Advent of 2021, Day 14 – Spark SQL query hints and executions

This article is originally published at https://tomaztsql.wordpress.com

Series of Apache Spark posts:

• Dec 01: What is Apache Spark
• Dec 02: Installing Apache Spark
• Dec 03: Getting around CLI and WEB UI in Apache Spark
• Dec 04: Spark Architecture – Local and cluster mode
• Dec 05: Setting up Spark Cluster
• Dec 06: Setting up IDE
• Dec 07: Starting Spark with R and Python
• Dec 08: Creating RDD files
• Dec 09: RDD Operations
• Dec 10: Working with data frames
• Dec 11: Working with packages and spark DataFrames
• Dec 12: Spark SQL
• Dec 13: Spark SQL Bucketing and partitioning

Caching data in most cases will improve your query performance and execution. Most commonly used command for caching table in Spark SQL is by using in-memory columnar format with dataFrame.cache(). This will tell Spark SQL to scan only required columns and will automatically tune compression to minimize memory usage.

To remove table from cache, you can call the dataFrame.unpersist() function.

Configuring the in-memory caching using the setConf method on SparkSession or by running SET key=value commands using SQL. Some of the optimisation can be done by tuning the parameters with key and value of couple of selected:

spark.sql.inMemoryColumnarStorage.compressed
spark.sql.inMemoryColumnarStorage.batchSize
spark.sql.files.minPartitionNum
spark.sql.shuffle.partitions
spark.sql.sources.parallelPartitionDiscovery.parallelism

Spark SQL can also be optimized with couple of JOIN hints. These are:

BROADCAST
MERGE 
SHUFFLE_HASH and 
SHUFFLE_REPLICATE_NL

And all can be used with different languages; Scala, R, Python, SQL and Java.

R will look like:

Data1 <- sql("SELECT * FROM table1")
Data2 <- sql("SELECT * FROM table2")
head(join(Data1, hint(Data2, "broadcast"), Data1$key == Data2$key))

Python will look like:

spark.table("Data1").join(spark.table("Data2").hint("broadcast"), "key").show()

SQL will look like:

SELECT  BROADCAST(r),* FROM Data1 AS d JOIN Data2 AS s ON r.key = s.key
-- OR with Broadcastjoin
SELECT BROADCASTJOIN (r) FROM Data1 AS d JOIN Data2 AS s ON r.key = s.key

This hint instructs Spark to use the hinted strategy on specified relation when joining tables together. When BROADCASTJOIN hint is used on Data1 table with Data2 table and overrides the suggested setting of statistics from configuration spark.sql.autoBroadcastJoinThreshold.

Spark also prioritise the join strategy, and also when different JOIN strategies are used, Spark SQL will always prioritise them.

Repartitioning Spark SQL hints are good for performance tuning and reducing the number of outputed results (or files).

The “COALESCE” hint only has a partition number as a parameter.
The “REPARTITION” hint has a partition number, columns, or both/neither of them as parameters.
The “REPARTITION_BY_RANGE” hint must have column names and a partition number is optional.

These hints are only available in Spark SQL language. The syntax is as following

SELECT  COALESCE(3) * FROM Data1;
SELECT  REPARTITION(3) * FROM Data1;
SELECT REBALANCE * FROM Data1;

There are also some features worth looking at to handle better optimisation.

Tomorrow we will make a gentle introduction into Spark Streaming.

Compete set of code, documents, notebooks, and all of the materials will be available at the Github repository: https://github.com/tomaztk/Spark-for-data-engineers

Happy Spark Advent of 2021!

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