Question 1

EXPLAIN ANALYZE shows Seq Scan on a 10M-row table — how do you diagnose and fix it?

Accepted Answer

**Target Audience:** Developers who have just seen `Seq Scan` in a plan and want a repeatable way to decide whether it is a problem, and candidates preparing for the "walk me through it" version of this question.
**Difficulty:** Medium
**Engines:** PostgreSQL 16.15 (all plans on this page were produced on it) and SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable example on this page uses). The diagnosis applies to MySQL and SQL Server too (`EXPLAIN` / actual execution plans); their plan word

Question 2

Seq Scan vs Index Scan vs Index-Only Scan — when does Postgres choose each?

Accepted Answer

**Target Audience:** Developers who have added an index and been surprised by the plan, and candidates who need to explain the planner choice with real thresholds instead of "it depends".
**Difficulty:** Medium
**Engines:** PostgreSQL 16.15 for every plan (a deterministic 100000-row table generated inside each example), plus SQLite 3.45.0 through sql.js 1.10.2 for the runnable example. The exact switch points are from this data and these settings; other data will switch at other points.

> **How

SQLDECODED.

Medium

EXPLAIN ANALYZE shows Seq Scan on a 10M-row table — how do you diagnose and fix it?

Seq Scan vs Index Scan vs Index-Only Scan — when does Postgres choose each?