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Friday, 31 August 2012

Deletes that Split Pages and Forwarded Ghosts

Deletes that Split Pages and Forwarded Ghosts

Can DELETE operations cause pages to split?

Yes. It sounds counter-intuitive on the face of it. Deleting rows frees up space on a page, and page splitting occurs when a page needs additional space. Nevertheless, there are circumstances when deleting rows causes them to expand before they can be deleted.

Friday, 17 August 2012

Temporary Table Caching Explained

Temporary Table Caching Explained

SQL Server (since 2005) caches temporary tables and table variables referenced in stored procedures for reuse, reducing contention on tempdb allocation structures and catalogue tables.

A number of things can prevent this caching (none of which are allowed when working with table variables):

  • Named constraints (bad idea anyway, since concurrent executions can cause a name collision)
  • DDL after creation (though what is considered DDL is interesting)
  • Creation using dynamic SQL
  • Table created in a different scope
  • Procedure executed using WITH RECOMPILE

Temporary objects are often created and destroyed at a high rate in production systems, so caching can be an important optimization.

Wednesday, 15 August 2012

Temporary Table Caching in Stored Procedures

Temporary Table Caching in Stored Procedures

Introduction

Ask anyone what the primary advantage of temporary tables over table variables is, and the chances are they will say that temporary tables support statistics and table variables do not.

This is true, of course. The indexes that enforce PRIMARY KEY and UNIQUE constraints on table variables do not have populated statistics associated with them. Neither do any non-constraint table variable indexes (using inline index definitions, available starting with SQL Server 2014). Finally, it is not possible to manually create statistics on table variables.

Intuitively, then, any query that has alternative execution plans to choose from ought to benefit from using a temporary table rather than a table variable. This is also true, up to a point.