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Category: T-SQL

Updating Statistics On System Tables

Published 2017-12-04 by Kevin Feasel

Dan Guzman shows that you can update statistics on system tables:

The solution in many cases to simply update statistics on the underlying system tables indexes used by the problem DMV queries. This can be done selectively by identifying the system table indexes referenced in execution plan seek and scan operators of the problem query execution plan and then executing UPDATE STATISTICS on each index. However, the task is somewhat tedious.

Alternatively, one can simply update stats on all the system tables. Below is a script that generates and executes DDL to update stats on all non-empty system tables, making quick work of this.

Click through for the script, as well as an interesting note if you try to use constructs like @sql = @sql + N’some string’ in your code.

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Don’t Forget NOCOUNT

Published 2017-12-04 by Kevin Feasel

Lonny Niederstadt shows just how expensive printing out result counts can be:

OK.  Now Aaron Bertrand has a post from February 2016…
Performance Surprises and Assumptions : SET NOCOUNT ON
https://sqlperformance.com/2016/02/t-sql-queries/nocount

In that blog post the potential performance benefit of NOCOUNT ON was elusive.  And, to be honest, this round of my tests was NOT to learn about NOCOUNT but other stuff.  Just happened to stumble on NOCOUNT when I was sifting through stuff after some early testing.  But with these results in hand, maybe NOCOUNT has a few more performance surprise for all of us 🙂

For a single run, printing out counts isn’t that expensive, but when doing a lot of work, it can add up.

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“Caveman” Graphs In SQL

Published 2017-12-01 by Kevin Feasel

Denis Gobo puts together some basic Management Studio data visualization:

I found this technique on Rich Benner’s SQL Server Blog: Visualising the Marvel Cinematic Universe in T-SQL and decided to play around with it after someone asked me to give him the sizes of all databases on a development instance of SQL Server

The way it works is that you take the size of the database and then divide that number against the total size of all databases. You then use the replicate function with the | (pipe) character to generate the ‘graph’  so 8% will look like this ||||||||

You can use this for tables with most rows, a count per state etc etc. By looking at the output the graph column adds a nice visual effect to it IMHO

It does the job and doesn’t require you to go out to a different product, so it works pretty well for occasional administrative queries.

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TOP And UNIONs

Published 2017-11-27 by Kevin Feasel

Andy Levy points out that the TOP operator doesn’t always apply to each element in a UNION:

This system uses TOP every now and then trying to limit the number of records it gets back (and the developers always seem to use the arbitrary 301 – I’m guessing some degree of cargo cult programming going on). I’d prefer a well-constructed WHERE clause to limit the result set but beggars can’t be choosers when working with legacy vendor code.

What I found odd was that the Ledger1 table didn’t get a lot of traffic – with the WHEREclause in use (omitted here for brevity), you’d only get a handful of records, maybe a dozen at most.

Click through to see the rest of the problem, as well as Andy’s solution.

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Fun With Dynamic SQL: Implicit Casting Can Allow SQL Injection

Published 2017-11-27 by Kevin Feasel

Remus Rusanu shows an example where implicit casting from NVARCHAR to VARCHAR can introduce a SQL injection vulnerability that you otherwise wouldn’t expect:

In both examples above the SQL executed apparently should had been safe from SQL injection, but it isn’t. Neither REPLACE nor QUOTENAME were able to properly protect and the injected division by zero was executed. The problem is the Unicode MODIFIER LETTER APOSTROPHE(NCHAR(0x02bc)) character that I used in constructing the NVARCHAR value, which is then implicitly cast to VARCHAR. This cast is converting the special ‘modifier letter apostrophe’ character to a plain single quote. This introduces new single quotes in the value, after the supposedly safe escaping occurred. The result is plain old SQL Injection.

Click through for the script.  The upside of this is that it’s entirely under your control and you should be able to get it right by using NVARCHAR consistently.

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Using The GROUPING SETS Operator

Published 2017-11-16 by Kevin Feasel

Alfonso Hernandez goes into detail with what you can do with GROUPING SETS:

In T-SQL, you summarize data by using the GROUP BY clause within an aggregate query. This clause creates groupings which are defined by a set of expressions. One row per unique combination of the expressions in the GROUP BY clause is returned, and aggregate functions such as COUNT or SUMmay be used on any columns in the query. However, if you want to group the data by multiple combinations of group by expressions, you may take one of two approaches. The first approach is to create one grouped query per combination of expressions and merge the results using the UNION ALLoperator. The other approach is to use the GROUPING SETS operator along with the GROUP BY clause and define each grouping set within a single query.

In this article I’ll demonstrate how to achieve the same results using each method.

Mastering GROUPING SETS makes reporting queries in T-SQL so much more effective.

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Searching Stored Procedures And Ad Hoc Queries

Published 2017-11-15 by Kevin Feasel

Bert Wagner has a couple queries to help you find references in T-SQL objects, as well as ad hoc statements which are currently in the plan cache:

Have you ever wanted to find something that was referenced in the body of a SQL query?

Maybe you need to know what queries you will have to modify for an upcoming table rename. Or maybe you want to see how many queries on your server are running SELECT *

Below are two templates you can use to search across the text of SQL queries on your server.

Click through for the scripts.  Finding references in T-SQL objects (views, procedures, functions, triggers, etc.) is a fairly straightforward process.  Finding references in ad hoc statements is much more hit-or-miss.

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Grouping Data With ROLLUP

Published 2017-11-09 by Kevin Feasel

Steve Jones shows how easy it is to use the ROLLUP function:

I was editing an article recently that talked about ROLLUP, and I wanted to play with this a bit more. I hadn’t used this much in my career, but it’s a handy operator that’s worth including in your toolkit.

ROLLUP is used to provide additional totals for your aggregates while using GROUP BY. Here’s a little example. I’ve got some data for American Football quaterbacks. In this case, I’ve extracted some stats for a few noteworthy players today.

I’ll probably end up using ROLLUP about once every three months and be very pleased when I remember that it exists.  I use GROUPING SETS more often and almost never use CUBE.

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String Splitting With Multi-Character Delimiters

Published 2017-11-01 by Kevin Feasel

Derek Colley shows a way of using the STRING_SPLIT function when your delimiter is more than one character:

But what happens if we try to use STRING_SPLIT with a multi-character separator, like this?

SELECT * FROM STRING_SPLIT(@GodawfulString, ‘,,’)

Msg 214, Level 16, State 11, Line 67
Procedure expects parameter ‘separator’ of type ‘nchar(1)/nvarchar(1)’.

Ah.  That’s a no, then.

Click through for the solution.

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Getting SQL Server Context Settings

Published 2017-11-01 by Kevin Feasel

Frank Gill has a script to find current Query Store context settings:

The following query will return the hex and integer value for each row in the table (NOTE: Query store must be enabled for the database to return values):

USE YourQueryStoreDatabase;
SELECT set_options, CONVERT(INT, set_options) AS IntSetOptions
FROM sys.query_context_settings;

The set_options value represents a bit mask, with each binary digit representing a specific set option. The full list of values can be found here. I created stored procedure ReturnSetOptions to take the IntSetOptions from the query above and return the set options represented. The code for the procedure is listed below.

Read on to get a script which breaks the bitmask field into human-readable results.

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