Can my DB?

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
ROLLBACK;

VACUUM accounts;

• Heap tuples carry visibility metadata; UPDATE creates a new row version instead of rewriting the old one in place.
• Readers work from transaction snapshots and usually do not block writers.
• WAL is the durability and crash-recovery path; dead tuples are reclaimed later by VACUUM and autovacuum.
• Long-running transactions delay cleanup and can increase bloat, vacuum cost, and replica recovery pressure.

START TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
ROLLBACK;

• InnoDB keeps the current version in the clustered record and reconstructs older versions through undo logs and read views.
• Redo logging provides the durability and recovery path for committed changes.
• Background purge removes history no longer needed by active transactions.
• Long-running transactions retain undo history, increase purge debt, and can degrade read performance.

START TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
ROLLBACK;

• For InnoDB tables, MariaDB follows the same broad model as MySQL: current rows plus undo history plus redo logging.
• Undo history chains and delete-marked rows support consistent reads and rollback.
• Purge threads remove old versions after they are no longer needed.
• This comparison is scoped to InnoDB; engine choice matters more in MariaDB than in PostgreSQL or Oracle.

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
ROLLBACK;

• Oracle uses multiversion read consistency built on undo data stored in undo segments or undo tablespaces.
• Readers reconstruct older versions from undo instead of blocking on active writers.
• Redo protects both data changes and undo changes, so forward recovery and rollback state are both durable.
• Undo retention sizing matters for long queries and flashback-style historical access.

ALTER DATABASE mydb SET READ_COMMITTED_SNAPSHOT ON;
SET TRANSACTION ISOLATION LEVEL SNAPSHOT;
BEGIN TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
ROLLBACK;

• SQL Server runs classic lock-based concurrency by default; row-versioning isolation is opt-in.
• READ_COMMITTED_SNAPSHOT and SNAPSHOT use copy-on-write row versioning backed by a version store.
• The version store lives in tempdb; Accelerated Database Recovery adds a persistent per-database version store and shortens rollback.
• Schema changes and write conflicts still take locks even with row versioning on.

PRAGMA journal_mode = WAL;
BEGIN IMMEDIATE;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
ROLLBACK;

• SQLite does not implement multi-writer row-version MVCC like PostgreSQL or InnoDB.
• Rollback-journal mode writes directly to the database file and uses a journal for atomicity and rollback.
• WAL mode gives readers snapshot isolation while still serializing writes to a single writer.
• Long-lived readers can delay checkpoints and WAL truncation.

BEGIN;
DELETE FROM fact_events WHERE event_date < '2025-01-01';
ROLLBACK;
VACUUM;

• Redshift exposes transaction snapshots and serializable-style guarantees, but its storage behavior is closer to a columnar warehouse than a classic OLTP MVCC engine.
• Deletes mark rows for deletion; cleanup and resorting happen later through background work and VACUUM.
• The default SNAPSHOT isolation level favors throughput over stricter SERIALIZABLE behavior.
• Heavy update and delete workloads create maintenance debt and can slow both DML and vacuum work.

SELECT balance FROM accounts WHERE id = 1 WITH CS;
ROLLBACK;

• Concurrency is lock-based, but the currently-committed semantics let readers see the last committed version without blocking on active writers for most read paths.
• This behavior is reconstructed from log information rather than from an always-visible heap-version chain model.
• The transaction log remains central for commit, rollback, and recovery.
• Behavior is more conditional than in PostgreSQL, Oracle, or InnoDB-style engines.

BEGIN;
UPDATE dim_customer SET segment = 'vip' WHERE customer_id = 42;
ROLLBACK;

• Snowflake is not a classic row-store MVCC engine; it uses immutable columnar micro-partitions and transaction snapshots.
• DML creates new micro-partition state instead of updating blocks in place.
• Historical states are retained through Time Travel and later Fail-safe.
• High-churn tables can accumulate more retained storage because updates rewrite affected partition state.

BEGIN TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
ROLLBACK TRANSACTION;

• BigQuery is not a classic OLTP MVCC engine; it offers snapshot-isolated multi-statement transactions on top of versioned columnar storage blocks.
• Modifying data creates new versions of affected storage blocks rather than updating rows in place.
• Readers and appends can proceed, but conflicting mutating operations on the same table are canceled or queued.
• Time travel provides the historical-access and recovery story rather than user-visible undo logs.