The Runtime Theory

topic / databases

Databases

Trace a query from SQL text to pages, indexes, and durable results.

Explore relational modeling, SQL, query planning, indexes, transactions, MVCC, write-ahead logging, recovery, replication, and storage engines.

1 article · 1 video · 1 trace · 1 request flow · 1 interview · 5 total

Articles

databasesSep 25, 2026

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.

Request Flows

request flow · 5 stagesSQL Becomes a Physical Query PlanFollow SQL through parsing and planning into scans, joins, buffers, visibility checks, and result rows.

Videos

databasesSep 25, 2026

How a Database Chooses a Query Plan

A planned visual lesson follows a SQL filter through planning estimates, scan alternatives, buffer pages, and observed rows.

Traces

trace 01 · systemTrace a PostgreSQL Indexed QueryFollow a selective SQL lookup through planning, a B-tree index, heap visibility checks, and the result rows.

Interviews

medium · t2When Is a Sequential Scan Better Than an Index Scan?A database prompt about selectivity, page access, planner estimates, visibility checks, and reading a real query plan.

Explore adjacent

backend engineering distributed systems performance engineering system design indexes and b trees transactions and mvcc query planning and execution

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