As an addendum to my year-end review of machine learning and deep learning, I offer this survey of SQL engines. SQL is the most widely used language for data science according to O’Reilly’s 2016 Data Science Salary Survey. Most projects require at least some SQL operations, and many need nothing but SQL.
This review covers six open source leaders: Hive, Impala, Spark SQL, Drill, HAWQ, and Presto; plus, for completeness, Calcite, Kylin, Phoenix, Tajo, and Trafodion. Omitted: two commercial options, Oracle Big Data SQL and IBM Big SQL, which IBM has not yet rebranded as “Watson SQL.”
(A reader asks: What about Druid? My response: erm. On inspection, I agree that Druid belongs in this category, so check it out.)
I use the term ‘SQL Engine’ loosely. Hive, for example, is not an engine; it’s a framework that uses the MapReduce, Tez, or Spark engines to run queries. And it doesn’t run SQL; it runs HiveQL, an SQL-like language that closely approximates SQL. ‘SQL-in-Hadoop’ is also inapt; while Hive and Impala work primarily with Hadoop, Spark, Drill, HAWQ, and Presto also work with a wide variety of other data storage systems.
Unlike relational databases, SQL engines operate independently of the data storage system. In contrast, relational databases bundle the query engine and storage into a single tightly coupled system, which permits certain types of optimization. Uncoupling them, on the other hand, provides greater flexibility, though at the potential loss of performance.
Figure 1, below, shows the relative popularity of the leading SQL engines according to DB-Engines, a website maintained by the Austrian consultancy Solid IT. DB-engines computes a monthly popularity score for more than 200 database systems. The score reflects search engine queries; mentions in online discussions; job offers; mentions in professional profiles, and tweets.
Although Impala, Spark SQL, Drill, Hawq, and Presto consistently beat Hive on measures such as runtime performance, concurrency, and throughput, Hive remains the most popular (at least by the DB-Engines metric). There are three reasons why that is so:
— Hive is the default option for SQL in Hadoop, supported in every distribution. The others align with specific vendors and cater to niche users.
— Hive has closed the performance gap to the other engines. Most of the Hive alternatives launched in 2012 when analysts would rather kill themselves than wait for a Hive query to finish. But while Impala, Spark, Drill, et.al. ran away like rabbits back then, Hive just kept chugging along, tortoise-like, with incremental improvements. Today, while Hive is not the fastest choice, it’s a lot better than it was five years ago.
— While bleeding-edge speed is cool, most organizations know that the world does not end if a junior marketing manager has to wait ten seconds to find out if the chicken wings outperformed the buffalo burgers in the Duxbury restaurant last Tuesday.
As you can see in Figure 2, below, the top SQL engines compete well for user interest compared to leading commercial data warehouse appliances.
The best measure of health for an open source project is the size of its active developer community. Hive and Presto have the largest base of contributors, as shown in Figure 3, below. (Data for Spark SQL is unavailable.)
In 2016, Cloudera, Hortonworks, Kognitio, and Teradata waded into the Battle of the Benchmarks Tony Baer summarizes. I’m sure that you will be shocked to learn that the vendor’s preferred SQL engine outperformed the others in each of these studies, which begs the question: are benchmarks bullshit?
AtScale‘s biannual benchmark is not BS. AtScale, a BI startup, markets software that brokers between BI front ends and SQL backends. The company’s software is engine-neutral — it seeks to run on as many as possible — and its broad experience in BI gives the testing a real-world flavor.
AtScale’s key findings from its most recent round, which included Hive, Impala, Spark SQL, and Presto:
— All four engines successfully ran AtScale’s BI benchmark queries.
— Each engine has its own performance “sweet spot” depending on data volume, query complexity, and concurrent users.
– Impala and Spark SQL outperform the others in queries against small data sets
– On large data sets, Impala and Spark SQL handle complex joins better than the others
– Impala and Presto demonstrate the best results in concurrency tests
— All engines showed 2X-4X performance gains in the six months since AtScale’s previous benchmark.
Let’s dive into the individual projects.
Apache Hive was the first SQL framework in the Hadoop ecosystem. Engineers at Facebook introduced Hive in 2007 and donated the code to the Apache Software Foundation in 2008; in September 2010, Hive graduated to top-level Apache project status. Every major player in the Hadoop ecosystem distributes and supports Hive, including Cloudera, MapR, Hortonworks, and IBM. Amazon Web Services offers a modified version of Hive as a cloud service in Elastic MapReduce (EMR).
Early releases of Hive used MapReduce to run queries. Complex queries required multiple passes through the data, which impaired performance. As a result, Hive was not suitable for interactive analysis. Led by Hortonworks, the Stinger initiative markedly enhanced Hive’s performance, notably through the use of Apache Tez, an application framework that delivers streamlined MapReduce code. Tez and ORCfile, a new storage format, produced a significant speedup for Hive queries.
More than 100 individuals contributed to Hive in 2016. The team announced Hive 2.0 in February and Hive 2.1 in June. Hive 2.0 includes improvements to several improvements to Hive-on-Spark, plus performance, usability, supportability and stability enhancements. Hive 2.1 includes Hive LLAP (“Live Long and Process”), which combines persistent query servers and optimized in-memory caching for high performance. The team claims a 25X speedup.
In September, the Hivemall project entered the Apache Incubator, as I noted in Part Two of my machine learning year-end roundup. Originally developed by Treasure Data and donated to the Apache Software Foundation, Hivemall is a scalable machine learning library implemented as a collection of Hive UDFs designed to run in Hive, Pig or Spark SQL with MapReduce, Tez or Spark. The team plans an initial release in Q1 2017.
Cloudera launched Impala, an open source MPP SQL engine, in 2012, as a high-performance alternative to Hive. Impala works with HDFS and HBase, and it leverages Hive metadata; however, it bypasses MapReduce to run queries. Mike Olson, Cloudera’s Chief Strategy Officer,
Mike Olson, Cloudera’s Chief Strategy Officer, argued in late 2013 that Hive’s architecture was fundamentally flawed. In Olson’s view, developers could only deliver high-performance SQL with a whole new approach, exemplified by Impala. In 2014 Cloudera released a series of benchmarks in January, May, and September. In these tests, Impala showed progressive improvement in query runtime, and significantly outperformed Hive on Tez, Spark SQL, and Presto. In addition to running fast, Impala performed particularly well in concurrency, throughput, and scalability.
In 2015, Cloudera donated Impala to the Apache Software Foundation, where it entered the Apache Incubator program. Cloudera, MapR, Oracle and Amazon Web Services distribute Impala; Cloudera, MapR, and Oracle provide commercial build and installation support.
Impala made steady progress in the Apache Incubator in 2016. The team cleaned up the code, ported it to Apache infrastructure and delivered Release 2.7.0, its first Apache release in October. The new version includes performance and scalability improvements, as well as some other minor enhancements.
In September, Cloudera published results of a study that compared Impala to Amazon Web Services’ Redshift columnar database. The report is interesting reading, though subject to the usual caveats about vendor benchmarks.
Spark SQL is a Spark component for structured data processing. The Apache Spark team launched Spark SQL in 2014 and absorbed Shark, an early Hive-on-Spark project. It quickly became the most widely used Spark module.
Spark SQL users can run SQL queries, read data from Hive, or use it as means to create Spark Datasets and DataFrames. (Datasets are distributed collections of data; DataFrames are Datasets organized into named columns.) The Spark SQL interface provides Spark with information about the structure of the data and operations to be performed; Spark’s Catalyst optimizer uses this information to construct an efficient query.
In 2015, Spark’s machine learning developers introduced the ML API, a package that leveraged Spark DataFrames instead of the lower-level Spark RDD API. This approach proved to be attractive and fruitful; in 2016, with Release 2.0, the Spark team placed the RDD-based API in maintenance mode. The DataFrames API is now the primary interface for Spark machine learning.
Also in 2016, the team released Structured Streaming, in an Alpha release as of Spark 2.1.0. Structured Streaming is a stream processing engine built on Spark SQL. Users can query streaming data sources in the same manner as static sources, and they can combine streaming and static sources in a single query. Spark SQL runs the query continuously and updates results as streaming data arrives. Structured Streaming delivers exactly-once fault-tolerance guarantees through checkpointing and Write Ahead Logs.
In 2012, a group led by MapR, one of the leading Hadoop distributors, proposed to build an open-source version of Google’s Dremel, a distributed system for interactive ad-hoc analysis. They named the project Apache Drill. Drill languished in the Apache Incubator for more than two years, finally graduating in late 2014. The team delivered its 1.0 release in 2015.
MapR distributes and supports Apache Drill.
More than 50 individuals contributed to Drill in 2016. The team delivered five dot releases in 2016. Key enhancements include:
- Web authentication
- Support for the Apache Kudu columnar database
- Support for HBase 1.x
- Dynamic UDF support
Pivotal Software introduced HAWQ as a commercially licensed high-performance SQL engine in 2012 and attempted to market it with minimal success. Changing strategy, Pivotal donated the project to Apache in June 2015, and it entered the Apache Incubator program in September 2015.
Fifteen months later, HAWQ remains in the Incubator. The team released HAWQ 184.108.40.206 in December, with a load of bug fixes. I suspect the project will graduate in 2017.
One small point in HAWQ’s favor is its support for Apache MADlib, the machine-learning-in-SQL project that is also still in the Incubator. The combination of HAWQ and MADlib should be a nice consolation to the folks who bought Greenplum and wonder what the hell happened.
Facebook engineers initiated the Presto project in 2012 as a fast interactive alternative to Hive. Rolled out in 2013, the software successfully supported more than a thousand Facebook users and more than 30,000 queries per day on petabytes of data. Facebook released Presto to open source in 2013.
Presto supports ANSI SQL queries across a range of data sources, including Hive, Cassandra, relational databases or proprietary file systems (such as Amazon Web Services’ S3.) Presto queries can federate data from multiple sources. Users can submit queries from C, Java, Node.js, PHP, Python, R and Ruby.
Airpal, a web-based query tool developed by Airbnb, offers users the ability to submit queries to Presto through a browser. Qubole provides a managed service for Presto. AWS delivers a Presto service on EMR.
In June 2015, Teradata announced plans to develop and support the project. Under an announced three-phase program, Teradata proposed to integrate Presto into the Hadoop ecosystem, enable operation under YARN and enhance connectivity through ODBC and JDBC. Teradata offers its own distribution of Presto, complete with a data sheet. In June, Teradata announced the certification of Information Builders, Looker, Qlik, Tableau, and ZoomData, with MicroStrategy and Microsoft Power BI on the way.
Presto is a very active project, with a vast and vibrant contributor community. The team cranks out releases faster than Miki Sudo eats hot dogs — I count 42 releases in 2016. Teradata hasn’t bothered to summarize what’s new, and I don’t plan to sift through 42 sets of release notes, so let’s just say it’s better.
Other Apache Projects
There are five other SQL-ish projects in the Apache ecosystem.
Apache Calcite is an open source framework for building databases. It includes:
— A SQL parser, validator and JDBC driver
— Query optimization tools, including a relational algebra API, rule-based planner, and a cost-based query optimizer.
Apache Hive uses Calcite for cost-based query optimization, while Apache Drill and Apache Kylin use the SQL parser.
The Calcite team pushed out five releases in 2016, with bug fixes and new adapters for Cassandra, Druid, and Elasticsearch.
Apache Kylin is an OLAP engine with a SQL interface. Developed by eBay and donated to Apache, Kylin graduated to top-level status in 2015.
A startup named Kyligence launched in 2016; it offers commercial support and a data warehousing product called KAP, FWIW. While the company has no funding listed in Crunchbase, a source tells me that it has strong backing and a large office in Shanghai.
Apache Phoenix is a SQL framework that runs on HBase and bypasses MapReduce. Salesforce developed the software and donated it to Apache in 2013. The project graduated to top-level status in May 2014. Hortonworks includes Phoenix in the Hortonworks Data Platform. Since the leading SQL engines all work with HBase, it’s not clear why we need Phoenix.
Apache Tajo is a fast SQL data warehousing framework introduced in 2011 by Gruter, a Big Data infrastructure company, and donated to Apache in 2013. Tajo graduated to top level status in 2014. The project has attracted little interest from prospective users and contributors outside of Gruter’s primary market in South Korea. Other than a brief mention by Gartner’s Nick Heudecker, the project isn’t on anyone’s dashboard.
Apache Trafodion is another SQL-on-HBase project, conceived by HP Labs, which tells you pretty much all you need to know. HP launched Trafodion in June 2014, a month after Apache Phoenix graduated to production. Six months later, it dawned on HP executives that there might be limited commercial potential for another SQL-on-HBase engine — I can see the facepalms — so they donated the project to Apache, where it entered the Incubator in May 2015.
Trafodion promises to be a transactional database if it ever gets out of incubation. Unfortunately, there are lots of options in that space, and the only competitive benefit the development team can articulate seems to be “it’s open source, so it’s cheap.”