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Cargo feature flags let you opt in to optional capabilities without pulling in dependencies you don’t need. better-duck uses feature flags extensively because it targets a wide range of environments — from Tauri desktop apps and iOS cross-builds to server-side async services and data-pipeline tools — and because some of its optional dependencies (like tokio or chrono) are large enough that you may want to exclude them. This page lists every feature flag for both better-duck-core and better-duck-diesel, explains what each one enables, and notes which are on by default.

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better-duck-core Features

The better-duck-core crate is the low-level DuckDB wrapper. Its feature flags control date/time conversions, decimal support, extensions, async, pooling, and user-defined functions.
FeatureDefaultDescription
chrono✓Enable chrono date/time conversions for DATE, TIME, TIMESTAMP, TIMESTAMPTZ, TIME_TZ, TIME_NS, and INTERVAL. When enabled, temporal DuckValue variants carry chrono types. When disabled, they carry types from better_duck_core::types::date_native and std::time.
decimal✓Enable rust_decimal::Decimal support for DECIMAL columns. Adds the DuckValue::Decimal variant and impl AppendAble for Decimal.
json—Enable DuckDB’s JSON extension. Needed only if you use json_extract, to_json, or other JSON functions in your queries.
parquet—Enable DuckDB’s Parquet extension. Needed only if you use read_parquet(…) or other Parquet functions in your queries.
async—Tokio-based async facade over spawn_blocking: adds AsyncConnection, AsyncDatabase, and AsyncPool. These types never block the async executor. Requires tokio with the rt feature.
pool—r2d2 connection pool backed by a shared Database handle. Adds DuckDbConnectionManager and re-exports r2d2::Pool. Unlike opening N independent connections, every pooled connection shares the same underlying database, which is important for in-memory databases.
udf—#[duckdb_scalar] and #[duckdb_table_function] proc-macro attributes for registering plain Rust functions as DuckDB scalar or table functions. Adds the better-duck-macros proc-macro crate as a dependency.

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better-duck-diesel Features

The better-duck-diesel crate is the Diesel 2.3 backend. Its feature flags are a smaller set — they mainly control whether optional chrono and decimal support flows through from better-duck-core, and whether r2d2 pooling is exposed via Diesel’s own ConnectionManager.
FeatureDefaultDescription
decimal✓Enable Diesel Numeric ↔ rust_decimal::Decimal mapping. Activates better-duck-core/decimal and the rust_decimal dependency.
chrono—Enable Diesel FromSql/ToSql impls for DATE, TIME, TIMESTAMP, TIMESTAMPTZ, INTERVAL, TIME_TZ, and TIME_NS. Activates better-duck-core/chrono, diesel/chrono, and the chrono dependency.
r2d2—r2d2 connection pool support via Diesel’s own diesel::r2d2::ConnectionManager<DuckDbConnection>. Activates diesel/r2d2.
The chrono feature is on by default in better-duck-core but off by default in better-duck-diesel. If you are using better-duck-diesel with Diesel date/time columns, you must explicitly enable chrono in your Cargo.toml dependency on better-duck-diesel. The reason for the asymmetry is that many Diesel users already pull chrono in through Diesel’s own feature system and want explicit control over which backend enables it.

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Enabling Features in Cargo.toml

Add better-duck-core or better-duck-diesel to your [dependencies] section and list the features you need. Because these crates are currently in pre-release, you must pin the full version string — Cargo’s default version requirement ("0.1") excludes pre-release versions. 
# Enable async and pool for better-duck-core
[dependencies]
better-duck-core = { version = "0.1.0-beta.4", features = ["async", "pool"] }

# Enable UDF support
[dependencies]
better-duck-core = { version = "0.1.0-beta.4", features = ["udf"] }

# Enable chrono and r2d2 for the Diesel backend
[dependencies]
better-duck-core   = "0.1.0-beta.4"
better-duck-diesel = { version = "0.1.0-beta.4", features = ["chrono", "r2d2"] }

# Disable default features (chrono and decimal) for a minimal build
[dependencies]
better-duck-core = { version = "0.1.0-beta.4", default-features = false }
Enable the json and parquet features only if you actually use DuckDB’s JSON or Parquet extensions in your queries. Both add meaningful compile-time cost. If your workload is purely relational, leave them disabled.

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Date/Time Without chrono

When the chrono feature is disabled, all temporal DuckValue variants and AppendAble impls use types from better_duck_core::types::date_native instead of chrono types. The native types are lightweight plain structs with no external dependencies:
DuckDB typeNo-chrono Rust typeFields
DATEDuckDateyear: i32, month: u8, day: u8
TIMEDuckTimehour: u8, min: u8, sec: u8, micros: u32
TIME_NSDuckTimeNshour: u8, min: u8, sec: u8, nanos: u32
TIME WITH TIME ZONEDuckTimeTzhour, min, sec, micros, offset_secs: i32
TIMESTAMP / TIMESTAMPTZstd::time::SystemTime
INTERVALstd::time::Duration
Only one set of implementations is compiled at a time — there is no runtime switching between chrono and native types. The decision is made at compile time by the presence or absence of the chrono feature. 
// With `chrono` disabled, date columns yield DuckDate
#[cfg(not(feature = "chrono"))]
use better_duck_core::types::date_native::DuckDate;

match value {
    DuckValue::Date(d) => println!("{}-{:02}-{:02}", d.year, d.month, d.day),
    DuckValue::Time(t) => println!("{:02}:{:02}:{:02}", t.hour, t.min, t.sec),
    DuckValue::Null    => println!("null"),
    _ => {}
}
For better-duck-diesel specifically, remember that the chrono feature must be enabled explicitly — see the note in the better-duck-diesel Features section above.

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