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SELECT FOR UPDATE

Lock rows for update.

SELECT FOR UPDATE

SELECT FOR UPDATE reads and locks a row. Others cannot change it until the transaction ends.

BEGIN;
SELECT * FROM accounts WHERE id = 1 FOR UPDATE;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
COMMIT;

A safe withdrawal.

SELECT FOR UPDATE — details

FOR UPDATE locks the read row. The lock stays until COMMIT or ROLLBACK.

For example, in a stock check you write SELECT stock FROM products WHERE id = 1 FOR UPDATE; nobody else can change that row until the transaction ends.

Practice SELECT FOR UPDATE

Lock and update a row with SELECT ... FOR UPDATE.

Starter code

query.sql
-- safe update
SELECT * FROM accounts WHERE id = 1

Sample solution

query.sql
BEGIN;
SELECT * FROM accounts WHERE id = 1 FOR UPDATE;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
COMMIT;

Expected output

id  balance
1   400

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 When does a FOR UPDATE lock end?

  • SELECT'te
  • COMMIT'te Correct answer
  • Never
  • UPDATE'te

Explanation: A FOR UPDATE lock ends at COMMIT.

2 Add FOR UPDATE

Question 
SELECT * FROM accounts WHERE id = 1 ____
Answer 
SELECT * FROM accounts WHERE id = 1 FOR UPDATE

Explanation: FOR UPDATE is the right answer.

3 SELECT FOR UPDATE: match the terms

Term Meaning
FOR UPDATE lock + read
BEGIN start
COMMIT release lock
safe consistent

All lessons in Advanced

1 CTE

  1. What is a CTE?
  2. Using WITH
  3. Multiple CTEs
  4. CTE vs subquery
  5. CTE with JOIN
  6. Summary - CTE

2 Recursive CTE

  1. What is a recursive CTE?
  2. Anchor and recursive parts
  3. Walk a hierarchy
  4. Tracking depth
  5. Stopping recursion
  6. Summary - Recursive CTE

3 Window functions

  1. What is a window function?
  2. ROW_NUMBER
  3. RANK and DENSE_RANK
  4. LAG and LEAD
  5. PARTITION BY
  6. Summary - Window functions

4 Project: Analytics report

  1. Analytics design
  2. CTE summary
  3. Top users with ROW_NUMBER
  4. Growth with LAG
  5. Final report

5 Pivot and unpivot

  1. What is pivot?
  2. Pivot with CASE
  3. FILTER clause
  4. Unpivot
  5. Dynamic pivot
  6. Summary - Pivot

6 Materialized view

  1. What is a materialized view?
  2. CREATE
  3. REFRESH
  4. View vs materialized view
  5. Index on materialized views
  6. Summary - Materialized view

7 Stored procedure

  1. What is a procedure?
  2. CREATE PROCEDURE
  3. Parameters
  4. CALL
  5. Control flow
  6. Summary - Stored procedure

8 Function

  1. What is a function?
  2. CREATE FUNCTION
  3. RETURN
  4. Calling a function
  5. Function vs procedure
  6. Summary - Function

9 Trigger

  1. What is a trigger?
  2. BEFORE and AFTER
  3. INSERT trigger
  4. UPDATE trigger
  5. Write an audit trigger
  6. Summary - Trigger

10 Transaction isolation

  1. What is isolation?
  2. READ UNCOMMITTED
  3. READ COMMITTED
  4. REPEATABLE READ
  5. SERIALIZABLE
  6. Summary - Isolation

11 Locking

  1. What is a lock?
  2. Shared and exclusive locks
  3. Deadlock
  4. SELECT FOR UPDATE
  5. Lock timeout
  6. Summary - Locking

12 Query optimization

  1. What is EXPLAIN?
  2. Seq scan vs index scan
  3. Cost
  4. Sargable
  5. N+1 problem
  6. Summary - Optimization

13 Partitioning

  1. What is partitioning?
  2. Range partition
  3. List partition
  4. Hash partition
  5. Partition pruning
  6. Summary - Partitioning

14 Advanced final

  1. Hierarchy report
  2. Window analysis
  3. Procedure and trigger
  4. Optimization