Why Fixing Part of Key Lookups Doesn’t Work In SQL Server

Adding columns to an index won’t always fix key lookups in SQL Server. It can lead to issues with space, memory, and performance. The cost of key lookups is the same whether you select all columns or just one. Avoid creating super wide indexes to cover various things. Evaluate queries before creating indexes. Cheers! 🍵

Key Takeaways 🌟

| Point |

|—————————————————|
| Adding columns to an index doesn’t always fix key lookups |
| Fixing key lookups with extensive columns can have downsides |
| The cost of key lookups is surprising and can lead to inefficient indexing |
| Beware of unnecessary wide indexes and evaluate queries first |

Why Partially Fixing Key Lookups Doesn’t Work In SQL Server 🚫

Eric darling with darling data still muddling through the holidays in today’s video talks about the limitations of fixing key lookups in SQL Server by adding columns to an index. The video discusses the repercussions of extensive indexing and the costing mechanism behind inefficient key lookups.

In a recent blog post, Eric discussed how fixing predicates in key lookups can be sufficient to avoid slow performance but it doesn’t cover cases where there are no predicates in the lookups. Hence, adding columns to the index may lead to various downsides.

The key lookups may lead to additional space on disk, memory usage, transaction logging, and locking and blocking issues. Additionally, extensive use of string columns can lead to larger and inefficient indexes, further affecting performance.

Thus, extensive index creation with multiple column inclusions should be evaluated thoroughly, to avoid possible downsides and performance issues. It is not always beneficial to create super wide indexes to cover various queries without analyzing their speed and efficacy.

The costing mechanism behind lookup operations brings to light surprising insights. The exact same cost applies whether every column or just one column is selected that isn’t part of the non-clustered index. Hence, the cost per loop of a key lookup remains constant irrespective of the number of columns included.

The large output list required in an extensive indexed table or a table with added columns is a fixed cost, irrespective of how many columns are involved in the query.

It is important to note that creating a non-clustered index with a key on reputation and including every other column is unnecessary, as having multiple cluster indexes on a table is not advisable or beneficial. Instead, the speed and efficacy of queries should be evaluated before creating super wide and extensive indexes.

The unnecessary creation of wide indexes without thoroughly evaluating the queries can lead to blocking and deadlocks, resulting in performance issues.

Finally, Eric thanks the viewers for watching and encourages them to like and subscribe for more content on SQL Server. It is a friendly reminder to appreciate the support and participation for future content.

Grab some hot chocolate and stay tuned for more informative and insightful SQL Server content.

The article provides valuable insights into the limitations of fixing key lookups in SQL Server by adding columns to an index. It also emphasizes the importance of evaluating queries and avoiding unnecessary extensive indexing. The repercussions of adding multiple columns to the index and the surprising costing mechanism behind lookup operations are discussed thoroughly.