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zzylol merged 2 commits into from
Oct 3, 2026
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docs: clarify common sub-DAG sharing terminology #558

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docs: clarify common sub-DAG sharing terminology
zzylol Oct 3, 2026
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docs: address sub-DAG sharing review comments
zzylol Oct 3, 2026
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8 changes: 7 additions & 1 deletion docs/design_docs/decisions/cse-cost-model.md
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@@ -1,9 +1,15 @@
# CSE sharing: rule-based vs. cost-based framework (issue #237)
# Common sub-DAG sharing: rule-based vs. cost-based framework (issue #237)

> Status: accepted decision for the implementation described here.

## Context

Sub-DAG sharing means multiple consumers reference one operator and its
upstream dependencies. Common sub-DAG sharing uses CSE (common subexpression
elimination) to identify eligible, structurally identical computations. A
shared logical node records an opportunity for reuse; selecting maintained
state or independent execution is a separate planning decision.

[`asap_types::pre_asap::cse::share_common_sub_dags`](../../../crates/types/src/pre_asap/cse.rs)
(issue #223 stages 1-2, PR #235) already *detects* every structurally-identical,
legally-shareable (`Schema::unique_keys`-gated) sub-DAG and shares it
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5 changes: 4 additions & 1 deletion docs/design_docs/proposals/planner-layering.md
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Expand Up @@ -261,7 +261,10 @@ exploits.

#### Pass 2: ASAP-aware common-subexpression elimination

ASAP-aware CSE extends traditional CSE with summary-specific sharing rules.
**Sub-DAG sharing** means several consumers reference one operator and its
upstream dependencies. Traditional CSE provides common sub-DAG sharing for
eligible, structurally identical computations. ASAP-aware CSE extends it with
summary-specific sharing rules.
Computations can share work when they use identical expressions, when one
summary build node supports several estimates, or when one window summary can answer
their overlapping windows.
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6 changes: 3 additions & 3 deletions docs/develop_docs/asap-aware-mapping-architecture.md
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Expand Up @@ -60,9 +60,9 @@ Terminology used in the diagram:
realizes an operation as a concrete ASAP realization; **post-ASAP** means
the resulting realization form.
- A **DAG** (directed acyclic graph) represents query operators whose sub-DAGs
may be shared. **CSE** (common subexpression elimination) finds equivalent
sub-DAGs and represents legal reuse by making them the same shared node.
Rust's `Rc<T>` (reference-counted pointer) records that shared node identity.
may be shared. See [sub-DAG sharing and ASAP-aware CSE](../design_docs/proposals/planner-layering.md#pass-2-asap-aware-common-subexpression-elimination)
for the sharing rules. Rust's `Rc<T>` (reference-counted pointer) records
shared node identity.
- A **target** is one replaceable site. A **candidate** is one valid alternative
for it. `Replacement::Summary` is a constructed post-ASAP summary—maintained state
such as an exact accumulator or an approximate sketch—while
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