Category: Indexing

Kendra Little explains how index modifications affect the plan cache:

Creating an index doesn’t remove a plan from cache

We run our Demo Query five times. Querying sys.dm_exec_query_stats and related DMVs, we see five executions, plan_generation_num = one.

Then we create an index on a table referenced by the query.

Querying the DMVs we still see five executions, plan_generation_num = one, and the query plan in cache. It hasn’t been removed!

Read on for more, including what happens when you run the query again, what happens when you rebuild indexes, and what happens when you drop an index.

Jeff Schwartz looks at the performance cost of indexes when it comes to deleting rows:

Many articles concerning SQL Server discuss how record insertion overhead increases with each additional index. They discuss b-tree manipulations and page splits in addition to leaf and non-leaf levels. However, few discuss the fact that deletion overhead increases as well, especially when large numbers of records are deleted by individual queries. Recently, I was working with a client who regularly purged large numbers of records from tables that ranged in size from large to gigantic. For example, one table contained over 6.5 billion records. I added an index (4^th overall) to one table expressly for the purpose of expediting the large deletion process, and the deletion run ran longer, despite using the new index! To determine how the numbers of indices and records to be deleted interact, I conducted an experiment to test several combinations. The specifics of the tests and their corresponding results are summarized below.

Check it out.  There’s certainly more to the story than “add indexes to improve performance.”

Paul Randal explains the difference between rebuilding and reorganizing rowstore indexes:

Rebuilding an index requires building a new index before dropping the old index, regardless of the amount of fragmentation present in the old index. This means you need to have enough free space to accommodate the new index.

Reorganizing an index first squishes the index rows together to try to deallocate some index pages, and then shuffles the remaining pages in place to make their physical (allocation) order the same as the logical (key) order. This only requires a single 8-KB page, as a temporary storage for pages being moved around. So an index reorganize is extremely space efficient, and is one of the reasons I wrote the original DBCC INDEXDEFRAG for SQL Server 2000 (the predecessor of ALTER INDEX … REORGANIZE).

If you have space constraints, and can’t make use of single-partition rebuild, reorganizing is the way to go.

Click through for the rest of the story.

Brent Ozar goes into detail on why you should not blindly trust missing index recommendations in SQL Server:

SQL Server’s telling us that it needs an index to do an equality search on LastAccessDate – because our query says LastAccessDate = ‘2016/11/10’.

But in reality, that’s not how you access datetime fields because it won’t give you everyone who accessed the system on 2016/11/10 – it only gives you 2016/11/10 00:00:00. Instead, you need to see everyone on that day, like this:

Read the whole thing.  The crux of this is that the missing index recommendation process only gets to see what you’re running at the time you run it, so it can’t generalize all that well; that’s your job.

Nate Johnson explains how to change the clustered index on a very large table:

I call this the “setup, dump, & swap”.  Essentially we need to create an empty copy of the table, with the desired index(es), dump all the data into it, and then swap it in.  There are couple ways you can do this, but it boils down to the same basic premise: It’s “better” (probably not in terms of speed, but definitely in terms of efficiency and overhead) to fill this new copy of the table & its indexes, than it is to build the desired index on the existing table.

As Nate notes, “very large” here will depend on your environment, but this is a useful technique because the old table can be live until the moment of the swap.  As Nate writes this, I’m actually in the middle of one of these sorts of swaps—one that will take a week or two to finish due to pacing.

Thomas Rushton has an example of a positive indexing experience:

100k runs of the query in a ten minute interval? yeowch. Yeah, this should be optimised if possible. The primary wait type was CPU – indicating that the data was all in RAM, but the CPU was having to schlep through the entire table to find what it needed. Or to find that it didn’t need anything. Or something.

The interesting story buried in here is that none of the tooling Thomas used indicated that things could be improved with a better index, and yet there was a tremendous opportunity here based on the graphics at the end.