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feat(engine): cost-based Expand dispatch chooser (C5)

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Replace the fixed frontier<=1024 && hops<=6 dispatch threshold with a pure,
IO-free cost model. choose_expand_mode compares the indexed path's
frontier-relative work (hops * frontier * fanout, or hops * |E| when BTREE
coverage is degraded) against the cost of building the whole-graph CSR
(BUILD_FACTOR * |E|), from cheap manifest row counts. Under good coverage this
reduces to a selectivity ratio independent of |E|, preserving the flat-in-|E|
indexed win for selective traversals while routing dense / deep / high-fanout
or degraded-and-expensive traversals to CSR.

execute_expand decides cardinality-first and only opens the edge dataset to
confirm coverage when it leans indexed (no open on a clearly-CSR traversal).
The two env knobs become hard ceilings layered on the model; the
OMNIGRAPH_TRAVERSAL_MODE override still forces a path; the chosen mode is
traced. Results are unchanged across modes — only the path differs.

Adds inline crossover unit tests and extends the traversal_indexed both_modes
harness with an auto pass asserting the chooser is result-preserving across
every traversal shape. Documents the new flag semantics in
docs/user/{constants,query-language}.md.
This commit is contained in:
Ragnor Comerford 2026-06-09 09:12:00 +02:00
parent 236e66c789
commit 787d41ec5f
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4 changed files with 367 additions and 31 deletions
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docs/user/query-language.md
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@ -79,7 +79,7 @@ Reason: under the staged-write rewire (MR-794), inserts and updates accumulate i
Pipeline operations:
- `NodeScan { variable, type_name, filters }`
- `Expand { src_var, dst_var, edge_type, direction (Out|In), dst_type, min_hops, max_hops, dst_filters }` — destination filters are pushed *into* the expand so Lance scalar pushdown can prune. Executed one of two ways, chosen per-expand by frontier size: selective traversals (small frontier) resolve neighbors from the persisted `src`/`dst` BTREE (one indexed scan per hop, cost ∝ frontier); dense / whole-graph traversals fall back to the in-memory CSR adjacency index. Both produce identical results. Tunable via `OMNIGRAPH_EXPAND_INDEXED_MAX_FRONTIER` / `OMNIGRAPH_EXPAND_INDEXED_MAX_HOPS`, with `OMNIGRAPH_TRAVERSAL_MODE=indexed|csr` forcing a mode (see [constants](constants.md)).
- `Expand { src_var, dst_var, edge_type, direction (Out|In), dst_type, min_hops, max_hops, dst_filters }` — destination filters are pushed *into* the expand so Lance scalar pushdown can prune. Executed one of two ways, chosen per-expand by a cost model over cheap manifest counts (frontier size, |E|, source-vertex count, hops) plus index coverage: selective traversals (small frontier relative to the source set) resolve neighbors from the persisted `src`/`dst` BTREE (one indexed scan per hop, cost ∝ frontier); dense / deep / large-frontier traversals — or those whose BTREE coverage is degraded so a full scan would be paid per hop — use the in-memory CSR adjacency index. Both produce identical results. Bounded by `OMNIGRAPH_EXPAND_INDEXED_MAX_FRONTIER` / `OMNIGRAPH_EXPAND_INDEXED_MAX_HOPS` (hard ceilings, beyond which CSR is always used), with `OMNIGRAPH_TRAVERSAL_MODE=indexed|csr` forcing a mode (see [constants](constants.md)).
- `Filter { left, op, right }`
- `AntiJoin { outer_var, inner: Vec<IROp> }` — for `not { … }`