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Tuesday, 14 December 2010

Beware Sneaky Reads with Unique Indexes

Beware Sneaky Reads with Unique Indexes

I saw a question asked recently on the #sqlhelp hash tag:

Might SQL Server retrieve (out-of-row) LOB data from a table, even if the column isn’t referenced in the query?

Leaving aside trivial cases like selecting a computed column that does reference the LOB data, one might be tempted to say that no, SQL Server does not read data you haven’t asked for.

In general, that is correct; however, there are cases where SQL Server might sneakily read a LOB column.

Friday, 10 December 2010

Heaps of Trouble?

Heaps of Trouble?

Brad Schulz recently wrote about optimizing a query run against tables with no indexes at all. The problem was, predictably, that performance was not very good. The catch was that we are not allowed to create any indexes (or even new statistics) as part of our optimization efforts.

In this post, I’m going to look at the problem from a different angle, and present an alternative solution to the one Brad found.