Readers don't block writers
On This Lesson 4 topics
This lesson is planned. The outline below lists the topics it will cover. The full lesson has not been published yet.
In this lesson
explain the core promise of MVCC.
Planned topics
- MVCC (multi-version concurrency control) keeps several versions of a row so readers and writers do not wait for each other.
- A reader sees a consistent snapshot; a writer creates a new version.
- Writers still block other writers on the same row.
- The price: old versions pile up and must be cleaned (vacuum).
Draft outline details
Demonstration
two sessions: session A starts a transaction and selects a product; session B updates its price and commits; session A still sees the old price (under REPEATABLE READ) and was never blocked.
Check your understanding
Does a long SELECT block an UPDATE of the same row?
Show answer
No.
Practice
Apply this lesson with the module's lab, exercises and quiz.
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