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🗄️ How Databases Work

Learn how indexes find rows, planners choose query strategies, transactions coordinate changes, and write-ahead logs make recovery possible.

lessons

~20 min

to learn

🤖 Technology

subject

Adults

level

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What you’ll learn

Finding RowsExplain how indexes and query planning reduce the work needed to find rows.Tables, indexes, B-trees, statistics, column order, and query plans shape the path from a query to its rows.
One Change at a TimeDescribe transactions, isolation, MVCC, locks, and deadlock handling.Transactions define reliable work while concurrency control manages what overlapping transactions can see and change.
Writing AheadExplain how write-ahead logging supports durable commits and log recycling.The database records changes before flushing data pages, creating an ordered recovery history.
Recovering After a CrashConnect checkpoints, replay, commit status, and durability settings to crash recovery.Recovery uses the log and its boundaries to restore committed work and define what a durability promise covers.

Questions this course answers

Why might a database choose a full table scan even when an index exists?

An index helps most for selective access. If many rows are needed, visiting the index and then the table can cost more than reading the table sequentially.

What does atomicity mean for a transaction?

Atomicity makes a defined unit of work all-or-nothing from the database's perspective.

Why must a WAL record be durable before its changed data page is written?

The log gives recovery a durable record of changes that may not yet have reached the table or index pages.

Grounded in trusted sources

https://www.postgresql.org/docs/current/indexes.html
https://www.postgresql.org/docs/current/transaction-iso.html
https://www.postgresql.org/docs/16/wal-intro.html
https://www.postgresql.org/docs/18/wal-configuration.html
https://dev.mysql.com/doc/refman/8.0/en/innodb-transaction-model.html
https://dev.mysql.com/doc/dev/mysql-server/latest/PAGE_INNODB_REDO_LOG.html

Every Wunder lesson is built from real, reputable sources — never invented.

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