Diving Into Spark’s Cost-Based Optimizer

Ron Hu, et al, explain how Spark’s cost-based optimizer works:

At its core, Spark’s Catalyst optimizer is a general library for representing query plans as trees and sequentially applying a number of optimization rules to manipulate them. A majority of these optimization rules are based on heuristics, i.e., they only account for a query’s structure and ignore the properties of the data being processed, which severely limits their applicability. Let us demonstrate this with a simple example. Consider a query shown below that filters a table t1 of size 500GB and joins the output with another table t2of size 20GB. Spark implements this query using a hash join by choosing the smaller join relation as the build side (to build a hash table) and the larger relation as the probe side 1. Given that t2 is smaller than t1, Apache Spark 2.1 would choose the right side as the build side without factoring in the effect of the filter operator (which in this case filters out the majority of t1‘s records). Choosing the incorrect side as the build side often forces the system to give up on a fast hash join and turn to sort-merge join due to memory constraints.

Click through for a very interesting look at this query optimzier.

Related Posts

Databricks Runtime 5.2 Released

Nakul Jamadagni announces Databricks Runtime 5.2: Delta Time TravelTime Travel, released as an Experimental feature, adds the ability to query a snapshot of a table using a timestamp string or a version, using SQL syntax as well as DataFrameReader options for timestamp expressions.Sample codeSELECT count() FROM events TIMESTAMP AS OF timestamp_expressionSELECT count() FROM events VERSION AS OF version Time travel looks a bit like temporal tables in SQL Server.

Read More

Kafka And The Differing Aims Of Data Professionals

Kai Waehner argues that there is an impedence mismatch between data engineers, data scientists, and ML production engineers: Data scientists love Python, period. Therefore, the majority of machine learning/deep learning frameworks focus on Python APIs. Both the stablest and most cutting edge APIs, as well as the majority of examples and tutorials use Python APIs. […]

Read More

Categories

September 2017
MTWTFSS
« Aug Oct »
 123
45678910
11121314151617
18192021222324
252627282930