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specterunseen asked

App with ETL like operations - pull back small sets or one large set?

**The problem** Have an application that needs to provide back a result set from a normalized database into a single result sheet that is highly normalized for processing changes in excel. - Must be pulled on demand - Users could pull several thousand results at a time. - Users could select fewer columns for the data, however, most are pulling all columns back on this, so the amount of data going across the network could be pretty heavy at busy times. - Right now clients are pulling in 200 accounts at time and it's pretty slow with the views and the amount of data coming back. **Actions I'm taking right now** Right now I'm refactoring some of these result sets that are currently views (some with 21 tables). I've been carefully evaluating the statistics io/time and the plans for optimization, breaking some of the initial tables into temp tables, and trying to improve the efficiency with all the nested loops taking place. From my predecessor, I'm hearing that they found breaking things into smaller chunks took longer overall, but they were using the views, and I'm using some tuned procs... so it's not a 1 for 1 comparison. With time limited, I'm hoping I could get some general design practice info for optimizing this for this initial performance patch I'm going to release. I can't redo the entire process right now, so I've worked on tuning the procs, and also am considering the amount of results to pull back per account. It's been running at 200 a pull. I reduced down to 50 and saw an exponential decrease in I/O being processed in the execution plan on some of the nested loops. (ie top 200 had 31 MB in one nested loop, but the top 50 had 5). **My questions** Is there a general design practice for these tough ETL type jobs with applications for amount to return back? One large query vs breaking up into much smaller top50 results? I'm leaning towards the smaller results due to all the cardinality estimation issues with the massive tables being returned in multiple joins, but also don't want to tank performance for customers that may deal with remote work and latency. I'm just not sure if it's considered less overhead to transmit smaller chunks 10x or 2 large chunks.
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Grant Fritchey avatar image
Grant Fritchey answered
It's really hard to say for certain with seeing the queries, the structure and the execution plans for what's happening. I would probably stay away from traditional views. I might look at materialized views for some aspects of the data where you might be able to pre-aggregate or pre-join the data to achieve a performance increase. Assuming you're on 2012 or better, you might look to columnstore indexes in some locations to use the pivoted data storage for improved performance in places where aggregation style queries are preferred over point lookups. With large scale data moves, and large query sets, I'd be a little concerned that you're getting lots of loops joins in your execution plans. You might want to make sure that your statistics are up to date, possibly with a full scan to ensure their accuracy. You may also need to have your queries recompile at each execution in order to avoid issues with bad parameter sniffing.
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