mediumpostgresql-docs#query-plans#indexes#measurement

Read a PostgreSQL Query Plan

Use EXPLAIN to predict scans and joins, then compare row estimates with execution observations on a safe SELECT query.

The Runtime Theory TeamSeptember 25, 20261 min read

Solve it

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Sample cases

inEXPLAIN SELECT * FROM tenk1 WHERE unique1 < 10

outIdentify scan node, estimated rows, and why selectivity matters

inCompare EXPLAIN with EXPLAIN ANALYZE for a SELECT

outExplain estimated vs actual work and note instrumentation overhead

Choose a table with a selective predicate and inspect the plan with EXPLAIN. Before running EXPLAIN ANALYZE, confirm the statement is a read-only query against data you may safely inspect; ANALYZE executes its statement.

For each plan node, write down its input, output, estimated rows, and role. Then compare estimated and actual row counts, look for filters applied after a scan, and explain whether an index scan or sequential scan is reasonable for the expected number of rows.

Do not treat planner cost units as milliseconds. Avoid adding an index until the plan and workload show what work is expensive. The linked PostgreSQL guide includes examples and caveats.

One dispatch a week

The trace behind each problem, the tradeoff that explains it, and one technical dispatch per week — no noise.

One technical dispatch per week. No noise.

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