`HAVING`, `OFFSET`, and post-`GROUP BY` `ORDER BY` are parsed then silently discarded

[mp-grind]

Summary

Three of the SQL features listed in one README bullet do not take effect. None of
them raise an error — the query succeeds and returns a wrong result, which makes
them easy to build on without noticing.

README (line 49) claims:

GROUP BY ... HAVING, LIMIT ... OFFSET, UNION/UNION ALL/INTERSECT/EXCEPT: standard aggregate filtering, pagination, and set operations across SELECTs

Observed on mq-db 0.2.1: HAVING is ignored, OFFSET is ignored, ORDER BY
after a GROUP BY is ignored, and the set operators error out.

Reproduction

mkdir -p /tmp/mqbug/docs && cd /tmp/mqbug
printf '# Alpha\n\nIntro paragraph.\n\n## Install\n\nRun the installer.\n' > docs/a.md
printf '# Beta\n\nAnother paragraph.\n\n## Usage\n\nUse it well.\n'       > docs/b.md
mq-db index docs -o s.mq-db

That yields 2 documents and 8 blocks, 4 per document (blocks.id = 0..7).

1. HAVING is ignored

$ mq-db sql -d s.mq-db -F json \
  "SELECT document_id, COUNT(*) AS n FROM blocks GROUP BY document_id HAVING COUNT(*) > 100"
[{"document_id":0,"n":4},{"document_id":1,"n":4}]

Expected: zero rows (no group has more than 100 blocks). Actual: every group,
identical to the same query without the HAVING.

2. OFFSET is ignored

$ mq-db sql -d s.mq-db -F json "SELECT id FROM blocks ORDER BY id LIMIT 2 OFFSET 4"
[{"id":0},{"id":1}]

Expected: [{"id":4},{"id":5}]. Actual: the same rows as LIMIT 2 with no
offset.

3. ORDER BY after GROUP BY is ignored

$ mq-db sql -d s.mq-db -F json \
  "SELECT document_id, COUNT(*) AS n FROM blocks GROUP BY document_id ORDER BY document_id DESC"
[{"document_id":0,"n":4},{"document_id":1,"n":4}]

Expected document_id 1 then 0. ORDER BY works correctly on an ungrouped
query, so this is specific to the aggregate path.

4. Set operators error (loud, but contradicts the README)

$ mq-db sql -d s.mq-db "SELECT path FROM documents UNION ALL SELECT path FROM documents"
Error: SQL execution error: unsupported query type
$ mq-db sql -d s.mq-db "SELECT path FROM documents EXCEPT SELECT path FROM documents"
Error: SQL execution error: unsupported query type

This one fails safely, so it's much less of a problem than 1–3 — but since it is
in the same README bullet, either the implementation or the bullet needs a
change.

EXPLAIN confirms the clauses are dropped, not mis-evaluated

$ mq-db sql -d s.mq-db "EXPLAIN SELECT document_id, COUNT(*) AS n FROM blocks GROUP BY document_id HAVING COUNT(*) > 100"
│ query:from     │ blocks (blocks)  │
│ query:where    │ none — full scan │
│ query:group-by │ 1 key(s)         │

$ mq-db sql -d s.mq-db "EXPLAIN SELECT id FROM blocks ORDER BY id LIMIT 2 OFFSET 4"
│ query:from     │ blocks (blocks)  │
│ query:where    │ none — full scan │
│ query:order-by │ id ASC           │
│ query:limit    │ 2                │

There is no having step and no offset step. The parser accepts the clauses;
the planner never emits them.

The underlying machinery already works

Doing the same filtering and sorting one level out, over a CTE, gives the right
answers — including the negative case:

$ mq-db sql -d s.mq-db -F json "WITH c AS (SELECT document_id, COUNT(*) AS n FROM blocks GROUP BY document_id) SELECT * FROM c WHERE n > 2"
[{"document_id":0,"n":4},{"document_id":1,"n":4}]

$ mq-db sql -d s.mq-db -F json "WITH c AS (...same...) SELECT * FROM c WHERE n > 4"
[]

$ mq-db sql -d s.mq-db -F json "WITH c AS (...same...) SELECT * FROM c ORDER BY document_id DESC"
[{"document_id":1,"n":4},{"document_id":0,"n":4}]

So aggregation, post-aggregate filtering, and post-aggregate sorting are all
correct. Only the inline HAVING / post-GROUP BY ORDER BY clauses are not
routed into them. That also gives users a workaround in the meantime.

Suggested resolution

Whatever the fix timeline, rejecting an unsupported clause is much better than
ignoring it. A HAVING that returns unfiltered rows is a wrong answer a caller
will act on; Error: HAVING is not supported is one they can see. The set
operators already behave this way, and that's the right behavior.

Environment

• mq-db 0.2.1, installed via cargo install
• macOS 26.6.2 (build 25G83), arm64

[harehare]

Thanks for the report.
The ordering fix is on main now, landing in the next release.