Function coerce_homogeneous_exprs

src/sql/src/plan/query.rs:5115–5151

Coerces a list of expressions such that all input expressions will be cast to the same type. If successful, returns a new list of expressions in the same order as the input, where each expression has the appropriate casts to make them all of a uniform type. If `force_type` is `Some`, the expressions are forced to the specified type via an explicit cast. Otherwise the best common type is guessed v

(
    ecx: &ExprContext,
    exprs: Vec<CoercibleScalarExpr>,
    force_type: Option<&SqlScalarType>,
)

/// [union-type-conv]:
/// https://www.postgresql.org/docs/12/typeconv-union-case.html
pub fn coerce_homogeneous_exprs(
    ecx: &ExprContext,
    exprs: Vec<CoercibleScalarExpr>,
    force_type: Option<&SqlScalarType>,
) -> Result<Vec<HirScalarExpr>, PlanError> {
    assert!(!exprs.is_empty());

    let target_holder;
    let target = match force_type {
        Some(t) => t,
        None => {
            let types: Vec<_> = exprs.iter().map(|e| ecx.scalar_type(e)).collect();
            target_holder = typeconv::guess_best_common_type(ecx, &types)?;
            &target_holder
        }
    };

    // Try to cast all expressions to `target`.
    let mut out = Vec::new();
    for expr in exprs {
        let arg = typeconv::plan_coerce(ecx, expr, target)?;
        let ccx = match force_type {
            None => CastContext::Implicit,
            Some(_) => CastContext::Explicit,
        };
        match typeconv::plan_cast(ecx, ccx, arg.clone(), target) {
            Ok(expr) => out.push(expr),
            Err(_) => sql_bail!(
                "{} could not convert type {} to {}",
                ecx.name,
                ecx.humanize_sql_scalar_type(&ecx.scalar_type(&arg), false),
                ecx.humanize_sql_scalar_type(target, false),
            ),
        }
    }
    Ok(out)
}

/// Creates a `ColumnOrder` from an `OrderByExpr` and column index.
/// Column index is specified by the caller, but `desc` and `nulls_last` is figured out here.