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  • Fun with Aggregates

    There are interesting things to be learned from even the simplest queries. For example, imagine you are given the task of writing a query to list AdventureWorks product names where the product has at least one entry in the transaction history table, but Read More...
  • Row Goals and Grouping

    You might recall (from my last post) that query plans containing a row goal tend to favour nested loops or merge join over hashing. This is because a hash join has to fully process its build input (to populate its hash table) before it can start probing for matches from its second input. Hash join therefore has a high start-up cost, which is balanced by a lower per-row cost once probing begins. In this post, I’ll take a look at how row goals affect grouping operations. Grouping Strategies While the start-up cost of hash join often makes it unsuitable for plans with a row goal, there are times when hashing operations may feature in such plans, since the Hash Match iterator also supports a streaming mode. As an example, say we are asked to list one hundred unique first names from the AdventureWorks Contacts table: Read More...
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