Enum FallbackReason 

#[non_exhaustive]
pub enum FallbackReason {
    NoSchema,
    PartitionKeyNotFullyConstrained,
    PartitionKeyEncodingFailed,
    MetadataScanPath,
    LegacyExecutorPath,
    ForcedFullScan,
    TombstonesBuildNoPrune,
}

The documented, closed set of reasons a SELECT fell back to a full scan.

Keep this list in sync with docs/access-paths.md. Adding a variant is a public-contract change: it documents a new, known reason a query cannot use a targeted path. It must never be used to paper over an unexpected fallback — an unexpected fallback should surface as a failing access-path assertion.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

NoSchema

No schema was available, so the partition-key columns cannot be identified to build a targeted lookup.

PartitionKeyNotFullyConstrained

The WHERE clause does not fully constrain the partition key with equality (partial key, no restriction, or a non-equality restriction such as a range). Mirrors Cassandra’s single-partition-read requirement.

PartitionKeyEncodingFailed

The fully-constrained partition-key values could not be encoded to the on-disk key form (e.g. a type mismatch). A full scan is the safe fallback.

MetadataScanPath

A WRITETIME/TTL metadata projection with NO usable partition-key restriction full-scans (there is no key to target). A fully-constrained WHERE pk = ? now routes through AccessPath::MetadataPartitionLookup (#962) and WHERE pk IN (...) fans out to AccessPath::MultiPartitionLookup (#1916); this reason is reserved for the genuinely unclassifiable metadata scan that remains.

LegacyExecutorPath

The legacy QueryExecutor issues an unconditional storage.scan; it does not consult the fast path. Since issue #1750 ad-hoc SELECTs route through the modern executor, so this reason now covers the remaining legacy SELECT surfaces (a cached legacy SELECT plan reused via the plan-cache HIT branch). Tracked by #962 (route the legacy/prepared surfaces through the modern executor) and #961 (param binding).

ForcedFullScan

The operator forced the full-scan path via the read-path forcing knob (CQLITE_READ_PATH=full or crate::config::ReadPathMode::Full, issue #1918). This is a forced fallback, distinct from every organic reason so a deliberately-forced full scan is never mistaken for one the classifier chose on its own. The rows are byte-identical to the auto result for the same query — forcing governs routing only, never decoding.

TombstonesBuildNoPrune

The tombstones build compiles out the partition-targeted prune. On that build the targeted storage surfaces (scan_partition, scan_partition_with_cell_metadata) are full-scan + retain fallbacks with NO bloom/BTI SSTable pruning, so a fully-constrained WHERE pk = ? (or IN (...)/WRITETIME-TTL) opens the whole table even though the rows are byte-identical to the pruned build. The executor reports this honest reason (rather than a fake targeted label) whenever the storage call returns engaged == false (Epic #951, honest access paths).

Implementations

impl FallbackReason

pub fn label(&self) -> &'static str

A stable, lowercase label suitable for EXPLAIN-style output and JSON.

Trait Implementations

impl Clone for FallbackReason

fn clone(&self) -> FallbackReason

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Copy for FallbackReason

impl Debug for FallbackReason

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

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for FallbackReason

fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Eq for FallbackReason

impl PartialEq for FallbackReason

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

Equality operator ==.

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

Inequality operator !=.

impl Serialize for FallbackReason

fn serialize<__S>(&self, __serializer: __S) -> Result<__S::Ok, __S::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.

impl StructuralPartialEq for FallbackReason