mediumSystem Design

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

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

A `Seq Scan` on a large table in `EXPLAIN ANALYZE` means no usable index or the planner thinks seq is cheaper. Check: 1) index exists, 2) `WHERE` matches index (no `LOWER(col)`), 3) selectivity, 4) stats freshness (`ANALYZE`), 5) parameter sniffing. Seq Scan vs Index Scan vs Index-Only Scan — planner chooses by selectivity + visibility map Seq Scan reads every heap page cheap if >5-10% rows no index overhead good for small tables <rect x="290" y="35"

963 views

Understand the problem

Trace sequential scans to missing indexes, function-wrapped columns, or stale statistics and verify the fix.

explainseq-scanindex-scan

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

Study the solution

A Seq Scan on a large table in EXPLAIN ANALYZE means no usable index or the planner thinks seq is cheaper. Check: 1) index exists, 2) WHERE matches index (no LOWER(col)), 3) selectivity, 4) stats freshness (ANALYZE), 5) parameter sniffing.

Solution ready — 2 min read

Classified // press E to declassify

Join the discussion

Discussion (0)

No responses yet. Be the first to share what you think.

Transmission complete // awaiting log

KEEP THE
STREAK ALIVE.

Dossier 25 of 70 decoded in the SQL track. One more won't hurt.

Back to track

Next up→

Why might EXISTS outperform a JOIN for existence checks?

Continue