Decimal precision is not maintained from postgres to rust_decimal

[jkleinknox]

Bug Description

Decimal precision does not match when converting a PgNumeric to a rust_decimal::Decimal. This is corrected by calling .normalize() on rust_decimal::Decimal.

Minimal Reproduction

#[test]
fn precision_match() {
    let m: i128 = 2147483647;
    let s = 2;

    let number = Decimal::new(m as i64, s);
    assert_eq!(m, number.mantissa());
    assert_eq!(s, number.scale());

    let pg_numeric = PgNumeric::try_from(&number).unwrap();
    let new_number : Decimal = pg_numeric.try_into().unwrap();

    assert_eq!(m, new_number.mantissa());
    // ---- postgres::types::decimal::decimal_to_pgnumeric::precision_match stdout ----
    //     thread 'postgres::types::decimal::decimal_to_pgnumeric::precision_match' panicked at 'assertion failed: `(left == right)`
    // left: `2147483647`,
    // right: `214748364700`', sqlx-core/src/postgres/types/decimal.rs:220:9

    assert_eq!(s, new_number.scale());
    // ---- postgres::types::decimal::decimal_to_pgnumeric::precision_match stdout ----
    //     thread 'postgres::types::decimal::decimal_to_pgnumeric::precision_match' panicked at 'assertion failed: `(left == right)`
    // left: `2`,
    // right: `4`', sqlx-core/src/postgres/types/decimal.rs:227:9
}

Info

• SQLx version: 0.6.2
• SQLx features enabled: runtime-tokio-rustls,decimal,postgres
• Database server and version: Postgres 15.1
• Operating system: Darwin Kernel Version 21.6.0: root:xnu-8020.140.49~2/RELEASE_ARM64_T6000 arm64
• rustc --version: rustc 1.65.0 (897e37553 2022-11-02)

[Flowneee]

This is because of https://github.com/launchbadge/sqlx/blob/main/sqlx-postgres/src/types/rust_decimal.rs#L56 (same for bigdecimal). But it seems like this is intended.

We encountered this problem when noticed that passing BigDecimal value from postgres to JSON always producing something like 1234.600, which is technically correct and shouldn't affect anything, but still annoying. I believe rust_decimal and bigdecimal features should call normalize() (normalized() for bigdecimal) at the end of TryFrom. But this will obviously add some overhead.

[Vrajs16]

Is there a workaround for this? I was thinking that I could use serde's custom deserializer, but that doesn't seem to work (I don't think sqlx deserializes into the struct field.) However I was able to serialize the field which has big decimal to 2.

struct x {
    ...
    #[serde(serialize_with = "size_serializer")]
    pub size: BigDecimal,
    ...
}

fn size_serializer<S>(big_decimal: &BigDecimal, serialize: S) -> Result<S::Ok, S::Error>
where
    S: Serializer,
{
    serialize.collect_str(&big_decimal.with_scale(2))
}

If anyone knows of a workaround that would be great.

[jkleinknox]

Completed in #2838