CharlesKWON
99f3ca5687
Correlated EXISTS with high-cardinality keys was stuck at O(outer × inner)
because memoization couldn't amortize across unique correlation values.
H3 in the subquery stress bench:
SELECT e.name FROM emp e
WHERE EXISTS (SELECT 1 FROM ord WHERE ord.emp_id = e.id AND ord.qty > 15)
500 outer rows × 500 distinct e.id values × 5000-row ord scan = 10s,
with no path to improvement from caching the subquery result.
Fix: detect the semi-join shape on the subquery and rewrite it at
runtime into a non-correlated DISTINCT scan whose result is cached
as a hash set. Each outer row then becomes an O(1) hash probe.
--- What we lift ---
SELECT ... FROM inner_table
WHERE inner.col = outer.col [AND other_non_correlated_preds]
Shape constraints (all must hold):
- single table, no JOIN
- no GROUP BY, no HAVING, no UNION
- WHERE is an AND tree containing an equi-term where one side is
a column with an alias prefix from the subquery's own FROM
and the other is a column from an outer alias
- the remaining AND terms (non-correlated residue) have no
outer references of their own — rules out patterns like
`WHERE e2.dept = e.dept AND e2.salary > e.salary` where the
second term can't live without the outer context
--- How the lift works ---
1. Walk the WHERE as a flat AND-term list
2. Find and remove the first correlated equi-term, remember the
inner column name and outer column reference
3. Verify residue is non-correlated via a recursive AST walker
(SemiJoinHasOuterRef) — bail to fallback if not
4. Clone hQuery with:
columns = {DISTINCT inner.col}
where = residue (or NIL)
distinct = .T.
limit / top / order_by / group_by / having cleared
5. Run the cloned subquery once via a nested TSqlExecutor — no
PushOuter because it's now non-correlated
6. Build a hash set keyed on SqlValToStr(each distinct inner value)
7. Per EXISTS probe: Resolve the outer column reference, look up
in the hash set
Cached in ::aSemiJoinSlots indexed by xSubNode identity so the
analysis + lifted scan runs exactly once per subquery expression.
Subqueries that don't match the shape store the sentinel "NO" so
subsequent probes skip re-analysis and fall through to the existing
SubqueryCached + LIMIT 1 path.
NOT EXISTS works through the same path — lNegate flag just flips
the final hash-lookup result.
--- Bench (emp=500, prod=100, ord=5k) ---
Pattern Before After Speedup
────────────────────────────────────────────────────────────
H3 EXISTS correlated 10.0s 4.5ms ~2200x
H8 NOT EXISTS self-join 900ms 890ms same (can't lift:
remainder
`e2.salary > e.salary`
is correlated)
H11 Scalar + EXISTS + derived 3.2s 1.0s 3.2x
H8 correctly falls through to the non-lifted path because the
remainder outer-reference check (SemiJoinHasOuterRef) rejects the
`e2.salary > e.salary` term. The 5-row answer is still correct.
Validation:
- FiveSql2 43/43
- Harbour compat 51/51
- go test ./... ALL PASS
- H3 returns 125 rows (matches pre-change correct result)
- H8 returns 5 rows (matches pre-change correct result)
Known pre-existing bug, unrelated: H7 (scalar correlated subquery
with inner INNER JOIN) returns zero for rows 2..N — workarea state
leaks between consecutive subquery invocations. Not touched here,
filed for follow-up.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
FiveSql2 — SQL Engine for Harbour DBF/NTX/CDX
Pratt parser + SQL:1992-2023 full standard support Supports both NTX (Clipper) and CDX (FoxPro/ADS) indexes
Architecture
five_SQL("SELECT ...")
│
├── TSqlLexer Tokenizer
├── TSqlParser2 Pratt parser (data-driven operators)
├── TSqlExecutor Query executor (Volcano model)
│ ├── TSqlAlias Central alias manager (no collisions)
│ ├── TSqlIndex NTX/CDX index optimization (auto-detect)
│ ├── TSqlAgg GROUP BY / aggregation
│ ├── TSqlSort ORDER BY / DISTINCT
│ ├── TSqlDDL CREATE/DROP/ALTER TABLE/INDEX
│ └── TSqlTxn BEGIN/COMMIT/ROLLBACK
├── TSqlExpr AST nodes + expression evaluation
└── TSqlFunc 60+ scalar functions
Build & Test
export PATH="/path/to/harbour-core/bin/linux/gcc:$PATH"
export HB_INSTALL_PREFIX="/path/to/harbour-core"
make # Build all tests
make test # Run all 157 tests
make bench # Parser benchmark
make clean # Clean
SQL Standard Coverage
| Standard | Features | Tests |
|---|---|---|
| SQL:1992 | SELECT, JOIN, GROUP BY, HAVING, Subquery, CASE, CAST | 43 |
| SQL:1999 | CTE, Recursive CTE, Window Functions, MERGE | 10 |
| SQL:2003 | SIMILAR TO, GROUPING SETS, LATERAL, Window frames | 64 |
| SQL:2008 | FETCH/OFFSET, FOR UPDATE, Extended MERGE | (incl.) |
| SQL:2016 | JSON functions, LISTAGG | (incl.) |
| SQL:2023 | ANY_VALUE, GREATEST/LEAST, BOOL_AND/OR | (incl.) |
| Challenge | LeetCode-level complex queries | 15 |
| Extreme | Production analytics stress tests | 15 |
Adding New Operators
Edit TSqlParser2.prg, method InitInfixTables():
::hInfixTT[ TK_MYOP ] := { "<=>", 40, 41, ND_BIN }
One line. No structural changes needed.
Copyright
Copyright (c) 2025-2026 Charles KWON (Charles KWON OhJun) Email: charleskwonohjun@gmail.com All rights reserved.