Struct IndexStats 

pub struct IndexStats {
    pub entries: usize,
    pub distinct_keys: usize,
    pub approx_bytes: usize,
    pub kind: IndexKind,
    pub has_bloom: bool,
    pub index_correlation: f64,
}

Per-index statistics used by the cost-based planner and diagnostics. All fields are best-effort; zero means “unknown”.

Fields

entries: usize

Total number of (key, value) pairs stored.

distinct_keys: usize

Number of distinct keys. For hash/btree this equals the key set size.

approx_bytes: usize

Approximate memory footprint in bytes (0 if not tracked).

kind: IndexKind

Family of index this originates from.

has_bloom: bool

Whether a bloom filter is attached (enables fast negative lookups).

index_correlation: f64

Physical correlation between index key order and heap row order. 1.0 = perfectly correlated (monotonic insert, timeseries) → sequential I/O. 0.0 = completely random → worst-case random I/O per row. Default 0.0 (conservative). Used by Mackert-Lohman I/O cost formula.

Implementations

impl IndexStats

pub fn point_selectivity(&self) -> f64

Rough selectivity estimate for an equality predicate. Returns the expected fraction of rows matching a random key, clamped to [0, 1].

Used by the planner to pick between index probes and full scans.

pub fn avg_values_per_key(&self) -> f64

Average number of values per distinct key.

pub fn correlated_io_cost(&self, result_rows: f64, heap_pages: f64) -> f64

Estimate the I/O cost (in arbitrary page-cost units) of fetching result_rows rows via this index from a heap_pages-page table.

Uses the Mackert-Lohman (1986) formula — the same model PostgreSQL uses in cost_index (optimizer/path/costsize.c:545-700):

pages_fetched = ML(selectivity, heap_pages)
io_cost = lerp(random_io, seq_io, correlation²)

Constants match PG GUC defaults:

• random_page_cost = 4.0
• seq_page_cost = 1.0

Trait Implementations

impl Clone for IndexStats

fn clone(&self) -> IndexStats

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for IndexStats

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

Formats the value using the given formatter.

impl Default for IndexStats

fn default() -> IndexStats

Returns the “default value” for a type.