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Temporary Tables in SQL Server

By: Phil Factor
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Temporary tables are just that. They are used most often to provide workspace for the intermediate results when processing data within a batch or procedure. They are also used to pass a table from a table-valued function, to pass table-based data between stored procedures or, more recently in the form of Table-valued parameters, to send whole read-only tables from applications to SQL Server routines, or pass read-only temporary tables as parameters. Once finished with their use, they are discarded automatically.

Temporary tables come in different flavours including, amongst others, local temporary tables (starting with #), global temporary tables (starting with ##), persistent temporary tables (prefixed by TempDB..), and table variables.(starting with (@)

Before we get too deep into the technology, Iíd advise that you should use table variables where possible. Theyíre easy, and SQL Server does the work for you. They also tend to cause fewer problems to a hard-working OLTP system. Just occasionally, you may need to fine-tune them to get good performance from them in joins, but I'll explain that in a moment,

Table Variables
Table variables are used within the scope of the routine or batch within which they are defined, and were originally created to make table-valued functions possible. However, they are good for many of the uses that the traditional temporary table was put to. They behave like other variables in their scoping rules. Once out of scope, they are disposed of. These are much easier to work with, and pretty secure, and they trigger fewer recompiles in the routines where theyíre used than if you were to use temporary tables. Table variables require less locking resources as they are 'private' to the process that created them. Transaction rollbacks do not affect them because table variables have limited scope and are not part of the persistent database, so they are handy for creating or storing data that ought to survive roll backs such as log entries. The downside of table variables is that they are often disposed of before you can investigate their contents for debugging, or use them to try out different SQL expressions interactively.

If your application is conservative and your data volumes light youíll never want anything else. However, you can hit problems. One difficulty is that table variables can only be referenced in their local scope, so you cannot process them using dynamic SQL as you might with a temporary table or table-valued parameter. This is because you canít refer an externally-defined table variable within dynamic SQL that you then execute via the EXEC statement or the sp_ExecuteSQL stored procedure because the dynamic SQL is executed outside the scope of the table variable. You can, of course, create, and then use, the table variable inside the dynamic SQL because the table variable would be in scope. However, once the dynamic SQL is run, there would be no table variable

There are a few anomalies to be aware of too. You canít, for example, change the table definition after the initial DECLARE statement; a table variable canít be the destination of a SELECT INTO statement or a INSERT EXEC; You canít call user-defined functions from CHECK constraints, DEFAULT values, and computed columns in the table variable. The only constraints that you're allowed beyond CHECK constraints are PRIMARY KEY, UNIQUE KEY, and NULL / NOT NULL

The trickiest problems, though, come with increasing size of the tables, because you canít declare an index explicitly and Distribution statistics arenít maintained on them. The Query Optimiser assumes that there is only one row in the table. You also cannot generate parallel query plans for a SQL expression that is modifying the table's contents. To get around the index restriction, you can use constraints to do the same thing. Most essential is the Primary Key constraint which allows you to impose a clustered index, but unique constraints are useful for performance. The Query optimiser will happily use them if they are around. The biggest problem with table variables is that statistics arenít maintained on the columns. This means that the query optimiser has to make a guess as to the size and distribution of the data and if it gets it wrong, then youíre going to see poor performance on joins: If this happens, there is little you can do other than to revert to using classic local temporary tables. One thing you can try is to add option (recompile) to the statement that involves the table variable joining with other tables. By doing this, SQL Server will be able to detect number of rows at recompile because the rows will have already been populated. In this demo, the join was reduced in time by three quarters simply by adding the OPTION (RECOMPILE)

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