Temporary Table Usage in SQL Server

Question

This is a bit of an open question but I would really like to hear people opinions.

I rarely make use of explicitly declared temporary tables (either table variables or r

Answer 1

Temporary tables are most useful for a complex batch process like a report or ETL job. Generally you would expect to use them fairly rarely in a transactional application.

If you're doing complex query with a join involving multiple large tables (perhaps for a report) the query optimiser may not actually be able to optimise this in one hit, so temporary tables become a win here - they decompose the query into a series of simpler ones that give the query optimiser less opportunity to screw up the plan. Sometimes you have an operation that cannot be done in a single SQL statement at all, so multiple steps for processing are necessary to do the job at all. Again, we're talking about more complex manipulations here.

You can also create a tempory table for an intermediate result and then index the table, possibly even putting a clustered index on it to optimise a subsequent query. This might also be a quick and dirty way to optimise a report query on a system where you are not allowed to add indexes to the database schema. SELECT INTO is useful for this type of operation as it is minimally logged (and therefore fast) and doesn't require to align the columns of a select and insert.

Other reasons might include extracting data from XML fields using CROSS APPLY and xpath queries. Generally it's much more efficient to extract this into a temp table and then work on the temp table. They're also much faster than CTE's for some tasks as they materialise the query results rather than re-evaluating the query.

One thing to note is that temporary tables are exactly the same structure that the query engine uses to store intermediate join results, so there is no performance penalty to using them. Temporary tables also allow multi-phase tasks using set operations and make cursors almost (not quite but almost) unnecessary in T-SQL code.

'Code Smell' is an overstatement but if I saw a lot of simple operations involving temporary tables I would be wondering what was going on.