Wednesday, February 10, 2010

Indexes and data fetching

Indexes can speed up not only record lookup but data fetching too. If all fields that participate in a query are present in the index, database will not start retrieving actual table data but will use index values only. It is quite important factor to consider during table’s indexes tuning.

select maxof(Quantity) from table1
    where  table1.Key >= 100000
        && table1.Key <= 300000;

The query above runs around 320 ms on my machine with 1M records in Table1 when index on the Table1 contains only Key field. If Quantity field is added to the index execution time drops to 70 ms.

Note: It is important to mention that the difference appears only if lookup happens using non-cluster index. Otherwise query execution time will be almost the same because cluster index is stored together with table data and it is cheap to retrieve it.


  1. It is worth a mention that you should not blindly be adding indexes to tables as well. For example, thanks to our var-layer reseller, we have 40+ indexes on SalesLine. No, that is not a typo. There is a point when indexes are no longer worth it.

    If you think about it from a performance standpoint, which you have to because you're probably adding the index to increase performance in the first place, for every index you have, so in our case 40+, that means the database now has to make 40+ writes in addition to the actual record itself.

    If you think about it from a disk space standpoint, same idea applies. Although, you will not be writing 40+ times the amount of disk space because you're only using small subsets of the original record. But, our SalesLine has 32+ million records, it adds up. And with our AX DB approaching 3TB, its something to keep in mind.

    Being more smart about what columns you choose to be in an index is key to a success in performance. Its bad enough AX shipped with nvarchars in the primary key, my point being just think about what you're doing before blindly adding indexes.

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