Database Management Systems · Storage and query processing

Query processing and optimization

How the database decides which order to run joins and filters in, to answer a query as cheaply as possible.

Also known as: query planner, join algorithms, query execution plans.

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Learn first

• SQL joins: Combining rows from two or more tables based on a matching column.
• Indexes and B-trees: A sorted, tree-shaped shortcut structure that lets the database find rows without scanning the whole table.

Leads to

• Database administration and performance tuning: The ongoing work of keeping a database backed up, monitored, and fast as real usage grows.

Related topics

• Index selectivity: How narrowly a condition filters the table, the reason a low-selectivity index gets ignored in favor of a full scan.
• Relational algebra: The formal, math-like operations SQL actually compiles down to under the hood.
• Database administration and performance tuning: The ongoing work of keeping a database backed up, monitored, and fast as real usage grows.