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Day: August 9, 2019

Application Caching at Stack Overflow

Nick Craver has a long post on how Stack Overflow does application caching:

For everyone who hates caching, this is the section for you! Yes, I’m totally playing both sides.

Given the above and how drastic the wins are, why wouldn’t we cache something? Well, because every single decision has trade-offs. Every. Single. One. It could be as simple as time spent or opportunity cost, but there’s still a trade-off.

This is a long but very useful post.

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Writing SQL Against Cosmos DB

Hasan Savran shows how to query Cosmos DB documents with SQL:

CosmosDB’s SQL API uses T-SQL like language to query the database. It looks like T-SQL language. You still need to use SELECT, FROM, WHERE clause to retrieve the data you are looking for. It looks similar but it works different because the data structure is different. CosmosDB is not a relational database that means you cannot join other containers.

     Let’s start with SELECT. You can use * to select all properties but just like T-SQL you shouldn’t, pick only what you need. Charge of a Request Units depends on how much data your queries retrieves from containers. ORMs like to select everything and that’s one of the reasons DBAs don’t like them. I know couple of ORM supports CosmosDB. Think twice if you want to use an ORM with CosmosDB. Don’t let an ORM to generate a query for CosmosDB. You will literally pay for it!

Feasel’s Law in force, with a bonus of “ORMs are expensive.”

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Guiding Your Users with Power BI

Marc Lelijveld continues a series on storytelling with Power BI:

Another thing to think about before building visuals, is the click path. In order to make sure we’re really telling the story, we need to get our click path in a logical order. This is most important for both, declarative and exploratory storytelling.

Maybe you want to provide all the information in your visuals right away. Sometimes I see things like this happening when people add a lot of visuals on only one page. In my experience it is better to make your report interactive and work with multiple pages. Spread the story you want to tell out over multiple pages. This is what Will Thompson also has done as well in this report for the Gartner Bakeoff in 2018.

This sort of planning helps you out in the long run.

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Statistics and Multiple Single-Column Indexes

Erik Darling is fusing together queries like Dr. Frankenstein in his lab:

You may have noticed that both queries get pretty bad estimates. You might even be thinking about leaving me a comment to update stats.

The thing is that I created these indexes, which means they get stats built with a full scan, and it’s a demo database where nothing changes.

We just get unfortunate histograms, in this case. If I create very specific filtered statistics, both plans perform a key lookup.

I snipped this part because it’s so ingrained in DBAs that when performance is bad, updating statistics is a panacea.

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Relating Nonclustered Indexes to the Clustered Index

Erin Stellato takes us through a bit of indexing strategy:

In the sp_SQLskills_helpindex output you’ll notice that OrderLineID is the only column in columns_in_tree, and both OrderLineID and OrderID_and_OrderLineID are in columns_in_leaf.

Remember that when you identify a nonclustered index as unique, it will not add the clustering key to the tree level, it just adds it to the leaf level. But if the nonclustered index is not declared as unique, then SQL Server adds the clustering key to the tree level to uniquely identify the row.

This post deserves a careful reading.

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SSIS Design Preferences

Meagan Longoria systematizes a set of preferences regarding Integration Services package and ETL process design:

– Every table should have InsertDateTime and UpdateDateTime columns. The UpdateDateTime column should be populated with the same value as the InsertDateTime column upon creation of the row, rather than being left null.
– Whatever you use to create tables, include primary keys, foreign keys, and indexes with your table definitions. Provide explicit constraint names to simplify database comparisons. You can disable your foreign keys, but they need to be there to provide that metadata.
– Separate your final dimensional/reporting tables from audit tables and staging tables. This can be done with separate schemas or even separate databases.

People have added some more thoughts in the comments as well.

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Monetizing Performance Tuning in the Cloud

Emanuele Meazzo points out a benefit from being in the cloud—performance tuning has a direct monetary tie:

A single step up for a single DB can cost from 10% to 25% more than the previous sizing, which translates in 15K or more when dealing with higher specs databases; remember that this is for a single scale up, on a single DB, on a single year.

It’s easy to understand that if your database and/or queries are not well tuned because “scaling will take care of it eventually”, then, you’ll be secretly bleeding money.

We can quantify performance improvements in terms of resource savings, but tying that to the specific business benefits can be hard because that hardware is a capital expenditure and so much of tuning benefits are “we avoided this worse case” rather than “we actively scaled something down.”

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Getting Max Column Value from Power Query

Chris Webb hits us with techniques to get the maximum value in a column using Power Query and M:

In this first part I’m going to set up the scenario and show you what I found out from my own experimentation. The really mind-blowing tricks shown to me by the ever-helpful Curt Hagenlocher of the Power Query dev team will be covered in part 2.

Let’s say you have a large csv file which contains a numeric column and you want to get the maximum value from that column. In this case I’m going to use the 2018 Price Paid data from the UK Land Registry available here. This csv file contains 1021215 rows, one for each property transaction in England and Wales in 2018; the second column in this file contains the the price paid for the property, so the aim here is to get the maximum price paid for all property transactions in 2018.

Read the whole thing.

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