ToSqlParam coverage: Numeric, Interval, Geography, JSON

[StefanSteiner]

Summary

ToSqlParam is implemented for primitives, Date/Time/Timestamp/OffsetTimestamp, byte slices, and strings, but not for Numeric, Interval, Geography, or JSON. Users who want to bind these types as query parameters must stringify them by hand — even though the types themselves already round-trip through IntoValue/RowValue and SqlType::Json is a recognized SQL type.

Current state

hyperdb-api/src/params.rs lists every implementing type starting at line 27. Confirmed missing impls:

$ grep -nE "impl ToSqlParam (for|<.+> for) (Numeric|Interval|Geography|serde_json|Json)" hyperdb-api/src/params.rs
# (no output)

Existing impls per params.rs:114-416:

• Numeric: i16, i32, i64, f32, f64, bool
• Strings/bytes: str, String, &str, [u8], Vec<u8>
• Time: Date, Time, Timestamp, OffsetTimestamp
• Generic: &T, Option<T>

Original gap analysis: §5 of docs/RUST_API_GAP_ANALYSIS.md (predecessor repo) — "Interval — exists in IntoValue/RowValue but not ToSqlParam. Numeric — same. Geography — neither ToSqlParam nor IntoValue. serde_json::Value / native JSON type — SqlType::Json exists, but no first-class reader/writer."

Proposed work

• impl ToSqlParam for Numeric — fixed-precision decimals.
• impl ToSqlParam for Interval.
• impl ToSqlParam for Geography (and add IntoValue if missing — the gap analysis flagged that gap too).
• First-class JSON support:
  • impl ToSqlParam for serde_json::Value (behind a serde-json feature flag, off by default to avoid pulling serde_json into builds that don't need it).
  • Companion reader path on Row::get so users can round-trip serde_json::Value instead of stringifying.
  • Document the feature flag and example in hyperdb-api/examples/.
• Tests verifying each new impl round-trips through query_params and the inserter path.

Backwards compatibility

Purely additive. New trait impls + one new (default-off) feature flag for the serde_json integration.

Open questions

• For Numeric: bind via the binary encoding path (encode_param) or text (to_sql_literal)? Binary is faster but query_params currently routes everything through text per params.rs:82. Worth deciding in scope of this issue.
• For Geography: which Rust geo type is canonical — geo_types::Geometry (already a dependency per DEVELOPMENT.md) or a hyperdb_api-owned newtype?