Granularity Design

Choosing the Right Summary Level The granularity of your summary table determines both its size and its usefulness. A daily summary has 365 rows per year. An hourly summary has 8,760. A per-minute summary has 525,600. The finer the grain, the more flexible the queries, but the larger the table and the more it costs to refresh. The key question: what is the finest grain that any dashboard or report needs? If no dashboard filters below daily, a daily summary is sufficient. If the operations team needs hourly metrics, you need hourly granularity. Do not build minute-level summaries unless someone actually queries by minute. Multiple Summary Levels Production warehouses typically maintain multiple summary tables at different granularities. A daily summary for operational dashboards. A monthly

About This Interactive Section

This section is part of the Pre-Aggregation lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live PostgreSQL database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

How DataDriven Lessons Work

DataDriven combines four interview rounds (SQL, Python, Data Modeling, Pipeline Architecture) with adaptive difficulty and spaced repetition. Easy problems get harder as you improve. Weak concepts resurface until you master them. Your readiness score tracks progress across every topic interviewers test. Every lesson section ends with problems you solve by writing and running real code, not by picking multiple-choice answers.