Why concurrency control matters in PostgreSQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the SQL query to select all columns from the orders table.

PostgreSQL

SELECT [1] FROM orders;

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A*

Bcolumns

Ceverything

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using words like 'all' or 'columns' instead of '*'

Forgetting to specify columns or using invalid keywords

The asterisk (*) selects all columns from the table.

2fill in blank

medium

Complete the SQL query to lock the row for update in the transactions table.

PostgreSQL

SELECT * FROM transactions WHERE id = 5 [1];

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AFOR SHARE

BWITH LOCK

CLOCK ROW

DFOR UPDATE

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'FOR SHARE' which locks for reading, not writing

Using invalid clauses like 'LOCK ROW' or 'WITH LOCK'

FOR UPDATE locks the selected rows to prevent concurrent modifications.

3fill in blank

hard

Fix the error in the transaction control statement to start a transaction.

PostgreSQL

[1] TRANSACTION;

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ABEGIN

BSTART

COPEN

DINITIATE

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid keywords like 'OPEN' or 'INITIATE'

Using non-standard keywords like 'OPEN' or 'INITIATE'

BEGIN TRANSACTION starts a new transaction block in PostgreSQL.

4fill in blank

hard

Fill both blanks to set the isolation level to serializable and start a transaction.

PostgreSQL

[2] TRANSACTION; SET TRANSACTION ISOLATION LEVEL [1];

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ASERIALIZABLE

BBEGIN

CREAD COMMITTED

DCOMMIT

Attempts:

3 left

💡 Hint

Common Mistakes

Using READ COMMITTED instead of SERIALIZABLE for strict control

Using COMMIT instead of BEGIN to start a transaction

Setting the isolation level to SERIALIZABLE ensures strict concurrency control. BEGIN TRANSACTION starts the transaction.

5fill in blank

hard

Fill the blanks to commit the transaction and release the lock.

PostgreSQL

[1]; [2] TRANSACTION;

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ACOMMIT

BEND

CSAVEPOINT

DLOCK

Attempts:

3 left

💡 Hint

Common Mistakes

Using SAVEPOINT instead of COMMIT

Using RELEASE SAVEPOINT which is for savepoints, not locks

COMMIT saves changes and releases locks. END TRANSACTION ends it (synonym for COMMIT).