Index Depth for Partitioned Tables

One really useful function that Microsoft provide with SQL Server is the INDEXPROPERTY function. With it you can get the depth of an index, which is the number of pages in the index that need to be read to get from the root to the leaf. Assuming a table called dbo.test with an index pk_test you could use a query like the one below: –

[code language=”sql”]
SELECT INDEXPROPERTY(OBJECT_ID(N’dbo.test’, N’U’), N’pk_test’, ‘IndexDepth’);

The problem with this in-built function is that it doesn’t work on partitioned tables, yielding a not so helpful NULL as a result. Now of course there is a way to get the index depth for each balanced tree structure of each partition with sys.dm_db_index_physical_stats. My issue here is that I don’t want that much information and I want to know how much load I will be putting on the server. Load here being measure in terms of reads and by extension buffer pool usage. So I rolled my own script to look up the index depth of all partitions of an index. We’ll get to that in a moment, first I thought I’d explain why I cared.

I was looking at some indexes that someone else had created with the DTA (Database Tuning Advisor). A very nice feature that works quite well when used properly. Not so great when someone feels that a scatter gun tactic of throwing all recommended indexes at a database would be clever. One place where the DTA falls down is that it doesn’t know what you were trying to achieve with your design. As such the DTA may well recommend a great index that breaks something else for you. For example with partitioned tables you can lose the ability to fast switch if you have non-aligned indexes. Here’s an example of something similar to what I had. Lets assume you have 100 partitions each of depth 3 (Which we don’t yet know but my script can help you there). You have a primary key which is a nice surrogate integer identity, often recommended but not always advisable. The partitioning key is a datetime column. Finally you also have a non partition aligned index on the primary key alone, depth 4. The clustering key is on the identity however in practice you have to be in the right partition so effectively it might as well be datetime, id instead.

Now if I wanted to check the existence of a record by looking up the ID, for example, then the optimiser would in this situation correctly conclude that the non-aligned index is ideal. 4 reads and we’re done. If we used the clustering key we’d need to access each partition and check each one because we cannot get the datetime field first (we only have the ID). Well that’s going to be 300 reads. 3 reads per partition. Yep them 4 reads sure are looking good right about now. Now the downside here was that this non-aligned index would prevent me fast switching partitions. So yes it was ideal for the data access but it was really bad for my data purge. Picture a very upset DBA about this time 🙂

I now had some work to do to sort this out, but before I did that I came across the annoyance with INDEXPROPERTY so I made a stored proc to help. The proc can be created in any DB and can then be pointed at any other DB, so I recommend putting it in a Tools or DBA database if you choose to use it. By the way it uses dynamic SQL. I really like dynamic SQL for it’s power and flexibility. In the right hands dynamic SQL is a really powerful tool, but only in the right places. I’ve seen been some really bad performance issues in the past where dynamic SQL was the right way to go and no other way would have been nearly as good. In this case I needed dynamic SQL to easily let me construct the query to get the root pages of each partition. I also needed dynamic SQL to capture the root page data with DBCC PAGE. The proc will check for the existence of the database, which should be the only part that might allow SQL injection, and bails out if the database cannot be found. Also there is error handling for this and anything deemed serious will be logged. Otherwise you should get a result set. This script should work in SQL Server 2005+ although I cannot test it on all versions. Please let me know if you use it and find it useful or find any errors.

Get index depth for partitioned tables

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