Category: Indexing

Kenneth Fisher discusses hypothetical indexes:

I saw something like this the other day. My first thought was “Hu, never seen that before.” My second thought was “Wow, that’s really cool. I wonder what a hypothetical index is?” A quick search later and I discovered that the DTA (database tuning adviser) uses them to test out what indexes will work best. A pretend (one might almost say hypothetical) index is created, with statistics, but without the actual index structure. Then a query plan is created allowing for that index.

This is pretty cool since creating a real index can take quite a bit of time, particularly on a really large table. It would be nice to be able to tell SQL that an index exists and try it out before actually spending the time creating it. I’d learned about a DB2 method of doing this a while back but wasn’t aware of one for SQL Server. In part that’s because it’s undocumented. Because the commands I’m going to use here are undocumented standard warnings apply.

That’s completely new to me.

Raul Gonzalez shows a case in which writing to a heap can be more efficient than writing to a clustered index:

Lots have been written, including myself, about the advantage and disadvantages of the different types of tables that exist in SQL Server, Clustered and Heaps.

Today I will again, because at the end of the day, a table is the most basic structure in a database and we need them to store the data, which is indeed the purpose of having a database, right?

Interesting reading.  Check it out.

Derik Hammer looks at computed columns with a bonus section on unique indexes (or maybe vice versa):

What can an index do that a constraint cannot?

• Set FILL FACTOR.
• Add non-key (INCLUDED) columns.
• Data compression.
• Lock management settings.

What can a constraint do that an index cannot?

The only potential benefit I was able to find, or think of, is that constraints can be disabled. If you could enable and disable a unique constraint, that could be one feature that the unique index does not. This is counter-intuitive knowing that the unique constraint is enforced with a unique index behind the scenes.

For me, the big benefit of unique constraints over unique indexes is that they provide a logical separation.  Unique constraints show up in the Keys section in Management Studio and let you say, “Yes, I made this thing because the data model requires uniqueness here.”  By contrast a unique index can tell the same thing, but could also say “for the subset of data in a filtered index, I can assume uniqueness” or “for performance reasons, this combination is unique, but its uniqueness is not important to the logical data model.”  I will happily admit that I’m splitting the hair pretty thin on this one and that in practice, there are benefits to using unique indexes across the board.

Kendra Little answers the age-old question:

With very large indexes, rebuilds take longer, generate more log, impact performance more while they’re running.

If you’re using high availability features like Availability Groups or database mirroring that stream the log between replicas, generating a lot of log data very quickly can create problems.

Your replica/ mirrors may fall behind. Depending on the latency between replicas, the size of the indexes rebuilt, and other operations in the database, they may be out of your Recovery Point Objective / Recovery Time objective for a long time.

In this situation, it’s particularly attractive to drip changes into the log more slowly. One of the ways to do this is to use REORGANIZE for those large indexes.

There’s a lot of nuance here, so give it a read (or watch the video).

Kenneth Fisher shows the downside cost of non-clustered indexes:

I want to make a couple of final points. I realize 99 indexes is a lot. It’s to emphasize the differences. However they were also fairly small indexes and this is a single table where a normal database might easily have hundreds. So take these results as an example. They aren’t going to match real life but will hopefully show you how all of this can play out.

Indexes are awesome but you want to be smart about adding them. My personal rule of thumb, with no scientific evidence behind it, is 5 indexes or less and I’m pretty easy. 5-10 indexes and you’ll have to convince me. I’m going to be reviewing the existing indexes and see what I can get rid of, or maybe I can combine something. Past 10 indexes and it had best be for a query that’s running a 100+ times a minute or something for the CEO.

Read on for demo code and specific results.