ACID & Basic Transactions — Series 2

Preview — 3 of 10 questions

After this block runs, what is accounts.balance for id = 1?

javascript

-- accounts.balance for id = 1 starts at 500
BEGIN;
UPDATE accounts SET balance = 100 WHERE id = 1;
ROLLBACK;

A500

B100

CNULL

DAn error — you cannot ROLLBACK after an UPDATE has already run

Session A runs BEGIN; UPDATE accounts SET balance = 100 WHERE id = 1; (no COMMIT yet). At that exact moment, Session B (a separate connection) runs SELECT balance FROM accounts WHERE id = 1;. What does Session B see?

javascript

-- Session A:
BEGIN;
UPDATE accounts SET balance = 100 WHERE id = 1;
-- (not yet committed)

-- Session B (concurrently):
SELECT balance FROM accounts WHERE id = 1;
-- sees the OLD value, e.g. 500 — Session A's change is invisible until COMMIT

A100 — Session B immediately sees Session A's uncommitted change

BThe original (pre-update) balance — Session A's change is not visible to any other session until Session A commits

CAn error — Session B's query is blocked and fails until Session A finishes

DNULL, because the row is temporarily locked

Which statement correctly describes PostgreSQL's syntax for starting a transaction?

javascript

BEGIN;
BEGIN WORK;
BEGIN TRANSACTION;
START TRANSACTION;   -- the SQL-standard spelling

AOnly BEGIN (with no suffix) is valid — BEGIN WORK and BEGIN TRANSACTION are syntax errors

BSTART TRANSACTION only works for SERIALIZABLE transactions

CBEGIN WORK, BEGIN TRANSACTION, and START TRANSACTION are all equivalent ways to start a transaction

DBEGIN WORK starts a read-only transaction, while BEGIN alone starts a read-write one

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