Fix Decimal read as System.Decimal losing the column scale

[retvain]

Summary

Fixes #654.

A Decimal column read as System.Decimal lost its scale whenever the value had trailing fractional zeros:
7 in a Decimal(18, 4) column came back as 7m instead of 7.0000m. DecimalType.ReadDecimal divided the
mantissa by 10^scale, and System.Decimal division drops trailing zeros. The value is now built from the
mantissa and the column scale directly, the same way (decimal)ClickHouseDecimal and the native TCP client
(DecimalColumnCodec.MakeDecimal) already do.

Affected paths:

• GetValue, GetDecimal and GetFieldValue<decimal> with UseCustomDecimals=false;
• a decimal property read through QueryAsync<T>, with any UseCustomDecimals value (the POCO fast path reads
  a decimal property through the typed decimal reader).

When a value does not fit System.Decimal at the column's scale (more than 28 fractional digits or a mantissa
wider than 96 bits), the existing behaviour is kept: trailing zeros are dropped only as far as needed
(toDecimal128(7, 30) reads as 7. followed by 28 zeros, toDecimal128('1000000000', 20) with scale 19), and a
value that still does not fit throws OverflowException.

Behaviour change

The numeric value does not change, but the scale does, so ToString(), decimal.GetBits and serialized output
do. In particular, since #467 a scalar decimal written to Dynamic (or to an unhinted JSON path with
JsonWriteMode.Binary) is stored with a padded scale: 1.2m is stored as Decimal(9, 8). Read back as
decimal, it was 1.2 before this change and is 1.20000000 now, which matches what (decimal)clickHouseDecimal
already returned.

Tests

ClickHouse.Driver.Tests/Types/DecimalReadScaleTests.cs: 18 cases against a real server (all four widths, zero,
negative values including the Decimal64 minimum, scale 0, scale 28 and above, a mantissa wider than 96 bits), each run through

• GetValue / GetDecimal / GetFieldValue<decimal> with UseCustomDecimals=false;
• QueryAsync<T> into a decimal property with the default settings.

The existing type matrix compares decimals by value only, so it does not catch a lost scale. Without the fix,
30 of the 36 tests fail. Passing on net9.0 and net6.0.

Benchmark

New SelectColumn.SelectDecimal{64,128,256}AsSystemDecimal benchmarks read through a UseCustomDecimals=false
connection; the existing SelectDecimal* ones read ClickHouseDecimal and do not reach this code. Local
ClickHouse 26.9, 500k rows, median:

Method	Before	After	Allocated
SelectDecimal64AsSystemDecimal	33.00 ms	27.87 ms	18 KB → 18 KB
SelectDecimal128AsSystemDecimal	42.48 ms	40.48 ms	14972 KB → 14972 KB
SelectDecimal256AsSystemDecimal	47.04 ms	45.80 ms	14972 KB → 14972 KB
SelectDecimal64 (unchanged code, for scale)	32.44 ms	33.06 ms	—

No regression and no new allocations. The differences are within run-to-run noise (about ±5% on the unchanged
rows), so I am not claiming a speed-up.

Checklist

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Copilot review overview

🟡 Changes recommended

The added benchmark connection owns resources but is never disposed.

Review effort: Balanced
Findings: 1

Open (1)

• Dispose ClickHouse clients during benchmark cleanup · New

What changed in this PR

Fixes HTTP client decimal deserialization to preserve the ClickHouse column scale when representable by System.Decimal.

Changes:

• Constructs decimals directly from mantissa and scale.
• Adds integration coverage and performance benchmarks.
• Updates documentation and changelog.

File	Description
ClickHouse.Driver/​Types/​DecimalType.cs	Preserves decimal scale during reads.
ClickHouse.Driver.Tests/​Types/​DecimalReadScaleTests.cs	Tests scale retention and range trimming.
ClickHouse.Driver.Benchmark/​SelectColumn.cs	Benchmarks System.Decimal decoding.
docs/​http.mdx	Documents scale-preserving behavior.
changelog.d/​654-decimal-scale.fixes.md	Records the bug fix.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation is consistent with existing decimal conversion logic and includes thorough tests, documentation, benchmarks, and resource cleanup.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Dispose ClickHouse clients during benchmark cleanup

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[alex-clickhouse]

LGTM, thank you for the PR!