Frequency Counting in Data Pipelines

Concepts: pyNullFrequency, pySkewDetection, pyHotKeyMonitor

Here is where interview prep becomes job prep. Frequency counting is not a LeetCode trick. It is the mechanism behind data quality checks, skew detection, and join optimization in every data system you will ever build. The Python patterns you learn in this lesson appear verbatim in production data pipelines. When you can talk about these connections in the interview, you go from 'knows algorithms' to 'has built real systems.' Counting NULLs per Column One of the most common data quality checks: for each column in a dataset, what fraction of rows is NULL? This is frequency counting applied to presence/absence. Build a Counter of (column, is_null) pairs, or more efficiently, count NULLs per column directly. Finding Skewed Keys in a Join When you are joining two large tables in Spark or any d