MVCC mental model in PostgreSQL - Step-by-Step Execution

Concept Flow - MVCC mental model in PostgreSQL

Transaction Start

↓

Read Snapshot of Data

↓

Perform Reads/Writes

↓

Create New Versions for Writes

↓

Commit or Rollback

↓

Old Versions Visible to Other Transactions Until Cleanup

This flow shows how a transaction in PostgreSQL reads a snapshot of data, creates new versions for writes, and commits or rolls back, while old versions remain visible to other transactions until cleaned up.

Execution Sample

PostgreSQL

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

A transaction reads an account balance, updates it by adding 100, and commits the change.

Execution Table

Step	Action	Data Version Read	New Version Created	Visibility to Other Transactions
1	BEGIN transaction	Snapshot of current committed data	No	No changes yet
2	SELECT balance WHERE id=1	Version visible at transaction start	No	No changes yet
3	UPDATE balance + 100 WHERE id=1	Reads old version	Creates new version with updated balance	New version visible only to this transaction
4	COMMIT transaction	N/A	New version becomes committed	New version visible to all new transactions
5	Other transactions still see old version until cleanup	Old version remains until vacuum	No	Old version visible to other active transactions

💡 Transaction commits, new version becomes visible; old versions remain until cleanup.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
balance_version	v1 (old)	v1 (read)	v2 (new created)	v2 (committed)	v2 (final visible)
transaction_state	idle	active	active	committed	committed

Key Moments - 3 Insights

Why does the transaction read the old version of data even after an update?

Because PostgreSQL uses MVCC, the update creates a new version invisible to other transactions until commit. The current transaction sees its own new version, but others still see the old version until cleanup. See execution_table step 3.

When do other transactions see the updated data?

Other transactions see the new version only after the committing transaction finishes and commits. Before that, they see the old version. See execution_table step 4.

What happens to old versions after commit?

Old versions remain in the database until a cleanup process (vacuum) removes them. This allows other transactions that started earlier to still see consistent data. See execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the new data version created?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Check the 'New Version Created' column in execution_table rows.

According to variable_tracker, what is the transaction_state after the COMMIT?

A idle

B committed

C active

D rolled back

💡 Hint

Look at the 'transaction_state' row after Step 4 in variable_tracker.

When do other transactions stop seeing the old data version?

A Immediately after UPDATE

B After BEGIN

C After COMMIT and cleanup

D Before COMMIT

💡 Hint

Refer to execution_table steps 4 and 5 about visibility and cleanup.

Concept Snapshot

MVCC in PostgreSQL:
- Each transaction reads a snapshot of data.
- Writes create new versions, not overwrite.
- New versions visible only after commit.
- Old versions kept for other transactions until vacuum.
- Ensures consistent reads without locking.

Full Transcript

In PostgreSQL, MVCC means each transaction works with a snapshot of the database. When a transaction starts, it sees the data as it was at that moment. If it updates data, PostgreSQL creates a new version of that data row instead of changing the old one. This new version is only visible to the transaction that made the change until it commits. Other transactions continue to see the old version until the first transaction commits and the old versions are cleaned up later. This system allows many transactions to work at the same time without blocking each other, keeping data consistent and safe.