How a Database Chooses a Query Plan
The lesson begins with a query for one user and draws the alternative sequential-scan and B-tree-index plans. It shows how table statistics inform row estimates, why an index scan can lose when many rows qualify, and how plan nodes feed rows into joins or aggregates.
The demonstration compares estimated and actual rows from EXPLAIN ANALYZE, including a note that the command executes the statement and adds instrumentation overhead. It closes with a practical debugging sequence: inspect the plan, compare estimates, verify indexes, and measure the whole request. Companion material: query-planning article, query trace, and PostgreSQL EXPLAIN.
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A Database Plans a Query Before It Reads the Rows
Trace SQL through parsing, logical rewrites, cost-based planning, physical operators, indexes, and the buffer pool.
How a B-Tree Index Narrows a Database Search
A database index is an auxiliary structure that helps find rows without scanning every table page.
Reading a Query Plan as an Execution Hypothesis
A query planner estimates alternative ways to produce a result and chooses one using statistics and cost assumptions.
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