Sujith Jay Nair Thinking Aloud

Prefer Unions over Or in Spark Joins

A common anti-pattern in Spark workloads is the use of an or operator as part of ajoin. An example of this goes as follows:

val resultDF = dataframe
 .join(anotherDF, $"cID" === $"customerID" || $"cID" === $"contactID",
   "left")

This looks straight-forward. The use of an or within the join makes its semantics easy to understand. However, we should be aware of the pitfalls of such an approach.

The declarative SQL above is resolved within Spark into a physical plan which determines how this particular query gets executed. To view the query plan for the computation, we could do:

resultDF.explain()

/* pass true if you are interested in the logical plan of the query as well */
resultDF.explain(true)
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Integration of Large-Scale Data Processing Systems and Traditional Parallel Database Technology

Integration of Large-Scale Data Processing Systems and Traditional Parallel Database Technology Abouzied, A., Abadi, D.J, Bajda-Pawlikowski, K., Silberschatz, A. (2019, August). Proceedings of the VLDB Vol. 12 (12).

HadoopDB was a prototype built in 2009 as a hybrid SQL system with the features from Hadoop MapReduce framework and parallel database management systems (Greenplum, Vertica, etc). This paper revisits the design choices for HadoopDB, and investigates its legacy in existing data systems. I felt it is a great review paper for the state of modern data analysis systems.

MapReduce is the most famous example in a class of systems which partition large amounts of data over multitude of machines, and provide a straightforward language in which to express complex transformations and analyses. The key feature of these systems is how they abstract out fault-tolerance and partitioning from the user.

MapReduce, along with other large-scale data processing systems such as Microsoft’s Dryad/LINQ project, were originally designed for processing unstructured data.

The success of these systems in processing unstructured data led to a natural desire to also use them for processing structured data. However, the final result was a major step backward relative to the decades of research in parallel database systems that provide similar capabilities of parallel query processing over structured data. 1

The MapReduce model of Map -> Shuffle -> Reduce/Aggregate -> Materialize is inefficient for parallel structured query processing.

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Datomic with Rich Hickey

This talk is an introduction to Datomic, by its creator Rich Hickey. My notes on this talk are linked below: