coalesce and broadcast join

Question

HI,

what exactly happen between coalesce and broadcast join in backend on databricks level

Answer 1

The coalesce function is used to reduce the number of partitions in a DataFrame. This is especially useful when you want to decrease the number of output files or manage the distribution of data across fewer nodes after filtering a large dataset down to a smaller one. When you use coalesce, Spark merges existing partitions into fewer partitions to reduce the shuffle of data across the nodes, which can be beneficial in terms of performance when the amount of data is reduced significantly.

Imagine you have a DataFrame with 100 partitions after performing a large filter operation, and only 10% of the data remains. You can use coalesce to reduce the number of partitions, like this:

filtered_df = df.filter("some_condition")

coalesced_df = filtered_df.coalesce(10)  # Reducing the number of partitions to 10

This does not shuffle all the data across nodes but combines existing partitions to reduce overhead.

In the other hand, broadcast join is a type of join operation used in Spark where the smaller of two DataFrames is sent to every node in the cluster so that it resides in the memory of each node. This eliminates the need for shuffling the smaller DataFrame when performing the join, which can greatly improve performance for large-scale join operations.

Suppose you have a large DataFrame transactions and a smaller DataFrame users. You want to join them on user ID without causing a huge shuffle of the transactions DataFrame across the cluster.

from pyspark.sql.functions import broadcast

# Assume transactions and users are DataFrames

joined_df = transactions.join(broadcast(users), transactions.user_id == users.id)

In this scenario, the entire users DataFrame is broadcasted to all nodes in the cluster. This means every node has a full copy of the users DataFrame, allowing each node to perform the join locally without needing to shuffle the transactions DataFrame.

Answer 2

Hi Vineet S,

Thank you for posting query in Microsoft Q&A Platform.

In Databricks, a coalesce operation is used to reduce the number of partitions in a DataFrame or RDD. The coalesce operation combines adjacent partitions into a single partition, which can improve the performance of subsequent operations by reducing the amount of data shuffling required.

A broadcast join is a type of join operation in which one of the tables is small enough to fit in memory, and is broadcast to all the worker nodes in the cluster. This allows the join operation to be performed locally on each worker node, rather than requiring a shuffle operation to redistribute the data.

When a coalesce operation is performed before a broadcast join, it can reduce the number of partitions in the larger table, which can improve the performance of the join operation. This is because the smaller table can be broadcast to each worker node more efficiently when there are fewer partitions in the larger table.

However, it is important to note that the optimal number of partitions for a DataFrame or RDD depends on a number of factors, including the size of the data, the available memory, and the number of worker nodes in the cluster. In some cases, reducing the number of partitions too much can actually decrease performance by reducing parallelism and increasing the amount of data shuffling required.

Therefore, it is important to carefully consider the partitioning strategy when using coalesce and broadcast join operations in Databricks, and to experiment with different partitioning strategies to find the optimal configuration for your specific use case.

Hope this helps. Please let me know if any further queries.

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