Struct Snapshot 

pub struct Snapshot {
    pub version: u64,
    pub in_progress: InProgressSet,
    pub oldest_active: u64,
    pub tx_id: u64,
    pub locked_out: Option<(RelId, BTreeSet<usize>)>,
}

Per-statement / per-transaction snapshot.

Cheap to clone (Vec inside an InProgressSet is the only non-Copy field; bounded at active-tx count which is small).

Fields

version: u64

The upper bound. A row whose xmin exceeds this is in the snapshot’s future — invisible.

in_progress: InProgressSet

In-flight transactions at snapshot time.

oldest_active: u64

Floor used by vacuum. Any row whose xmax < oldest_active is dead to every live snapshot.

tx_id: u64

The reading transaction’s OWN id. Used to implement “see your own writes” — a row your transaction inserted is visible to you even before commit. 0 for non- transactional reads (autocommit SELECT).

locked_out: Option<(RelId, BTreeSet<usize>)>

v7.39 (round 297, E3 Phase 1b) — rows a SKIP LOCKED pre-pass found held by another transaction, as (relation, row indices).

It rides the SNAPSHOT because that is the only channel every row source already threads. Adding the filter at individual scan sites missed the real path three times running — index_access alone performs the visibility test in ten places. A row that someone else holds is, for this statement, exactly as unavailable as a row the snapshot cannot see.

Implementations

impl Snapshot

pub const fn unbounded() -> Snapshot

A “see everything visible” snapshot — version at the current upper-bound u64, in-progress empty. Equivalent to the pre-v7.37.15 “Arc-snapshot reads the entire catalog” behaviour; useful for phase-A migration where the engine doesn’t yet track per-tx state.

pub fn new( version: u64, in_progress: InProgressSet, oldest_active: u64, tx_id: u64, ) -> Snapshot

Construct from explicit fields. The version cursor and in-progress set come from the engine’s per-process version counter at snapshot time; oldest_active is the MIN of every live snapshot’s version (vacuum reads it).

pub fn visible(&self, h: &RowHeader) -> bool

Should the row be visible to a reader holding this snapshot? The five-step rule mirrors PG’s HeapTupleSatisfiesMVCC.

1. Self-write: if the row’s writer is THIS reader’s own tx, the row is visible (READ COMMITTED sees its own writes).
2. xmin in the future: invisible.
3. xmin still in-progress: invisible.
4. Alive (xmax == ALIVE): visible.
5. xmax in the future or in-progress: visible (the delete hasn’t committed yet from this reader’s point of view).
6. xmax in the past + committed: invisible (deleted before this reader’s snapshot).

pub fn visible_with_status<O>(&self, h: &RowHeader, xact: &O) -> bool

where O: XactStatusOracle + ?Sized,

v7.37.15 (Phase C.2) — abort-aware visibility. Same as Self::visible but consults a XactStatusOracle to distinguish a committed version from an aborted one when neither is in the snapshot’s in_progress set. Phase C.3’s in-place write path needs this: a rolled-back or crash-orphaned version leaves its xmin/xmax stamp physically present until vacuum reclaims it, and the two-state Self::visible would wrongly treat it as committed.

Two extra branches vs visible (marked NEW):

• xmin aborted → the insert never happened → invisible.
• xmax aborted → the delete never happened → still visible.

The frozen-and-alive fast path returns before any oracle call, so steady-state scans over old data pay no oracle cost — preserving the Phase B “≤5% overhead” result. Passing AllCommitted makes this behave exactly like visible.

Trait Implementations

impl Clone for Snapshot

fn clone(&self) -> Snapshot

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Snapshot

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl Default for Snapshot

fn default() -> Snapshot

Returns the “default value” for a type.

impl Eq for Snapshot

impl PartialEq for Snapshot

fn eq(&self, other: &Snapshot) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for Snapshot