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Index selectivity and index scans

There was a question raised some time back ‘If an index is not selective, will the query operators that use it always be index scans’?

It’s an interesting question and requires a look at what’s going on behind the scenes in order to answer properly..

Short answer: No, not always. Read more

1 comments, 2,122 reads

Posted in SQL In The Wild on 6 October 2015

Repairing a replication subscriber

It’s a question which has come up a couple of times. If a subscriber of a transactional replication publication becomes corrupt, is running CheckDB with repair allow data loss safe?

The theory is, since the subscriber is a copy of another database, allowing CheckDB to discard data in the process… Read more

2 comments, 673 reads

Posted in SQL In The Wild on 22 September 2015

Unusual errors with Distributed Replay

I’ve come to really like Distributed Replay in the last couple of years. I’ve used it to do a scale test, I’ve used it to test a workload for performance regressions before upgrading. It has a lot of possibilities.

One problem with it is there’s no GUI, so configuring it… Read more

4 comments, 477 reads

Posted in SQL In The Wild on 8 September 2015

How not to do a Masters degree

There are many, many guides to successfully completing a post grad degree, so I am not going to add to them. Instead, this is a list of things, based on personal experience, that you can do to make it somewhere between very difficult and impossible to complete that Masters or… Read more

6 comments, 443 reads

Posted in SQL In The Wild on 11 August 2015

It's not you; it's us

Mirrored from http://www.sqlservercentral.com/articles/Editorial/101894/

By now I expect this news is no longer new: the MCM, MSA, and renamed versions of those certifications are no longer being offered after Oct 1, 2013. The announcement was made in email, late on Friday night is the US, which was Saturday morning for those… Read more

0 comments, 1,685 reads

Posted in SQL In The Wild on 4 September 2013

SQL University: Advanced Indexing – Indexing Strategies

Right, I know it’s Friday and everyone’s tired and looking forward to the weekend, but I do need to finish off this indexing section and I’ll try to keep this short and interesting and hopefully keep everyone awake.

There’s no shortage of information available on how to create indexes. Hell,… Read more

0 comments, 1,365 reads

Posted in SQL In The Wild on 11 November 2011

SQL University: Advanced Indexing – Filtered Indexes

Welcome back to day 2 of Advanced Indexing. Today we’re going to look at a feature that was added in SQL Server 2008 – filtered indexes.

In versions previous, indexes were always on the entire table. An index would always have the same numb of rows as the table it… Read more

2 comments, 1,636 reads

Posted in SQL In The Wild on 9 November 2011

SQL University: Advanced indexing – Sorting and Grouping

Good day everyone and welcome to another week of SQL University. I know we’re getting close to the end of the year and everyone’s looking forward to a nice long vacation soaking up the sun at the beach, but a little bit of attention would be nice. Thank you.

This… Read more

3 comments, 1,937 reads

Posted in SQL In The Wild on 7 November 2011

24 Hours of PASS Questions

I finally found the time to work through the questions from the 24 Hours of PASS session that I did. Thanks to everyone that attended the event

Q1: Can you filter execution plans for sort warnings?

No. The sort and hash warnings don’t appear in the execution plan. You’d have… Read more

1 comments, 666 reads

Posted in SQL In The Wild on 27 September 2011

Compiles and recompiles

I want to spend some time over the next few months looking at query compilation and the plan cache, and there’s a couple concepts that I want to get cleared up beforehand. The first of those is around two terms that are often used interchangeably, compile and recompile.


A… Read more

2 comments, 1,606 reads

Posted in SQL In The Wild on 16 August 2011

All about Execution Plans

Coming to the PASS Summit in October this year? Excellent!

I say excellent, because Grant (blog|twitter) and I are presenting a full-day seminar on the Monday, all about execution plans. Excited already? If not, let me give you a taste of what we’ll be discussing. Read more

2 comments, 663 reads

Posted in SQL In The Wild on 9 August 2011

SQLSkills Immersion training in London

Just short of the winter solstice, I bailed out of a freezing cold Johannesburg for warmer climates; well, actually for London where the weather could almost have been mistaken for a South African winter, except much wetter.

However I wasn’t going to London for the weather (fortunately), nor even for… Read more

2 comments, 1,056 reads

Posted in SQL In The Wild on 22 July 2011

Converting OR to Union

When I looked at indexing for queries containing predicates combined with OR, it became clear that the are some restrictive requirements for indexes for the optimiser to consider using the indexes for seek operations.

  • Each predicate (or set of predicates) combined with an OR must have a separate index

Read more

0 comments, 1,354 reads

Posted in SQL In The Wild on 5 July 2011

Are all updates split into delete-insert?

This should be another quick one.

Earlier I saw a forum post where someone asserted that SQL always executes an update as a delete of the row and then an insert of the row with the new values. Now I need another excuse to play with undocumented features, so let’s… Read more

11 comments, 3,217 reads

Posted in SQL In The Wild on 21 June 2011

T-SQL Tuesday: Disasters don’t just come in huge

So we’re supposed to talk about disasters we’ve had or discuss disaster recovery technologies. I’m going to take a slightly different approach…


I’m in the fortunate position of living in a region of the world that’s relatively free of natural disasters. We’re reasonably geologically stable, the nearest fault lines… Read more

1 comments, 661 reads

Posted in SQL In The Wild on 14 June 2011

On the exorcism of Ghost Records

There was a question earlier on one of the SQL forums as to whether or not Ghost Cleanup overwrote the deleted rows when it ran so as to ensure no one could read that data again.

Now I could just reference Paul Randal‘s blog post where he described Ghost… Read more

3 comments, 1,634 reads

Posted in SQL In The Wild on 7 June 2011

PASS Summit Session Votes

I blogged a week ago about the sessions that I submitted for the PASS Summit this year. At the point that I scheduled the post I hadn’t seen the announcement from PASS, hence a second post on the same topic.

This year, you can vote for sessions that you want… Read more

0 comments, 905 reads

Posted in SQL In The Wild on 18 May 2011

On Transactions, errors and rollbacks

Do errors encountered within a transaction result in a rollback?

It seems, at first, to be a simple question with an obvious answer. Transactions are supposed to be atomic, either the entire transaction completes or none of it completes.

Maybe too simple…

CREATE TABLE TestingTransactionRollbacks (

Read more

15 comments, 11,567 reads

Posted in SQL In The Wild on 17 May 2011

Pass summit abstracts

I missed the Pass summit last November for a number of reasons, but I’m hoping I can attend this year. Given that, I submitted a number of abstracts for consideration.

The limits on number were as follows: Up to 4 abstracts for regular, spotlight or half-day sessions (ie main conference… Read more

1 comments, 810 reads

Posted in SQL In The Wild on 10 May 2011

Sql Saturday #83

Frikkie Bosch opened the day with a short keynote giving an overview of Denali, talking mostly from a marketing point of view about the major features coming in the next version.

The virtual sessions were very popular, we don’t get many good international speakers down in our part of the… Read more

4 comments, 688 reads

Posted in SQL In The Wild on 9 May 2011

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