Serve descending ORDER BY from ascending index tags

[chrisrichards]

Closes #294.

Summary

ORDER BY col DESC is now index-served whenever the column has a usable ascending tag — previously any descending order meant a full scan plus in-memory sort:

select * from setup.dbf order by KEY_NAME desc                  → index order scan (descending) on tag 'KEY_NAME'
select CALL_ID from calls.dbf where CALL_ID >= 5 order by CALL_ID desc → index range scan on tag 'CALL_ID'   (no in-memory sort)
select CALL_ID from calls.dbf order by CALL_ID desc limit 3     → 16, 15, 14

How

The planner reverses the ascending tag's entry list — and then re-stabilizes each run of duplicate keys by record index, because a stable descending sort keeps tied rows in their original ascending row order, while a naive reversal would flip them. The duplicate-heavy CONTACT_ID tag (runs of five equal keys) pins this: the reversed index order must match the forced-scan stable sort byte-for-byte, and does. Works for pure order scans, for WHERE candidates on the same column (seeks and ranges), with TOP/LIMIT applied after the order, and through DbfQuery<T>.OrderByDescending. Multi-key orderings and untagged columns still sort in memory, as before.

Deliberate remainder

Tags that are themselves DESCENDING stay excluded from planning. No fixture anywhere contains one, and their on-disk key semantics are unverified (VFP may store complemented key bytes keeping the tree byte-ascending, or store a genuinely reversed tree) — searching them on a guess could silently miss rows, which is the one failure mode this planner never accepts. A follow-up issue tracks that remainder with the verification plan; contributing a VFP file with INDEX ON … TAG … DESCENDING would unblock it immediately.

Test plan

8 new tests in QueryDescendingOrderTests, all differential against forced scans: descending order scans (character and integer tags), the stable-duplicate-run case, descending order satisfied after equality seeks and ranges, LIMIT after descending order (exact sequence asserted), untagged-column fallback, and both builder paths. The phase-6 test that pinned DESC to a full scan is repurposed to a genuinely unsatisfiable multi-key ordering. Full suite: 440 passed, 1 pre-existing skip, zero warnings on both TFMs.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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