How Rust native types round-trip through different SQL column types.
Tested via entity insert + read-back (Table API).
Legend:
- exact = lossless round-trip, value identical after insert + read
- lossy = value stored but precision reduced (e.g. f64 rounding)
- truncated = value stored but digits beyond column precision are cut
- err = database rejects the insert or entity conversion fails on read
tests/mysql/1_chrono.rs
| Column \ Rust type | NaiveDate | NaiveTime | NaiveDateTime | DateTime<Utc> | DateTime<FixedOffset> |
|---|
| VARCHAR | exact | exact | exact | exact | exact |
| DATE | exact | err | err | err | — |
| TIME | err | truncating µs | err | err | — |
| TIME(6) | err | exact | err | err | — |
| DATETIME | err | err | truncating µs | truncating µs | offset → +00:00 |
| DATETIME(6) | err | err | exact | exact | — |
| TIMESTAMP | err | err | truncating µs | truncating µs | offset → +00:00 |
| TIMESTAMP(6) | err | err | exact | exact | — |
- Format:
"2025-01-10 12:00:00" (space separator, no T); FixedOffset appends +05:30
- TIME/DATETIME/TIMESTAMP default to 0 fractional digits — use
(6) for microseconds
DateTime<FixedOffset>: VARCHAR preserves offset; DATETIME/TIMESTAMP normalize to UTC- MariaDB 11 rejects
DateTime<FixedOffset> into DATETIME/TIMESTAMP (no offset literals) — use DateTime<Utc>
- Cross-type coercions (e.g. NaiveTime → DATE) fail with variant mismatch
tests/postgres/1_chrono.rs
| Column \ Rust type | NaiveDate | NaiveTime | NaiveDateTime | DateTime<Utc> | DateTime<FixedOffset> |
|---|
| VARCHAR | exact | exact | exact | exact | offset → +00:00 |
| DATE | exact | err | err | err | — |
| TIME | err | exact | err | err | — |
| TIMESTAMP | err | err | exact | exact | offset → +00:00 |
| TIMESTAMPTZ | err | err | exact | exact | offset → +00:00 |
- Format:
"2025-01-10 12:00:00+00" (space separator, abbreviated tz offset)
- Microsecond precision by default — no
(6) suffix needed
- Typed binds (chrono types, not text) required for DATE/TIME/TIMESTAMP/TIMESTAMPTZ
DateTime<FixedOffset>: offset always normalized to UTC (typed binds), no column preserves it- Cross-type coercions fail with variant mismatch
tests/sqlite/1_chrono.rs
| Column \ Rust type | NaiveDate | NaiveTime | NaiveDateTime | DateTime<Utc> | DateTime<FixedOffset> |
|---|
| TEXT | exact | exact | exact | exact | exact |
- All dates stored as TEXT — format: ISO 8601 with T separator (
"2025-01-10T12:00:00Z")
- Subsecond precision and timezone offsets preserved as-is
tests/mysql/1_decimal.rs
| Column \ Rust type | Decimal | i64 | f64 |
|---|
| VARCHAR | exact | exact | exact |
| DECIMAL(20,6) | truncated to 6 places | err | err |
| DECIMAL(38,15) | exact | err | err |
| DOUBLE | lossy (~15 digits) | err | exact |
| FLOAT | lossy (~7 digits) | err | lossy (~7 digits) |
| BIGINT | fractional part lost | exact | err |
- No cross-conversion between Integer, Float, and Decimal CBOR types
- VARCHAR works for all types —
from_cbor parses text as fallback
tests/postgres/1_decimal.rs
| Column \ Rust type | Decimal | i64 | f64 |
|---|
| VARCHAR | exact | exact | exact |
| NUMERIC(20,6) | truncated to 6 places | err | err |
| NUMERIC(38,15) | exact | err | err |
| DOUBLE PRECISION | lossy (~15 digits) | err | exact |
| REAL | lossy (~7 digits) | err | lossy (~7 digits) |
| BIGINT | fractional part lost | exact | err |
- Typed binds (
rust_decimal::Decimal) used for NUMERIC columns
tests/sqlite/1_decimal.rs
| Column \ Rust type | Decimal | i64 | f64 |
|---|
| TEXT | exact | exact | exact |
| NUMERIC | lossy (~15 digits, stored as REAL) | exact | exact |
| REAL | lossy (~15 digits) | err | exact |
| INTEGER | fractional part lost | exact | err |
- SQLite infers CBOR type from stored value, not declared column type
- NUMERIC/REAL affinities coerce to float —
i64 in REAL comes back as Float and fails
- Store Decimal in TEXT for lossless precision