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Query Execution Plan

A query plan is a tree of physical operators — scans at the leaves, joins/sorts/aggregates above — through which rows flow upward, typically pulled one batch at a time by each parent. The same SQL can compile to many different plans; the plan, not the SQL text, determines performance.

Questions this Concept answers

Why does knowing that a plan exists change how you approach a slow query?

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Reading a PostgreSQL EXPLAIN Plan to Spot Sequential Scans and Index-Only Scan Opportunities

EXPLAIN in PostgreSQL prints the plan the query planner chose before any row is read, letting you tell a sequential scan (reading every page of the table) apart from an index scan or an index-only sc…