Struct ApproxCountDistinctConstraint 

pub struct ApproxCountDistinctConstraint { /* private fields */ }

A constraint that validates the approximate count of distinct values in a column.

This constraint uses DataFusion’s APPROX_DISTINCT function which provides an approximate count using HyperLogLog algorithm. This is much faster than exact COUNT(DISTINCT) for large datasets while maintaining accuracy within 2-3% error margin.

Examples

use term_guard::constraints::{ApproxCountDistinctConstraint, Assertion};
use term_guard::core::Constraint;

// Check for high cardinality (e.g., user IDs)
let constraint = ApproxCountDistinctConstraint::new("user_id", Assertion::GreaterThan(1000000.0));
assert_eq!(constraint.name(), "approx_count_distinct");

// Check for low cardinality (e.g., country codes)
let constraint = ApproxCountDistinctConstraint::new("country_code", Assertion::LessThan(200.0));

Implementations

impl ApproxCountDistinctConstraint

pub fn new<S: Into<String>>(column: S, assertion: Assertion) -> Self

Creates a new approximate count distinct constraint.

Arguments

• column - The column to count distinct values in
• assertion - The assertion to apply to the approximate distinct count

Trait Implementations

impl Clone for ApproxCountDistinctConstraint

fn clone(&self) -> ApproxCountDistinctConstraint

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Constraint for ApproxCountDistinctConstraint

fn evaluate<'life0, 'life1, 'async_trait>( &'life0 self, ctx: &'life1 SessionContext, ) -> Pin<Box<dyn Future<Output = Result<ConstraintResult>> + Send + 'async_trait>>

where Self: 'async_trait, 'life0: 'async_trait, 'life1: 'async_trait,

Evaluates the constraint against the data in the session context.

fn name(&self) -> &str

Returns the name of the constraint.

fn column(&self) -> Option<&str>

Returns the column this constraint operates on (if single-column).

fn metadata(&self) -> ConstraintMetadata

Returns the metadata associated with this constraint.

fn description(&self) -> Option<&str>

Returns a description of what this constraint validates.

impl Debug for ApproxCountDistinctConstraint

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

Formats the value using the given formatter.