data file per IAM? – w0051977 Feb 13 '16 at 16:31
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@w0051977 msdn.microsoft.com/en-us/library/ms189563.aspx – Martin Smith Feb 13 '16 at 16:41
@martin smith, thanks. Are both iams and pages linked using doubley linked lists? – w0051977 Feb 13 '16 at 17:06
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@w0051977, you can have as many data files in a database as you want and as many log files as you want. These two are not correlated, although you need at least one of each. You should probably read up on this (pretty basic) stuff before you start learning about IAMs, extents and pages. – Daniel Hutmacher Feb 13 '16 at 17:52
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@w0051977, "The pages in each level of the index are linked in a doubly-linked list.". This is the storage structure that allows SQL Server to perform forward and backward scans through an index. – Daniel Hutmacher Feb 14 '16 at 20:25

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