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Tuesday, June 5, 2012

Sql Server : Differences b/w CTE/Temp/Table Variable

It depends on the circumstances.

Advantages of CTE:

1. You'll really have to performance test - There is no Yes/No answer. As per Andy Living's post above links to, a CTE is just shorthand for a query or subquery.
2. If you are calling it twice or more in the same function, you might get better performance if you fill a table variable and then join to/select from that. However, as table variables take up space somewhere, and don't have indexes/statistics (With the exception of any declared primary key on the table variable) there's no way of saying which will be faster.
3. They both have costs and savings, and which is the best way depends on the data they pull in and what they do with it. I've been in your situation, and after testing for speed under various conditions - Some functions used CTEs, and others used table variables.

Advantages of temp tables

1. The table "exists" - that is, it's materialized as a table, at least in memory, which contains the result set and can be reused.
2. In some cases, performance can be improved or blocking reduced when you have to perform some elaborate transformation on the data - for example, if you want to fetch a 'snapshot' set of rows out of a base table that is busy, and then do some complicated calculation on that set, there can be less contention if you get the rows out of the base table and unlock it as quickly as possible, then do the work independently. In some cases the overhead of a real temp table is small relative to the advantage in concurrency.

Advantages of derived tables:

4. A derived table is part of a larger, single query, and will be optimized in the context of the rest of the query. This can be an advantage, if the query optimization helps performance (it usually does, with some exceptions). Example: if you populate a temp table, then consume the results in a second query, you are in effect tying the database engine to one execution method (run the first query in its entirety, save the whole result, run the second query) where with a derived table the optimizer might be able to find a faster execution method or access path.

5. A derived table only "exists" in terms of the query execution plan - it's purely a logical construct. There really is no table.

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