Deadlock detection and prevention in PostgreSQL - Step-by-Step Execution

Concept Flow - Deadlock detection and prevention

Transaction A requests Lock on Resource 1

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Transaction B requests Lock on Resource 2

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Transaction A requests Lock on Resource 2

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Transaction B requests Lock on Resource 1

↓

Deadlock detected by system

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System chooses a victim transaction to rollback

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Victim transaction rolled back, locks released

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Other transaction proceeds

↓

Deadlock resolved

Transactions request locks on resources; if each waits for the other, a deadlock occurs. The system detects this cycle and aborts one transaction to resolve it.

Execution Sample

PostgreSQL

-- Session A:
BEGIN;
LOCK TABLE accounts IN ACCESS EXCLUSIVE MODE;
-- A then tries: LOCK TABLE orders IN ACCESS EXCLUSIVE MODE; (will wait)

-- Session B:
BEGIN;
LOCK TABLE orders IN ACCESS EXCLUSIVE MODE;
-- B then tries: LOCK TABLE accounts IN ACCESS EXCLUSIVE MODE; (will wait)

-- Deadlock detected and resolved

Two transactions acquire locks on different tables (accounts and orders) first, then request each other's table in exclusive mode, causing a deadlock cycle that PostgreSQL detects and resolves by aborting one.

Execution Table

Step	Transaction	Action	Lock Requested	Lock Granted	Waits For	Deadlock Detected	Result
1	A	Begin transaction	None	None	None	No	Transaction A started
2	A	Lock accounts table	Access Exclusive	Granted	None	No	Lock on accounts acquired by A
3	B	Begin transaction	None	None	None	No	Transaction B started
4	B	Lock orders table	Access Exclusive	Granted	None	No	Lock on orders acquired by B
5	A	Request lock on orders table	Access Exclusive	Waiting	Held by B	No	A waits for B
6	B	Request lock on accounts table	Access Exclusive	Waiting	Held by A	Yes	B waits for A; deadlock detected
7	System	Choose victim	N/A	N/A	N/A	Yes	Transaction B aborted
8	B	Rollback	N/A	N/A	N/A	No	Locks released by B
9	A	Proceed	N/A	Granted	None	No	A acquires orders lock and continues
10	A	Commit	N/A	Released	None	No	Transaction A committed

💡 Deadlock detected at step 6; system aborts Transaction B to resolve.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8	Final
Transaction A locks	None	accounts: Granted	accounts: Held	accounts: Held, orders: Waiting	accounts: Held, orders: Granted	Released
Transaction B locks	None	None	orders: Granted	orders: Held, accounts: Waiting	Aborted	None
Deadlock Status	No	No	No	Yes	No	No

Key Moments - 3 Insights

Why does Transaction A wait at step 5 even though it requested a lock?

Because Transaction B already holds an Access Exclusive lock on the orders table, so A must wait until B releases it (see execution_table step 5).

How does the system detect the deadlock at step 6?

The system notices that Transaction A waits for a lock held by B (orders), and Transaction B waits for a lock held by A (accounts), forming a cycle, which is a deadlock (execution_table step 6).

What happens to Transaction B after deadlock detection?

Transaction B is chosen as the victim and aborted to release its locks, allowing Transaction A to continue (execution_table steps 7 and 8).

Visual Quiz - 3 Questions

Look at the execution table, what lock does Transaction A hold after step 2?

A No lock

B Shared lock

C Access Exclusive lock on accounts table

D Access Exclusive lock on another table

💡 Hint

Check the 'Lock Granted' column at step 2 for Transaction A.

At which step is the deadlock detected according to the execution table?

A Step 4

B Step 6

C Step 8

D Step 10

💡 Hint

Look at the 'Deadlock Detected' column for the first 'Yes' value.

If Transaction A released its lock earlier, how would the execution table change?

A Transaction B would not wait and no deadlock would occur

B Deadlock would still occur at step 6

C Transaction A would be aborted instead

D Both transactions would wait indefinitely

💡 Hint

Refer to the 'Waits For' and 'Deadlock Detected' columns and consider lock availability.

Concept Snapshot

Deadlock occurs when two transactions wait for each other's locks.
PostgreSQL detects deadlocks automatically.
It aborts one transaction to break the cycle.
Use explicit locking carefully to avoid deadlocks.
Monitor and handle deadlock errors in applications.

Full Transcript

Deadlock detection and prevention in PostgreSQL involves transactions requesting locks on resources. When two transactions each hold locks the other needs, they wait indefinitely, causing a deadlock. PostgreSQL detects this cycle and resolves it by aborting one transaction, releasing its locks so the other can proceed. This process ensures the database does not freeze. The execution trace shows Transaction A locking accounts, B locking orders, then cross-requests causing waits, deadlock detection, and B rolled back. Understanding this helps avoid and handle deadlocks in real applications.