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commands: the $group accumulators

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Nine of them -- `$sum`, `$avg`, `$min`, `$max`, `$first`, `$last`, `$push`,
`$addToSet`, `$count` -- and the corpus goes 9 pass / 10 fail to 18 pass /
1 fail. The one left is the compound `_id`, which needs the expression
evaluator.

Landed before that evaluator, against the tier order the design review set
out, and the corpus is why: every one of these takes a single value per
document, a path or a constant, so nine of its ten failures turned out to be
reachable without one. `classify_expr` already produced exactly that value.

What the recording caught, which is the argument for measuring expectations
rather than writing them:

  - `$avg` over a group with no numeric value is **null**, not `0`. A divisor
    that counted documents rather than numbers would pass every test anybody
    would think to write by hand, and be wrong on the one group that matters.
  - `$min`/`$max` compare across types in canonical BSON order, so the maximum
    of `30`, `7` and `"not a number"` is the string.
  - `$push` skips an absent field but would push an explicit null, so "resolved
    to nothing" and "resolved to null" cannot be the same value internally --
    which is why the accumulators take `?bson.Value` and not `.null`.
  - `$first`/`$last` follow input order, including when the value is absent:
    `$last` of a missing field is null, not the last present one.

`AccState` is one struct rather than a union: the fields are small and every
site already switches on the kind, so a union would add a tag test where a
switch was going to be anyway. Its arrays are the gpa's, the values inside them
the reply arena's -- they outlive the group and travel with the documents.

`numeric_value` is the int32-or-double narrowing MongoDB reports, shared now
between the accumulators and `cmd_aggregate`'s count fast path. It was written
twice before; a divergence between them would make `countDocuments` disagree
with the pipeline it is a shortcut for.

`$avg` and `$push` came out of the Tier 0 refusal test, replaced by
`$stdDevPop` and `$mergeObjects`. The refusal is a property of what is missing
rather than of a list, and the test should read that way.

191/191 unit tests in ReleaseFast and ReleaseSafe, 83/83 fuzz, e2e 49, e2e3 16,
e2e4 17, e2e6 72, e2e7 86, crud corpus unchanged at 201/90/196.
This commit is contained in:
A.Shakhmatov
2026-08-09 22:06:57 +03:00
parent 3044a38d1c
commit 76afa75efe
2 changed files with 163 additions and 41 deletions
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13
tests/spec/aggregate/README.md
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@@ -58,12 +58,15 @@ answer measures the version gap, not the engine.
## Where it stands
Recorded against mongod 8.3.7, run against the M2 tip:
Recorded against mongod 8.3.7. At the M2 tip it read 9 pass / 10 fail; with the
accumulators in:
```
group-accumulators.json 9 pass 10 fail 0 skip
group-accumulators.json 18 pass 1 fail 0 skip
```
The nine include the four refusals M2 added, which answer with mongod's own
codes. The ten are M2.5's work: `$avg`, `$min`, `$max`, `$first`, `$last`,
`$push`, `$addToSet`, `$count`, and a compound `_id`.
The one that remains is the compound `_id`, which needs the expression
evaluator and is the next tier. The corpus found its first real disagreement on
the way there: `$avg` over a group with no numeric value is `null`, not `0`,
and a divisor that counted documents rather than numbers would have passed
every test anybody would think to write by hand.