04 February 2010

Just In CASE – Part III: Quest for Performance

Arranging branches

    Given the fact that the CASE function stops after the first match, in case there are no two expressions that evaluate to true for any same value, the CASE could be optimized for the best performance by arranging the branches in the order of the highest number of matches. For example taking the logic from the 3rd example from Part I and checking the number of occurrences for each value, surprisingly (or maybe not) there are 257 NULL values, 97 ‘L’ values, 82 ‘H’ values, respectively 68 ‘M’ values. If we change the branches in this order, the number of evaluations will decrease from 82*1+68*2+97*3+257*3=1280 to 257*1+97*2+82*3+68*3=901, the difference is small though when applied to big data sets the improvement could pay the effort. Here is the modified logic for Example 3:
    This technique is unfortunately not bulletproof – even if less probable in many cases, the order of the values’ occurrences could change overnight, the number of total evaluations varying in time on the number of occurrences for each value.

Rewriting Simple Unions

    Have you ever seen a set of similar queries whose output is merged with the help of UNION (ALL) operator? I’ve seen so many queries like the below one, in which the only difference between the sub-queries was residing in the WHERE constraint(s) applied and maybe a few additional calculations!

    The example is quite simple, though I think it demonstrates the point. The UNION could be translated to a simple CASE as in the 3rd example. The problem with such queries is that if there is no index on the attribute(s) used is the WHERE constraint, it will be performed a full table scan for each sub-query, quite expensive from a performance standpoint. Even if the respective attribute is indexed, there’s still a pay in performance. Such an approach could be maybe acceptable when in each scenario different tables are joined to the main table, though even then should be checked which one of the two approaches has better performance. A recent example I can recall and in which could have been used several CASE functions instead of the UNION was based on the JOIN of two tables, the query looking something like:

Rewriting Complex Unions

    Even if when the base table is used in several union-based merged queries, it doesn’t sense to apply this technique all the times, especially when dealing with complex queries using multiple joins. Now it depends, there are scenarios and scenarios, is must be always considered the trade in performance, readability and usability of a query when applying any technique. The Person.Address from AdventureWorks is the best way to exemplify two scenarios in which it makes sense and doesn’t makes sense to combine the logic in two union queries. The respective table stored the addresses for Customers (Sales & Individuals), Vendors and Employees, thus supposing that we would like to see the Owners and Owner Type for each Address we would need to create several unions.

    Let’s consider first the Customer-based Addresses – as there are two types of Customers and because the details for each type are stored in different tables, we might end up creating the following query:

    As can be seen the two queries are quite similar, most of the tables joined are the same with 3 exceptions: Sales.Store vs. Sales.Individual & Person.Contact. The query could be rewritten using a CASE and left joins instead of inner joins as follows:

    Similar queries could be written in order to get the Vendor and Employee details, and as can be seen also these queries share many similarities.
    Does it makes sense to use the above technique also in this case? Maybe it would be acceptable to merge the Vendor with Employee Addresses queries, though does it make sense to do merge also the Customer Addresses using CASE & left joins. The queries are not so complex so it’s possible to do that, though I think a decision should be taken only when has been studied the performance of the two approaches.

Pivoting Behavior

    Another technique when a CASE could help eliminate several joins to the same table is presented in a post on List Based Aggregations, the CASE function being used together with aggregated functions in order to cumulate the On Hand for several locations and show it on the same line. This approach was quite useful before the PIVOT operator was introduced in SQL Server 2005 (see Pivot operator example), and is still is in case is needed to select more than 1 attribute for each line.

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