Deadlock detection and prevention in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check for deadlocks using PostgreSQL system views.

PostgreSQL

SELECT * FROM pg_locks WHERE [1] = true;

Drag options to blanks, or click blank then click option'

Awaiting

Blocked

Cgranted

Dactive

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column name that does not exist in pg_locks.

Confusing 'waiting' with 'granted'.

The pg_locks view shows lock information. The granted column indicates if the lock is currently held. Checking for granted locks helps identify potential deadlocks.

2fill in blank

medium

Complete the query to find transactions waiting for locks in PostgreSQL.

PostgreSQL

SELECT pid, blocked_pid FROM pg_blocking_pids([1]);

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Atransaction_id

Bsession_id

Clock_id

Dpid

Attempts:

3 left

💡 Hint

Common Mistakes

Passing transaction or session IDs instead of process IDs.

Using a column that does not exist in the context.

The function pg_blocking_pids(pid) returns the process IDs blocking the given process ID. So, you need to pass a pid to check who is blocking it.

3fill in blank

hard

Fix the error in the query to detect deadlocks by joining pg_locks and pg_stat_activity.

PostgreSQL

SELECT a.pid, a.query, l.locktype FROM pg_stat_activity a JOIN pg_locks l ON a.[1] = l.pid WHERE NOT l.[2];

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Apid

Bprocess_id

Csession_pid

Dgranted

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect column names for join.

Filtering on the wrong lock status column.

The correct join is on pid columns from both views. The condition NOT l.granted filters locks that are waiting, which helps detect deadlocks.

4fill in blank

hard

Fill both blanks to write a query that detects deadlocks by checking waiting and granted locks.

PostgreSQL

SELECT blocked_locks.pid AS blocked_pid, blocking_locks.pid AS blocking_pid FROM pg_locks blocked_locks JOIN pg_locks blocking_locks ON blocked_locks.[1] = blocking_locks.[2] WHERE NOT blocked_locks.granted AND blocking_locks.granted;

Drag options to blanks, or click blank then click option'

Atransactionid

Bpid

Clocktype

Dmode

Attempts:

3 left

💡 Hint

Common Mistakes

Joining on process IDs instead of transaction IDs.

Using locktype or mode for join which is incorrect.

Deadlocks often occur when transactions wait on the same transactionid. Joining on transactionid helps find blocking and blocked transactions.

5fill in blank

hard

Fill all three blanks to write a query that prevents deadlocks by ordering lock acquisition.

PostgreSQL

BEGIN; LOCK TABLE [1] IN [2] MODE; LOCK TABLE [3] IN SHARE MODE; COMMIT;

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Atable_a

BEXCLUSIVE

Ctable_b

DACCESS SHARE

Attempts:

3 left

💡 Hint

Common Mistakes

Locking tables in different orders causing deadlocks.

Using incorrect lock modes.

To prevent deadlocks, acquire locks in a consistent order. Here, table_a is locked first in EXCLUSIVE mode, then table_b in SHARE mode.