MVCC & Distributed Tx

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In PostgreSQL's MVCC implementation, what do the system columns xmin and xmax represent for a heap tuple?

javascript

-- Inspect MVCC system columns directly
SELECT xmin, xmax, ctid, * FROM orders WHERE id = 1;

Axmin is the row's creation timestamp; xmax is its deletion timestamp in Unix epoch format

Bxmin and xmax are lock identifiers used internally by the advisory lock subsystem

Cxmin is the minimum transaction ID in the current snapshot; xmax is the maximum transaction ID assigned so far

Dxmin is the transaction ID that inserted the row; xmax is the transaction ID that deleted or superseded the row (0 if the row is still current)

In PostgreSQLs snapshot isolation, what does a transactions snapshot actually capture?

AA full physical copy of all table data at the moment the transaction starts

BThe latest committed values for all rows in the database at snapshot creation time

CA lightweight structure containing: xmin (lowest active XID), xmax (next XID to be assigned), and a list of in-progress transaction IDs — used to determine row visibility without copying data

DThe list of all table-level and row-level locks held at snapshot creation time

What mechanism does PostgreSQL's Serializable Snapshot Isolation (SSI) use to detect dangerous concurrency anomalies?

AIt uses exclusive row-level locks to prevent all concurrent reads during a serializable transaction

BIt tracks read-write anti-dependency edges between concurrent transactions and aborts a transaction when a dangerous cycle is detected, indicating execution that cannot be explained by any serial order

CIt replays all committed serializable transactions serially after commit to verify correctness retrospectively

DIt uses a global lock table that serializes all SERIALIZABLE transactions through a single bottleneck

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