Bare ::numeric / CAST(... AS numeric) cast (no typmod) truncates to scale 0

[StefanSteiner]

Summary

A cast to unconstrained NUMERIC (no precision/scale typmod) rounds the value to an integer (scale 0) instead of preserving the fractional digits. Specifying a typmod works correctly.

Discovered alongside #84 (the negative-sub-unit Display sign bug) during an analysis session; filing separately so it can be tracked after #84 closes.

Reproduction

SELECT
  0.859::numeric        AS bare_cast,    -- returns 1.0    (expected 0.859)
  CAST(0.859 AS numeric) AS bare_cast2,  -- returns 1.0    (expected 0.859)
  123.456::numeric      AS bare_big,     -- returns 123.0  (expected 123.456)
  0.859::numeric(10,4)  AS with_typmod;  -- returns 0.859  (correct)

For comparison, PostgreSQL treats unconstrained numeric as "any precision/scale" and preserves 0.859.

Why this matters

It is an easy, silent footgun in analytical SQL: AVG(...)::numeric, (a/b)::numeric, correlation/ratio expressions, etc. all quietly round to integers unless the author remembers to add a typmod. Combined with #84 it produced sign- and magnitude-wrong numbers in an LLM-driven analysis.

Open question / where to look

Need to determine the layer:

1. Is hyperd itself returning the cast result typed as NUMERIC(_, 0) for an unconstrained cast, or returning a value already rounded to scale 0?
2. Or is the Rust API defaulting scale = 0 when the RowDescription typmod is -1 (unconstrained) during SqlType::Numeric { scale, .. } parsing? See the numeric scale extraction in the protocol/types layer and how row.get::<Numeric>() obtains scale.

If (2), the fix is to carry through the actual decimal scale (or treat unconstrained numeric as a high default scale) rather than defaulting to 0. If (1), it is an upstream hyperd semantics question.

Notes

• Workaround for users today: always specify a typmod, e.g. ::numeric(38, 10).
• Related: NUMERIC values in (-1, 0) lose their sign in Display (e.g. -0.5 renders as 0.5) #84 (negative sub-unit NUMERIC sign loss in Display).