Polybase Design Patterns On Azure SQL DW

Simon Whiteley continues his Polybase on Azure SQL Data Warehouse series.  First, he covers data loading patterns:

That’s enough about data loading for now, there’s another major use case for Polybase that we haven’t yet discussed. Many data processing solutions have a huge, unwieldy overnight batch job that performs aggregates, lookups, analytics and various other calculations.

However, it is often the case that this is not timely enough for many business requirements. This is where Polybase can help.

If we have an External Table over the newest files, this will read these new records at query time. We can write a view that combines the External Table with our batch-produced table. This will obviously go a little slower than usual, given it has to read data from flat files each time, however the results returned will be up to date.

Simon then covers the Create Table As Select statement:

In order to utilise SQLDW effectively, we write SQL for our transformations, rather than relying on external tools such as SSIS. This ensures the work is being done by our compute nodes and, therefore, can be scaled up to increase performance.

General best practice, therefore, would be write stored procedures for each of the data movements we want to occur. This allows us to add in auditing, logging etc. But what we’re interested in here is the core data movement itself.

Writing a traditional INSERT statement isn’t the fastest way to get data into a table. There is a special syntax which creates a new table and inserts into it, that is automatically configured for optimal bulk loading, this is the CTAS, or “Create Table as Select” statement.

This is a pair of interesting posts from Simon.

Polybase In Azure SQL Data Warehouse

Simon Whiteley loves Polybase as much as I do:

“Polybase is by far the fastest way to import/export data from SQLDW. If you’re loading any reasonably sized data volume, you should be using Polybase”

That’s not a quote – I just thought it would be more impactful looking like one!

For those of a traditional “Big Data” background, Polybase is essentially the same as an external Hive table, embedded into a traditional relational database.

For everyone else – Polybase is a special kind of SQL table. It looks like a table, it has columns and you can run normal T-SQL on it. However, instead of holding any data locally, your query is converted into map-reduce jobs against flat files – even those in sub-folders. For me, this is mind-bogglingly cool – you can query thousands of files at once by typing “select * from dbo.myexternaltable”.

Simon also covers limitations in Polybase:

Push-down predicates

This one is a biggie – if you’re querying over a whole range of flat files that are organised into [YEAR]/[MONTH] folders, for example, you should be able to write a query like the following:

SELECT * FROM dbo.MyExternalTable WHERE [YEAR] > 2016

This filter would be pushed down to the polybase engine and tell it to ignore any files that have been vertically partitioned outside of our chosen range. Instead, all files are read and returned to the SQL Server engine and the filtering is done in-memory on the returned dataset. This is obviously hugely inefficient in some cases – especially when you’re using Polybase as a data loading mechanism. This feature is available in HIVE tables and you can do it in U-SQL – hopefully it’s only a matter of time before a similar feature is implemented here.

It’s an interesting look at Polybase, focusing on Azure SQL Data Warehouse.

Loading Azure SQL DW: Embrace The CTAS

John Hoang, Joe Sack, and Martin Lee explain different loading patterns for loading Azure SQL Data Warehouse:

SQL Data Warehouse supports many loading methods, including SSIS, BCP, the SQLBulkCopy API, and Azure Data Factory (ADF). These methods all share a common pattern for data ingestion. By comparison, the PolyBase technology uses a different approach that provides better performance.

PolyBase is by far the fastest and most scalable SQL Data Warehouse loading method to date, so we recommend it as your default loading mechanism. PolyBase is a scalable, query processing framework compatible with Transact-SQL that can be used to combine and bridge data across relational database management systems, Azure Blob Storage, Azure Data Lake Store and Hadoop database platform ecosystems (APS only).

If you’re curious about Azure SQL Data Warehouse, this is a must-read.

Polybase Use Cases

James Serra talks about Polybase use cases:

For federated queries: “N” requires all data from the source to be copied into SQL Server 2016 and then filtered.  For “Y”, the query is pushed down into the data source and only the results are returned back, which can be much faster for large amounts of data.

I mention “Maybe” for age out data in SQL DW as you can use PolyBase to access the aged-out data in blob or Azure Data Lake Storage (ADLS), but it will have to import all the data so may have slower performance (which is usually ok for accessing data that is aged-out).  For SQL Server 2016, it will have to import the data unless you use HDP/Cloudera, in which case the creation of the MapReduce job will add overhead.

The thing that I like about this chart is that the new Polybase sources (SQL Server, Oracle, Teradata, Mongo, and generic ODBC) do support predicate pushdown.  For large data sets, that’s huge:  it lets the database engine on the opposite end do as much filtering as possible before sending results back to your SQL Server head node.

Integrating Data Lake Storage With SQL Data Warehouse

Sachin Sheth alerts us to a new integration point between Azure Data Lake Storage and Azure SQL Data Warehouse via Polybase:

Most common patterns using Azure Data Lake Store (ADLS) involve customers ingesting and storing raw data into ADLS. This data is then cooked and prepared by analytic workloads like Azure Data Lake Analytics and HDInsight. Once cooked this data is then explored using engines like Azure SQL Data Warehouse. One key pain point for customers is having to wait for a substantial time after the data was cooked to be able to explore it and gather insights. This was because the data stored in ADLS would have to be loaded into SQL Data Warehouse using tools row-by-row insertion. But now, you don’t have to wait that long anymore. With the new SQL Data Warehouse PolyBase support for ADLS, you will now be able to load and access the cooked data rapidly and lessen your time to start performing interactive analytics. PolyBase support will allow to you access unstructured/semi-structured files in ADLS faster because of a highly scalable loading design. You can load the files stored in ADLS into SQL Data Warehouse to perform analytics with fast response times or you use can the files in ADLS as external tables. So get ready to unlock the value stored in your petabytes of data stored in ADLS.

I’ve been waiting for this support, and I’m happy that they were able to integrate the two products.

Polybase And Azure SQL Data Warehouse

I have a post on using Polybase with Azure SQL Data Warehouse:

That’s a header row, and I’m okay with it not making its way in.  As a quick aside, I should note that I picked tailnum as my distribution key.  The airplane’s tail number is unique to that craft, so there absolutely will be more than 60 distinct values, and as I recall, this data set didn’t have too many NULL values.  After loading the 2008 data, I loaded all years’ data the same way, except selecting from dbo.Flights instead of Flights2008.

Click through for more details, including the CETAS statement, which I’d love to see in on-prem SQL Server.

Partitioned Views With Polybase

I look at using SQL 2000-style partitioning with a set of external tables:

Using a view, we were able to create a “partitioned” Polybase experience, similar to what we had in SQL Server 2000.  This form of poor man’s partitioning allows us to segment out data sets and query them independently, something which can be helpful when storing very large amounts of data off-site and only occasionally needing to query it.  The thing to remember, though, is that if you store this in Azure Blob Storage, you will need to pull down the entire table’s worth of data to do any processing.

This leads to a concept I first heard from Ginger Grant:  pseudo-StretchDB.  Instead of paying for what Stretch offers, you get an important subset of the functionality at a much, much lower price.  If you do store the data in Azure Blob Storage, you’re paying pennies per gigabyte per month.  For cold storage, like a scenario in which you need to keep data around to keep the auditors happy but your main application doesn’t use that information, it can work fine.  But if you need to query this data frequently, performance might be a killer.

For Polybase tables without the ability to perform external pushdown, coming up with a good partitioning strategy is probably one of the two best ways to improve performance, with creating a Polybase scale-out cluster the other method.

Split Query Processing In Polybase

David DeWitt, et al, describe the Polybase engine in an academic article:

When compiling a SQL query that references an external table stored in an HDFS file, the PDW Engine Service contacts the Hadoop Namenode for information about the file. This information, combined with the number of DMS instances in the PDW cluster, is used to calculate the portion (offset and length) of the input file(s) each DMS instance should read from HDFS. This information is passed to DMS in the HDFS Shuffle step of the DSQL (distributed SQL) plan along with other information needed to read the file, including the file’s path, the location of the appropriate Namenode, and the name of the RecordReader that the bridge should use.

The system attempts to evenly balance the number of bytes read by each DMS instance. Once the DMS instances obtain split information from the Namenode, each can independently read the portion of the file it is assigned, directly communicating with the appropriate Datanodes without any centralized control.

This is a very clear paper which helps describe the core constructs of Polybase.  Highly recommended.

Polybase: Inserting Into Azure Blob Storage

I have a post which uses Polybase to insert into Azure Blob Storage:

One additional question I have involves whether the process for loading data is round-robin on a row-by-row basis.  My conjecture is that it is not (particularly given that our first example had 4 files with zero records in them!), but I figured I’d create a new table and test.  In this case, I’m using three fixed-width data types and loading 10 million identical records.  I chose to use identical record values to make sure that the text length of the columns in this line were exactly the same; the reason is that we’re taking data out of SQL Server (where an int is stored in a 4-byte block) and converting that int to a string (where each numeric value in the int is stored as a one-byte character).  I chose 10 million because I now that’s well above the cutoff point for data to go into each of the eight files, so if there’s special logic to handle tiny row counts, I’d get past it.

Read on for the exciting(?) conclusion.

Polybase Execution Plan With Blob Storage

I look at an execution plan and packet capture of a Polybase query which reads from Azure Blob Storage:

In this case, all of those packets were 1514 bytes, so it’s an easy multiplication problem to see that we downloaded approximately 113 MB.  The 2008.csv.bz2 file itself is 108 MB, so factoring in TCP packet overhead and that there were additional, smaller packets in the stream, I think that’s enough to show that we did in fact download the entire file.  Just like in the Hadoop scenario without MapReduce, the Polybase engine needs to take all of the data and load it into a temp table (or set of temp tables if you’re using a Polybase scale-out cluster) before it can pull out the relevant rows based on our query.

The upshot is that Polybase behaves very similarly on Azure Blob Storage as it does with on-prem Hadoop for non-MapReduce queries.

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