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Database Statistics: How Query Planners Build Histograms

Why does the same query use a different index today than yesterday? Deep dive into PostgreSQL statistics, histograms, and query plan costs.

Sachin Sarawgi•April 20, 2026•1 min read•1 minute lesson
#postgresql#query-planner#statistics#performance
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Database Query Planner Statistics

Your database doesn't "guess" how to run a query. It calculates the Cost of every possible execution path. It does this using Histograms stored in the system catalog.

1. How Histograms Work

Postgres samples your data to build a frequency distribution. If your is skewed (e.g., one has 90% of rows), the planner uses the histogram to realize that an Index Scan is actually slower than a Sequential Scan.

2. The 'ANALYZE' Trap

If you run too rarely, your stats are stale. The planner will think a query returns 10 rows when it actually returns 10 million. Result: Slow query, high memory usage, system crash.

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Sachin Sarawgi

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Sachin Sarawgi

Engineering Manager and backend engineer with 10+ years building distributed systems across fintech, enterprise SaaS, and startups. CodeSprintPro is where I write practical guides on system design, Java, Kafka, databases, AI infrastructure, and production reliability.

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