Read committed behavior in PostgreSQL - Step-by-Step Execution

Concept Flow - Read committed behavior

Start Transaction T1

↓

T1 reads data

↓

Start Transaction T2

↓

T2 modifies and commits data

↓

T1 reads data again

↓

T1 sees T2's committed changes

↓

T1 commits or rolls back

In Read Committed, each query sees only data committed before it started, so a transaction can see changes committed by others between its queries.

Execution Sample

PostgreSQL

BEGIN;
SELECT balance FROM accounts WHERE id=1;
-- T2 updates balance and commits
SELECT balance FROM accounts WHERE id=1;
COMMIT;

Transaction T1 reads a balance, then after T2 commits an update, T1 reads the updated balance.

Execution Table

Step	Transaction	Action	Data Read	Data State
1	T1	BEGIN transaction	N/A	Balance=100
2	T1	SELECT balance	100	Balance=100
3	T2	BEGIN transaction	N/A	Balance=100
4	T2	UPDATE balance=150	N/A	Balance=150 (uncommitted)
5	T2	COMMIT	N/A	Balance=150 (committed)
6	T1	SELECT balance	150	Balance=150 (committed)
7	T1	COMMIT	N/A	Balance=150

💡 T1 ends after seeing T2's committed update in its second read.

Variable Tracker

Variable	Start	After Step 2	After Step 5	After Step 6	Final
balance	100	100	150	150	150

Key Moments - 2 Insights

Why does T1 see the updated balance 150 in step 6 but not in step 2?

Because in Read Committed, each query sees only data committed before it starts. At step 2, T2's update was not committed yet, but by step 6, T2 committed, so T1 sees the new value.

Does T1 see uncommitted changes from T2 during its transaction?

No. T1 only sees committed data at the start of each query, so uncommitted changes by T2 are invisible to T1.

Visual Quiz - 3 Questions

Look at the execution table, what is the balance read by T1 at step 2?

A 150

B 100

C NULL

D Uncommitted value

💡 Hint

Check the 'Data Read' column at step 2 in the execution_table.

At which step does T1 see the updated balance of 150?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look for the step where T1 reads balance after T2 commits in the execution_table.

If T2 did not commit at step 5, what would T1 read at step 6?

A 100

B NULL

C 150

D Uncommitted 150

💡 Hint

Read Committed only shows committed data; uncommitted changes are invisible (see key_moments).

Concept Snapshot

Read Committed isolation means:
- Each query sees only data committed before it starts.
- Changes committed by other transactions between queries become visible.
- Uncommitted changes are never visible.
- Prevents dirty reads but allows non-repeatable reads.
- Default isolation level in PostgreSQL.

Full Transcript

This visual trace shows how Read Committed isolation works in PostgreSQL. Transaction T1 starts and reads a balance of 100. Meanwhile, T2 starts, updates the balance to 150, and commits. When T1 reads again, it sees the updated balance 150 because the change is now committed. This demonstrates that each query in Read Committed sees only committed data at its start, so T1 does not see uncommitted changes but can see committed changes made by others between its queries.