Inserting Into External Tables

Paul Hernandez shows how to insert data into an external table using Polybase:

One of the most interesting use cases of Polybase is the ability to store historical data from relational databases into a Hadoop File System. The storage costs could be reduced while keeping the data accessible and still can be joined with the regular relational tables. So let`s do the first steps for our new archiving solution.

Archival is a very good use case for external table insertion, and if you don’t have a Hadoop cluster, you could insert into Azure blob storage.

Polybase DMVs

I look at the DMVs associated with Polybase and external table creation:

Let’s walk through this one step at a time and understand what the DMV is telling us.  Unfortunately, the DMV documentation is a little sparse, so some of this is guesswork on my part.

  1. A RandomIDOperation appears to create a temporary table.  In this case, the table (whose name is randomly generated) is named TEMP_ID_53.  I’m not sure where that name comes from; the session I ran this from was 54, so it wasn’t a session ID.

  2. After the table gets created, each Compute node gets told to create a table called TMP_ID_53 in tempdb whose structure matches our external table’s structure.  One thing you can’t see from the screenshot is that this table is created with DATA_COMPRESSION = PAGE.  I have to wonder if that’d be the same if my Compute node were on Standard edition.

  3. We add an extended property on the table, flagging it as IS_EXTERNAL_STREAMING_TABLE.

  4. We then update the statistics on that temp table based on expected values.  629 rows are expected here.

  5. Then, we create the dest stat, meaning that the temp table now has exactly the same statistics as our external table.

  6. The next step is that the Head node begins a MultiStreamOperation, which tells the Compute nodes to begin working.  This operator does not show up in the documentation, but we can see that the elapsed time is 58.8 seconds, which is just about as long as my query took.  My guess is that this is where the Head node passes code to the Compute nodes and tells them what to do.

  7. We have a HadoopRoundRobinOperation on DMS, which stands for “Data Movement Step” according to the location_type documentation.  What’s interesting is that according to the DMV, that operation is still going.  Even after I checked it 40 minutes later, it still claimed to be running.  If you check the full query, it’s basically a SELECT * from our external table.

  8. Next is a StreamingReturnOperation, which includes our predicate WHERE dest = ‘ORD’ in it.  This is a Data Movement Step and includes all of the Compute nodes (all one of them, that is) sending data back to the Head node so that I can see the results.

  9. Finally, we drop TEMP_ID_53 because we’re done with the table.

This post was about 70% legwork and 30% guesswork.  That’s a bit higher a percentage than I’d ideally like, but there isn’t that much information readily available yet, so I’m trying (in my own small way) to fix that.

Where Polybase Stats Live

I dig into where the statistics against a Polybase table actually live:

Today, we learned that Polybase statistics are stored in the same way as other statistics; as far as SQL Server is concerned, they’re just more statistics built from a table (remembering that the way stats get created involves loading data into a temp table and building stats off of that temp table).  We can do most of what you’d expect with these stats, but beware calling sys.dm_db_stats_properties() on Polybase stats, as they may not show up.

Also, remember that you cannot maintain, auto-create, auto-update, or otherwise modify these stats.  The only way to modify Polybase stats is to drop and re-create them, and if you’re dealing with a large enough table, you might want to take a sample.

The result isn’t very surprising in retrospect, and it’s good that “stats are stats are stats” is the correct answer.

Polybase Setup Errors

Murshed Zaman on the Azure CAT team covers a number of Polybase configuration errors:

SSMS Error:

Any Select query fails with the following error.
Msg 106000, Level 16, State 1, Line 1
Java heap space

Possible Reason:

Illegal input may cause the java out of memory error.  In this particular case the file was not in UTF8 format. DMS tries to read the whole file as one row since it cannot decode the row delimiter and runs into Java heap space error.

Possible Solution:

Convert the file to UTF8 format since PolyBase currently requires UTF8 format for text delimited files.

I imagine that this page will get quite a few hits over the years, as there currently exists limited information on how to solve these issues if you run into them, and some of the error messages (especially the one quoted above) have nothing to do with root causes.

Polybase Statistics

I dig into a non-trivial Polybase query:

Polybase offers the ability to create statistics on tables, the same way that you would on normal tables.  There are a few rules about statistics:

  1. Stats are not auto-created.  You need to create all statistics manually.

  2. Stats are not auto-updated.  You will need to update all statistics manually, and currently, the only way you can do that is to drop and re-create the stats.

  3. When you create statistics, SQL Server pulls the data into a temp table, so if you have a billion-row table, you’d better have the tempdb space to pull that off.  To mitigate this, you can run stats on a sample of the data.

Round one did not end on a high note, so we’ll see what round two has to offer.

Connecting SQL Server To Hadoop Using Polybase

I have a post up on using Polybase to create an external table which points to Hadoop:

An interesting thing about FIELD_TERMINATOR is that it can be multi-character.  MSDN uses ~|~ as a potential delimiter.  The reason you’d look at a multi-character delimiter is that not all file formats handle quoted identifiers—for example, putting quotation marks around strings that have commas in them to indicate that commas inside quotation marks are punctuation marks rather than field separators—very well.  For example, the default Hive SerDe (Serializer and Deserializer) does not handle quoted identifiers; you can easily grab a different SerDe which does offer quoted identifiers and use it instead, or you can make your delimiter something which is guaranteed not to show up in the file.

You can also set some defaults such as date format, string format, and data compression codec you’re using, but we don’t need those here.  Read the MSDN doc above if you’re interested in digging into that a bit further.

It’s a bit of a read, but the end result is that we can retrieve data from a Hadoop cluster as though it were coming from a standard SQL Server table.  This is easily my favorite feature in SQL Server 2016.

Configuring Polybase

I have a blog post up on configuring Polybase:

Microsoft’s next recommendation is to make sure that predicate pushdown is enabled.  To do that, we’re going to go back to the Hadoop VM and grab our yarn.application.classpath from there.  To do that, cd to /etc/hadoop/conf/ and vi yarn-site.xml (or use whatever other text reader you want).  Copy the value for yarn.application.classpath, which should be a pretty long string.  Mine looks like:

1
<value>$HADOOP_CONF_DIR,/usr/hdp/current/hadoop-client/*,/usr/hdp/current/hadoop-client/lib/*,/usr/hdp/current/hadoop-hdfs-client/*,/usr/hdp/current/hadoop-hdfs-client/lib/*,/usr/hdp/current/hadoop-yarn-client/*,/usr/hdp/current/hadoop-yarn-client/lib/*</value>

Now that you have a copy of that value, go to your SQL Server installation directory (by default, C:\Program Files\Microsoft SQL Server\MSSQL13.MSSQLSERVER\MSSQL\Binn\Polybase\Hadoop\conf) and open up yarn-site.xml.  Paste the value into the corresponding yarn.application.classpath setting and you’re good to go.

This is part one of a series on using Polybase.

Polybase Row Size Limits

Manoj Pandey notes that Polybase has a row size limit:

With the error description its quiet evident that the External tables does not support row size more than 32768 bytes. But still I take a look online and found in Azure Documentation that this is a limitation right now with Polybase. The Azure document mentions:

Wide rows support is not supported yet, “If you are using Polybase to load your tables, define your tables so that the maximum possible row size, including the full length of variable length columns, does not exceed 32,767 bytes. While you can define a row with variable length data that can exceed this figure, and load rows with BCP, you will not be be able to use Polybase to load this data quite yet. Polybase support for wide rows will be added soon. Also, try to limit the size of your variable length columns for even better throughput for running queries.”

You can still use varchar(max) and nvarchar(max) for data types (unlike the Hive provider, which has a strict limit of 8000 characters for a single column) but can’t break that 32K mark.

Polybase

Ayman El-Ghazali talks Polybase:

HDFS is a distributed file system that works differently than what we’re used to in the Windows OS side of things; the general principle is to use cheap commodity hardware that replicates data in order to account for availability and to prevent loss of data. With that in mind, it makes a great use case to store a lot of data cheaply for archiving purposes or can be used to store large quantities of data that been to be processed in large quantities as well.

For more information please visit: https://msdn.microsoft.com/en-us/library/mt143171.aspx

Now if you want to try it out for yourself, make sure you install the PolyBase Engine (from the SQL Server setup) and feel free to try the modified code sample below.

Polybase is, without a doubt, my favorite SQL Server 2016 feature.  I am excited to put this through its paces in a production environment.

Configuring Polybase

David Benoit walks through how to configure a Polybase cluster:

When you get to the PolyBase configuration screen, you have the option to run this as a standalone instance, or to use the SQL Server as part of a scale-out group. Personally, I don’t think that anyone should ever choose the standalone instance option. You can always run a SQL Server configured for a scale-out group as a standalone instance, BUT you can’t change (at least not today) a SQL Server configured as a standalone PolyBase instance to run as part of a PolyBase cluster once you have completed the install.

This note sounds like the argument for clustering all SQL Server instances.

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