Uniqueness And Multiple NULL Values

Dennes Torres shows how to allow an indefinite number of NULL values  while guaranteeing non-NULL values are unique:

Regardless of using unique constraint or unique index, the field can accept null values, however the uniqueness will result in only accepting a single row with null value.

The solution to allow nulls in unique fields is create a unique filtered index excluding the nulls of the index, due to that the uniqueness of the nulls will not be validated and multiple rows with nulls will be accepted.

Click through for the code.  I enjoy asking this as an interview question.  It’s a non-trivial problem with a non-trivial solution and isn’t a trick question.

When Was That Index Modified?

Kendra Little looks at index creation and modification dates:

SQL Server doesn’t really track index create or modification date by default

I say “really”, because SQL Server’s default trace captures things like index create and alter commands. However, the default trace rolls over pretty quickly on most active servers, and it’s rare that you’re looking up the creation date for an index you created five minutes ago.

I think it’s fine that SQL Server doesn’t permanently store the creation date and modification date for most indexes, because not everyone wants this information — so why not make the default as lightweight as possible?

That said, Kendra has several methods for answering the question of when a particular index was created.

Tuning Indexes For Stored Procedures

Kendra Little offers advice on index tuning within stored procedures:

The best way is to run the stored procedure yourself to generate and save an “actual” execution plan, which contains the estimates SQL Server used when it generated the plan as well as actual rowcounts, actual memory granted, etc. It will also contain a green tooltip with a “missing index request” if SQL Server thinks an index would help.

This is an introductory-level post which contains good advice.

Non-Clustered, Memory-Optimized Indexes

Raul Gonzalez takes a look at non-clustered indexes on memory-optimized tables:

Wow, what happened there? This is something new I wasn’t expecting.

The first query matches one of our expected query plans, “Index Seek + Lookup” but just an “Index seek” doesn’t make sense, or it does?

Read on for the answer.

The Risk Of Custom Indexes

David Klee shows the risk of creating custom indexes on third-party software:

The error file tells you specifically which indexes it does not like. The error file is found at:

C:\Users\(youraccount)\AppData\Local\Temp\vcsUpgrade\vcdb_req.err

You’ll find the error message towards the bottom of the document. My specific item was:

1 [42000](50000) [Microsoft][SQL Server Native Client 11.0][SQL Server]ERROR ! Extra indexes: VPX_EVENT.HFX_VPX_EVENT_Cover01; VPX_STAT_COUNTER.IX_VPX_STAT_COUNTER_STAT; VPX_TASK.HFX_VPX_TASK_Cover01;

For well-maintained third-party vendor software which doesn’t require you to add indexes to support the product at any scale beyond what a developer needs for basic testing, this isn’t an issue.  And if you ever find that piece of software, write the company a note of congratulations for being the first…

Index Create Dates

Kenneth Fisher looks to see when his indexes were created (or at least updated):

SQL Server stores a create date and a change date for each object in the sys.objects system view.

Unfortunately while tables, views and even constraints are objects, indexes are not. Or at least they aren’t stored in the sys.objects system view. And the sys.indexes system view doesn’t have any dates associated with it. So how do we get the create/update date on an index? Well, short answer is you don’t. Long answer is that in some cases you can get some information.

These aren’t ideal answers, but they can be better than nothing.

Filtered Indexes

Kendra Little explains the two types of filtered indexes:

These two filtered indexes are very different – and the SQL Server optimizer can use them very differently!

While classic filtered nonclustered rowstore indexes must reliably “cover” parts of the query to be used to the optimizer, filtered nonclustered columnstore indexes may be combined with other indexes to produce a plan returning a larger range of data.

This sounds a little weird. I’ll show you what I mean using the WideWorldImporters database.

Kendra exposes an interesting difference in the two types of index and a case where filtered indexes simply fail  (though that’s not a situation you want to be in anyhow!).

The Value Of Unused Indexes

Erik Darling provides a scenario in which an index which does not get used in an execution plan can nonetheless help query performance:

We can see an example of this with unique indexes and constraints, but another possibility is that the created index had better statistical information via the histogram. When you add an index, you get Fresh Hot Stats, whereas the index you were using could be many modifications behind current for various reasons. If you have a big table and don’t hit auto-update thresholds often, if you’re not manually updating statistics somehow, or if you’re running into ascending key weirdness. These are all sane potential reasons. One insane potential reason is if you have autocreate stats turned off, and the index you create is on a column that didn’t have a statistics object associated with it. But you’d see plan warnings about operators not having associated statistics.

Again, we’re going to focus on how ADDING an index your query doesn’t use can help. I found out the hard way that both unique indexes and constraints can cease being helpful to cardinality estimation when their statistics get out of date.

This is sort of like a triple bank shot solution:  even if it works that one time, there are easier ways to do it—and those ways are more likely to succeed to boot.

Check Those Estimates

Grant Fritchey runs into a statistics issue:

While the number of rows for 1048 was the lowest, at 3, unfortunately it seems that the 1048 values were added to the table after the statistics for the index had been updated. Instead of using something from the histogram, my value fell outside the values in the histogram. When the value is outside histogram the Cardinality Estimator uses the average value across the entire histogram, 258.181 (at least for any database that’s in SQL Server 2014 or greater and not running in a compatibility mode), as the row estimate.

Figuring out those boundaries can make the difference between a good plan and a bad plan.

Hypothetical Indexes

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.

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