Press "Enter" to skip to content

Curated SQL Posts

Gradient Boosting In R

Published 2017-08-28 by Kevin Feasel

Anish Sing Walia walks us through a gradient boosting exercise using R:

An important thing to remember in boosting is that the base learner which is being boosted should not be a complex and complicated learner which has high variance for e.g a neural network with lots of nodes and high weight values.For such learners boosting will have inverse effects.

So I will explain Boosting with respect to decision trees in this tutorial because they can be regarded as weak learners most of the times.We will generate a gradient boosting model.

Click through for more details.  H/T R-Bloggers

Comments closed

Columnstore Predicate Pushdown Tipping Point

Published 2017-08-28 by Kevin Feasel

Lonny Niederstadt is hot on the trail, looking for evidence of a tipping point for COUNT(*) aggregates performing pushdown against a clustered columnstore index:

Below is what we want post-execution plans to look like when counting rows in a range – the thin arrow coming out of the Columnstore scan is a hint that predicate pushdown was successful.  I didn’t specify MAXDOP in a query hint, and Resource Governor isn’t supplying MAXDOP; MAXDOP 8 is coming from the Database Scoped Configuration.  The degree of parallelism turns out to be a significant factor in determining the tipping point.  The [key] column is a BigInt.  Maybe its surprising that I’m using 27213.9 as the upper bound.  But… check out the estimated number of rows 🙂 Again – this estimate is coming from the Legacy CE, specified in the database scoped configuration for my database.

Interesting findings, although it looks like the specific values are going to be more settings-dependent than the usual finding of this nature.

Comments closed

On Global Temp Tables

Published 2017-08-28 by Kevin Feasel

Denis Gobo riffs on global temp tables:

SQL Azure has added something called database scoped global temporary tables.

Azure SQL Database supports global temporary tables that are also stored in tempdb and scoped to the database level. This means that global temporary tables are shared for all users’ sessions within the same Azure SQL database. User sessions from other Azure SQL databases cannot access global temporary tables.

They way you add these is by using a double pound sign

Click through for more.  I will occasionally use a global temp table, mostly in conjunction with Central Management Server queries going over a set of databases on each instance, but I limit my usage to manual operations and nothing as part of a normal application or administrative process.

Comments closed

Columnstore Segment Alignment

Published 2017-08-28 by Kevin Feasel

Niko Neugebauer walks us through a new segment alignment detection function he has written:

There are 3 important factors that I use for the determination of the column that is really good for the Segment Elimination and hence the Segment Alignment:
– The support for the Segment Elimination (and then for the Predicate Pushdown). If the data type does not support Segment Elimination, than why would someone optimise for it ?
– The frequency with which the column is used in the predicates (not in the joins, because this is where generally the Segment Elimination/Predicate Pushdown does not function)
– The number of the distinct values within a table/partition (if we have more Segments than distinct values, it is not a very good sign generally: example – 10 million rows with 5 distinct values)

Read on for more details.

Comments closed

Waits With Outsized Importance

Published 2017-08-28 by Kevin Feasel

Brent Ozar has a few wait types whose appearance in your “uh-oh” list is disproportionate to the wait type’s relative percentage:

RESOURCE_SEMAPHORE_QUERY_COMPILE – this means a query came in, and SQL Server didn’t have an execution plan cached for it. In order to build an execution plan, SQL Server needs a little memory – not a lot, just a little – but that memory wasn’t available. SQL Server had to wait for memory to become available before it could even build an execution plan. For more details and a reproduction script, check out my Bad Idea Jeans: Dynamically Generating Ugly Queries post. In this scenario, cached query plans (and small ones) may be able to proceed just fine (depending on how much pressure the server is under), but the ugly ones will feel frozen.

Read on for a couple more examples of poison wait types.

Comments closed

In-Memory OLTP In SQL Server 2017

Published 2017-08-28 by Kevin Feasel

Ned Otter walks us through all of the changes to memory-optimized objects coming in SQL Server 2017:

  • Up to and including SQL 2016, the maximum number of nonclustered indexes on a memory-optimized table was eight, but that limitation has been removed for SQL 2017. I’ve tested this with almost 300 indexes, and it worked. With this many supported indexes, it’s no wonder they had to….

  • Enhance the index rebuild performance for nonclustered indexes during database recovery. I confirmed with Microsoft that the database does not have be in SQL 2017 compatibility mode (140) to benefit from the index rebuild enhancement. This type of rebuild happens not only for database restore and failover, but also for other “recovery events” – see my blog post here.

Read on for several more improvements, as well as a couple of things which Ned would like to see but aren’t there yet.

Comments closed

When CHECKDB Snapshots Run Out Of Disk Space

Published 2017-08-28 by Kevin Feasel

Andy Galbraith walks through an error message in DBCC CHECKDB when the snapshot runs out of disk space:

Looking in the SQL Error Log there were hundreds of these combinations in the minutes immediately preceding the job failure:

—

The operating system returned error 665(The requested operation could not be completed due to a file system limitation) to SQL Server during a write at offset 0x000048a123e000 in file ‘E:\SQL_Data\VLDB01.mdf:MSSQL_DBCC17‘. Additional messages in the SQL Server error log and system event log may provide more detail. This is a severe system-level error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.

—

Error: 17053, Severity: 16, State: 1.

—

E:\SQL_Data\VLDB01.mdf:MSSQL_DBCC17: Operating system error 665(The requested operation could not be completed due to a file system limitation) encountered.

Read on for more information, including a rough idea of how much space the snapshot requires as well as a few workarounds and hints.

Comments closed

Classifying Time Series Data With TensorFlow

Published 2017-08-25 by Kevin Feasel

Burak Himmetoglu applies a time series data set to two different types of neural networks using TensorFlow:

In this blog post, I will discuss the use of deep leaning methods to classify time-series data, without the need to manually engineer features. The example I will consider is the classic Human Activity Recognition (HAR) dataset from the UCI repository. The dataset contains the raw time-series data, as well as a pre-processed one with 561 engineered features. I will compare the performance of typical machine learning algorithms which use engineered features with two deep learning methods (convolutional and recurrent neural networks) and show that deep learning can surpass the performance of the former.

I have used Tensorflow for the implementation and training of the models discussed in this post.  In the discussion below, code snippets are provided to explain the implementation. For the complete code, please see my Github repository.

Click through for the samples, or check out the repo, linked above.

Comments closed

Evaluating A Data Science Project

Published 2017-08-25 by Kevin Feasel

Tom Fawcett gives us an interesting evaluation of a data science case study:

The model is a fully connected neural network with three hidden layers, with a ReLU as the activation function. They state that data from Google Compute Engine was used to train the model (implemented in TensorFlow), and Cloud Machine Learning Engine’s HyperTune feature was used to tune hyperparameters.

I have no reason to doubt their representation choices or network design, but one thing looks odd. Their output is two ReLU (rectifier) units, each emitting the network’s accuracy (technically: recall) on that class. I would’ve chosen a single Softmax unit representing the probability of Large Loss driver, from which I could get a ROC or Precision-Recall curve. I could then threshold the output to get any achievable performance on the curve. (I explain the advantages of scoring over hard classification in this post.)

But I’m not a neural network expert, and the purpose here isn’t to critique their network design, just their general approach. I assume they experimented and are reporting the best performance they found.

Read the whole thing.

Comments closed

Reversing Dynamic Data Masking

Published 2017-08-25 by Kevin Feasel

Joe Obbish shows how easy it is to reverse Dynamic Data Masking:

Armed with our new knowledge, we can create a single SQL query that decodes all of the SSNs. The strategy is to define a single CTE with all ten digits and to use one CROSS APPLY for each digit in the SSN. Each CROSS APPLY only references the SSN column in the WHERE clause and returns the matching prefix of the SSN that we’ve found so far. Here’s a snippet of the code:

Click through for progressively faster solutions.  This is the main reason I do not care for DDM as a feature.  Its main benefit seems to be preventing shoulder-surfing on reports; any concerted attacker with a little bit of access to writing queries can subvert it.

Comments closed