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Showing posts from November, 2014

Temp Tables vs Table Varibles: The Great Debate

There seems to be a lot of confusion around the differences between  temp tables and table variables (in the classic sense; I'm not talking about in memory table types or anything here). Some people say you should only use table variables. Some people say you should only use temp tables. Most people caution a more nuanced approach. Here I want to cover what I think are the biggest issues in an attempt to shed some light on this surprisingly tricky topic.

Statistics This is usually the most important factor when considering which one to use. It boils down to two facts, and everything else flows from these: Temp Tables maintain statistics, Table Variables do not.

Table Variables Because table variables maintain no statistics, the Query Optimizer always assumes the contain exactly one row. As a result, any joins against a table variable, unless explicitly told to behave otherwise will (probably) alwaysbe a nested loop join.

As long as the number of rows in the table variable are small…

NOEXEC

This is a cool little feature I found the other day doing some research on a SQL Server Central article. Someone asked if they could essentailly check for syntax errors in dynamically generated maintenance scripts. The first thought I had was to simply wrap each statement in a try/catch and an explicit transaction, but he also said that these were index maintenance statements and it wasn't feasible to be doing all the "heavy lifting" of rebuilding and reorganizing indices just to validate the SQL.

Enter the obscure NOEXEC setting. As the name suggests, when set ON, it will parse/validate sql but not execute it. While this is admittedly a pretty fringe case you'd need this for, it's none the less a very elegant solution to problems like the aforementioned.

set noexec on alter index IXN__myIndexondbo.myTablerebuild set noexec off