Module pushdown 

Predicate pushdown: translate Kit Expr predicates into MongrelDB core Conditions so the native engine can resolve them via indexes instead of a full-scan + Rust evaluation (Kit Priority 1).

Design

The Kit’s query layer materializes every visible row and evaluates predicates in Rust. This module is the bridge that lets simple, index- served predicates bypass that full scan. It produces PushdownPlan — a list of native Conditions that the core engine can resolve via HOT / bitmap / range indexes, plus a flag indicating whether the translation covered the entire predicate.

Core’s native Query is a conjunction (AND of conditions) that resolves to a row-id set. Conditions always return a superset of the matching rows; the Kit must still re-apply the original Expr filter in Rust on the survivors unless PushdownPlan::fully_translated is true. This makes partial translation safe by construction.

Supported translations

Kit Expr	Core Condition	Requirement
Eq(Column, Literal) on PK	Pk	single-column PK
Eq(Column, Literal)	BitmapEq	bitmap-indexed column
In(Column, [literals])	BitmapIn	bitmap-indexed column
Lt/Lte/Gt/Gte(Column, Literal) int	Range	int-typed column
Lt/Lte/Gt/Gte(Column, Literal) float	RangeF64	float-typed column
Contains(Column, needle)	FmContains	FM-indexed column (residual re-check)
Like(Column, pattern)	FmContainsAll	FM-indexed column (residual re-check)
IsNull / IsNotNull(Column)	IsNull / IsNotNull	page-stat-aware (residual re-check)
And([sub-exprs])	recurse each	each part translated independently

Unsupported (Or, Not, Ne, NotIn, InSubquery, Exists, cross-column comparisons) are left as residual Rust evaluation — the caller falls back to a full scan for those branches.

Structs

PushdownPlan

The result of translating a Kit predicate into core conditions.

Functions

pk_conditions

Build a Condition::Pk (or bitmap fallback) from a PK value map. Used by get_by_pk_internal and FK parent_exists — the highest-value pushdown targets since they currently do O(N) linear scans with the PK in hand.

translate_predicate

Attempt to translate expr into native Conditions for table. Returns None when no part of the expression could be translated (caller falls back to a full scan + Rust evaluation).