[coverage] Conformance findings: PARAMQUERY-022

從 DecimalParameter._cast_expr 開始，檢查 issue 中描述的 CAST_EXPR.format 呼叫。使用 coverage PR 的 tests/ diff 定位 test_decimal_target_carries_declared_precision_and_scale，然後執行該測試；完成的標準是 SELECT ? AS v 保留宣告的 DECIMAL(10,2) 形狀，且預期的 123.45 Decimal128 結果通過。

Summary

Surfaced by the multi-language coverage fan-out while conformance-testing these SPEC-IDs against databricks/databricks-sql-python. Each finding is committed as an expected-failure (xfail) test in the coverage PR — the test asserts the CORRECT (post-fix) behavior and stays red until THIS driver (databricks/databricks-sql-python) is fixed, then flips green as a tripwire.

Findings

• PARAMQUERY-022 [thrift]: DecimalParameter._cast_expr formats the DECIMAL cast with its arguments transposed — CAST_EXPR.format(self.scale, self.precision) — so a declared precision=10/scale=2 target emits DECIMAL(2,10) (scale greater than precision, invalid) instead of DECIMAL(10,2), losing the declared decimal shape on the wire
  • failing test: test_decimal_target_carries_declared_precision_and_scale (see the coverage PR diff under tests/)

Reproduce & Expected

PARAMQUERY-022 — Verify a DECIMAL/NUMERIC target with a declared precision and scale sends the parameter as a DECIMAL carrying THAT precision/scale, so the bare-marker result column is a decimal of the declared shape and the fractional digits survive.

Reproduce:

SELECT ? AS v

Expected (per the shared spec):

• result has exactly 1 row(s)
• col 0, row 0 == '123.45' (type Decimal128)
• full assertion contract:

result:
- row_count: 1
- column:
    index: 0
    row: 0
    type: Decimal128
    equals: '123.45'
- result_column_type_matches_target: true

Context

• The behavior was first fixed in a DIFFERENT driver — reference PR: https://github.com/databricks/databricks-odbc/pull/167 — which seeded the shared language-neutral spec. This issue tracks the same conformance gap in databricks/databricks-sql-python; the reference PR is for cross-referencing the intended behavior, NOT a change to this repo.
• Coverage PR carrying the reproducing xfail test(s): https://github.com/databricks/databricks-driver-test/pull/1061