`mergeByDimensions` joins its dimension key with no delimiter, so two distinct groups can merge into one row

[os-zhuang]

Found while implementing #4708 (packages/services/service-analytics/src/dataset-executor.ts); unrelated to that fix, so filed rather than fixed in it.

Mechanism

mergeByDimensions is the seam every multi-query dataset result is assembled through — supplementary measure-scoped queries and the compareTo pass both merge back onto the grid with it. Its key function:

const keyOf = (row: Record<string, unknown>) =>
  dimensions.map((d) => String(row[d] ?? '')).join('');

Two problems in one line:

1. No delimiter. With two or more dimensions, the concatenation is ambiguous. Rows { region: 'ab', segment: 'c' } and { region: 'a', segment: 'bc' } both key as "abc", so the second row's measures are merged onto the first — one row silently absorbs another group's numbers, and the other group's measure column is left absent (which then renders blank, or gets an empty-group 0 filled into it after A filtered dataset measure returns ABSENT (not 0) for a group its filter excludes, so every derived ratio over it blanks — on exactly the worst-performing row #4708).
2. null and '' collapse. String(row[d] ?? '') maps a genuinely null dimension value and an empty-string one onto the same key, merging "unassigned" into "blank".

The neighbouring cross-object-rebucket.ts already solved exactly this and documents why:

// JSON-encoded, so the empty bucket (`null` on both aggregation paths since
// …) … `"null"` — plain interpolation renders both as `null` and would merge two
for (const f of baseDimFields) keyParts.push(`${f}=${JSON.stringify(row[f] ?? null)}`);

So the repo has the right pattern one file over; mergeByDimensions predates it.

Impact

Only bites datasets grouped by two or more dimensions where the concatenation of adjacent values is ambiguous — a matrix report by region × segment, owner × stage, or any pair of free-text/select values. Single-dimension grids (the common case) are unaffected, which is presumably why it has survived. When it does bite, it is silent: the grid keeps the right number of columns and a plausible number of rows.

Suggested direction

Key the same way cross-object-rebucket does — dimensions.map((d) => ${d}=${JSON.stringify(row[d] ?? null)}).join('&') — which fixes both the delimiter and the null/empty-string conflation in one change. Needs a test with a colliding pair ('ab'|'c' vs 'a'|'bc') and one with a null-vs-empty dimension value.

Note the fix changes only the internal join key, never a value in the response, so it is contained.

Related: #4708.

[os-zhuang]

⚠️ 派发前必读:正文建议的那个键法会引入一个静默的新缺陷;另外本单优先级已被 #4820 抬高

来自 #4820(PR #4870,已复核通过入队)实施过程中的发现。dev 按指示没碰本单,只写进报告 —— 但这条足以改变本单的修法,所以搬到正文旁边。

1. JSON.stringify 键法会顺带改掉类型强制语义

正文建议 ${d}=${JSON.stringify(row[d] ?? null)}。问题在于:

写法	数字 1 与字符串 '1'
现行 String(row[d])	强制成同一个键 → 合并成一组
JSON.stringify(row[d])	1 vs "1" → 分成两组

这不是理论风险 —— dataset-executor.ts 自己的 compareValues 注释就写着:

some drivers return SUM results as numeric strings

而本文件的合并路径要把主查询与每个带 filter 的 measure 的补充子查询按维度键对齐。两条查询若对同一个维度返回不同的 JS 类型(某些驱动确实如此),换成 JSON.stringify 会让原本能正确合并的行不再合并 —— 结果是分组莫名其妙裂开、数字分散到两行,而且完全静默。

用一个新的静默缺陷换掉旧的静默缺陷,不算修好。

2. 建议的替代方向

保留 String() 的逐段强制(维持现有类型合并语义,那是一个已在生产上依赖的行为),只把拼接改成无歧义形式:

// 现在:无分隔符拼接 —— ['a','bc'] 与 ['ab','c'] 撞键
// 建议:长度前缀,天然无歧义
key = dims.map(d => { const s = String(row[d] ?? ''); return s.length + ':' + s; }).join('')

null 与 '' 的区分单独用一个哨兵值处理,不要靠 JSON.stringify 顺带解决 —— 那正是把两件事(消歧 / 类型语义)耦合进一个改动的地方。

⚠️ 若要用 � 之类做分隔符,注意仓内有 check:nul-bytes 门禁:必须写转义序列,不能写裸字节(裸 NUL 会让 grep/ripgrep 把整个文件当二进制、静默返回零匹配,git 也会把它判为二进制)。参考 packages/rest/src/rest-server.ts:1065 的既有写法。#4820 的 dev 刚在这上面红过一轮 CI。

3. 影响面比正文估计的大 —— 优先级上调

#4870 合并后,compare 路径也会扇出并逐 measure 合并(runCompare 现在与 executeSelection 共用 runMeasurePass)。所以一次维度键碰撞现在不只污染当前期的列,还会污染 __compare 列。

正文写这条时,compare 路径还是一条扁平查询、碰不到合并逻辑;现在不是了。

4. 排期

本单与 #4820 同文件(packages/services/service-analytics/src/dataset-executor.ts),我刻意没有同批派发以避开撞车。#4870 合并后本单即可派发,届时请基于合并后的代码工作 —— runMeasurePass / splitMeasuresByFilter 是新抽出的,合并键的调用点已经变了。

Blocked-by: objectstack-ai/objectstack#4820

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