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A PostgreSQL transaction can delete a row without committing, while another transaction still reads the row’s previous committed version. The row is not contradictory or instantly erased: visibility depends on the reader’s snapshot, and multiple row versions can coexist physically.
How can a deleted row still be visible?
Imagine Session A deletes a row but has not committed. Session B runs a query and still sees the row as it existed before that delete. Session B is reading according to its snapshot: the delete is not yet part of the committed history visible to that statement.
PostgreSQL describes this model in its PostgreSQL 17 documentation: “each SQL statement sees a snapshot of data (a database version) as it was some time ago, regardless of the current state of the underlying data.” PostgreSQL 17: Introduction to MVCC.
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Ordinary MVCC reads and writes avoid the read/write lock conflicts associated with some simpler concurrency schemes, but MVCC does not eliminate locks. Writers can conflict with writers, and PostgreSQL also provides explicit table- and row-level locks.
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Think of a row as a history of versions
The following is an educational representation, not PostgreSQL’s literal internal data structure. A logical row can be understood as a chain of versions, with each version carrying a creator transaction ID (xmin) and, when applicable, a deleting transaction ID (xmax).
- Insert: a transaction creates a version.
- Update: a transaction marks the old version as superseded and creates a new version.
- Delete: a transaction marks the current version as deleted; it does not mean every concurrent reader immediately loses access to that version.
These labels are inspired by PostgreSQL tuple metadata. A teaching model can track them to reason about visibility without reproducing the database’s full machinery.
Transaction status determines whether a version counts
A transaction ID alone is not enough to decide visibility. If the transaction that created a version has not committed, another transaction must not treat that version as committed data. If the creating transaction aborts, its change must be ignored. Similarly, an uncommitted delete should not hide a previously committed version from other transactions’ snapshots.
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For the simplified model, track transaction status alongside version stamps. A transaction’s own writes are a special case: a transaction should be able to see its own changes, including its own deletes, even though those changes are not yet committed for other observers.
A snapshot is a rule set, not a copied table
A snapshot does not need to copy every row. In this model, it records a transaction-ID boundary and the set of transactions that were active when the snapshot was taken. Together with transaction status and version stamps, those facts let the reader decide which versions count.
The active set handles an important timing case: Transaction X is active when a snapshot is created, then commits afterward. That later commit does not retroactively make X’s changes visible to the already-created snapshot. The snapshot preserves the view established at its own creation.
Decide whether a version is visible
Visibility has two parts: does the version’s creation count for this observer, and does its deletion count? Jagdish Parihar’s concise teaching summary is: “A version is visible when its birth is inside your snapshot and its death is not.” This is a useful mental model, not an official PostgreSQL specification or complete implementation rule.
- Check whether the version’s creating transaction is committed and falls within the snapshot’s view. Exclude changes from transactions that were still active at snapshot time, as well as aborted transactions.
- Check whether a deleting transaction counts as committed within that same snapshot. If it does, the old version is no longer visible to that observer; if not, it can remain visible.
- Apply the observer’s own-write rules in the teaching model so a transaction sees its own changes consistently.
For the opening example, Session B’s snapshot does not count Session A’s uncommitted delete. The old version therefore remains visible to B, while A sees the effect of its own delete. Once A commits, a later snapshot can account for the deletion. Whether an existing transaction also sees that commit depends on its isolation level and snapshot timing.
Snapshot timing differs by isolation level
PostgreSQL’s snapshot schedule changes the view a transaction uses. The PostgreSQL Professional training material describes the following behavior for PostgreSQL 16:
| Isolation level | Snapshot timing | What that means |
|---|---|---|
| Read Committed | A fresh snapshot for each statement | A later statement in the same transaction can see commits that occurred after an earlier statement began. |
| Repeatable Read | A transaction-wide snapshot established by the first statement | Subsequent statements keep the same snapshot through transaction completion. |
| Serializable | A transaction-wide snapshot established by the first statement | It uses a stable snapshot and additionally monitors conflicts to prevent serialization anomalies. |
The PostgreSQL 16 training material describes Repeatable Read as providing repeatable reads and preventing phantoms. Serializable adds protection against serialization anomalies; it is not simply another name for snapshot isolation.
In PostgreSQL, Serializable uses Serializable Snapshot Isolation (SSI) to detect dangerous read-write conflict structures. A transaction may be aborted when needed to prevent a result that cannot be explained by a serial execution. See the PostgreSQL Wiki’s Serializable overview for supporting context.
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A compact implementation can make the core idea concrete by representing transaction IDs, commit status, version chains, snapshots, and a visibility check. It is useful for seeing why one logical row can have multiple versions and why a transaction’s later commit does not change an older snapshot.
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It is not a PostgreSQL implementation. Real PostgreSQL behavior includes details that the small model leaves out:
- Snapshot scheduling: actual statement and transaction snapshot rules vary by isolation level.
- Write-write conflicts and locking: concurrent writers and explicit locks require conflict handling beyond a visibility function.
- Command IDs and statement behavior: visibility can depend on details within a transaction that a basic transaction-ID boundary does not capture.
- Serializable conflict detection: SSI monitors conflict patterns and can abort transactions.
- Tuple metadata and optimizations: transaction status and hint bits help PostgreSQL determine visibility efficiently.
- Cleanup: old versions are not retained forever. VACUUM and autovacuum reclaim versions once they are no longer needed by relevant snapshots.
Use the model to understand snapshot-relative visibility, not to predict every edge case, locking outcome, or performance characteristic of a running PostgreSQL server.
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