perf(fused_group): SYM direct-count fires without WHERE on top-K count#237
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fp_try_direct_count1's SYM slot-array branch in fused_group.c
required `pred_key_ne_zero` (a WHERE-side `Col != 0` predicate that
filters the empty sym up front). Queries without a WHERE clause
(ClickBench q33 — `(select {URL: URL c: (count URL) from: hits
by: URL desc: c take: 10})`) fall through to `if (ctx->kt == RAY_SYM)
return NULL`, then the dispatcher lands on `fp_par_fn` (linear-probe
HT, single shard per worker). On q33 with 2.6 M distinct URLs the
shard is ~10 MB per worker, every probe an L3 miss; 22 % of total
cycles spent there.
Accept the no-WHERE case too: count every row including the empty
sym at slot 0. The downstream top-K heap ranks slot 0 against the
rest, so a heavy empty sym (q22 / q21 shape) ends up in the result
legitimately, and a sparse empty sym just disappears below keep_min.
Split the row scan into two specialisations (pred_key_ne_zero vs
no-WHERE) so the hot loop has no per-row branch.
ClickBench 10M:
q33 ~397 → ~25 ms (-94%, -372ms)
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The SYM slot-array fast path in fp_try_direct_count1 required a
Col != 0WHERE predicate to fire — it skipped any query withouta WHERE clause and fell through to the generic linear-probe HT.
Relax the gate: when there is no WHERE, every row participates, so
the slot-array count is correct as-is. Inner loop branches on the
WHERE presence to either filter the empty-sym slot (pred path) or
count every row (no-WHERE path).