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+- Start Date: 2026-02-25
+- RFC PR: [vortex-data/rfcs#0000](https://github.com/vortex-data/rfcs/pull/19)
+
+## Summary
+
+Replace the current execution model with a scheduler-driven design. `reduce` and
+`reduce_parent` remain metadata-only. `execute` returns an `ExecutionStep` — requesting a child
+be executed or declaring completion. The scheduler drives all iteration using an explicit work
+stack, runs reduce rules between steps, and caches results for shared sub-expressions.
+
+## Motivation
+
+**Stack overflow from recursion.** The current executor recurses into children. Deep encoding
+trees overflow the stack. The 128-iteration limit applies to the outer loop, not inner
+recursive calls.
+
+**Repeated decompression.** Some operations access the same compressed child multiple times.
+Each access decompresses independently. For example, binary search on RunEnd's PCodec-compressed
+ends calls `scalar_at` per probe, each decompressing the full array.
+
+**Shared sub-expressions.** `a < 10 & a > 5` references the same array `a` twice. The current
+model executes it independently for each reference.
+
+**Unclear execute/reduce boundary.** Some `execute_parent` implementations are metadata-only
+and belong in `reduce_parent`. The boundary isn't enforced.
+
+## Design
+
+### VTable methods
+
+```rust
+// Metadata-only rewrites. Never read data buffers.
+fn reduce(array: &Self::Array) -> VortexResult<Option<ArrayRef>>;
+fn reduce_parent(
+    array: &Self::Array, parent: &ArrayRef, child_idx: usize,
+) -> VortexResult<Option<ArrayRef>>;
+
+// Execution. May read data buffers.
+fn execute(
+    array: &Self::Array, ctx: &mut ExecutionCtx,
+) -> VortexResult<ExecutionStep>;
+fn execute_parent(
+    array: &Self::Array, parent: &ArrayRef, child_idx: usize, ctx: &mut ExecutionCtx,
+) -> VortexResult<Option<ArrayRef>>;
+```
+
+```rust
+pub enum ExecutionStep {
+    /// Execute the child at this index to columnar, replace it,
+    /// then call execute on this array again.
+    ExecuteChild(usize),
+
+    /// Execution is complete.
+    Done(Columnar),
+}
+```
+
+**reduce / reduce_parent** are unchanged. Strictly metadata-only. The framework runs them to a
+fixpoint before execution and between execution steps. Implementations currently misplaced in
+`execute_parent` that are metadata-only (Dict + Compare, ALP + Compare, FoR + Compare, FSST +
+Compare) move to `reduce_parent`.
+
+**execute** returns `ExecutionStep`. The encoding never recurses into children — it yields
+control to the scheduler.
+
+- `ExecuteChild(i)` asks the scheduler to execute child `i` to columnar, replace it, and call
+  `execute` again.
+- `Done(columnar)` returns the final columnar result.
+
+**execute_parent** returns `Option<ArrayRef>`. `None` means the child can't handle this parent.
+`Some(result)` means it handled the parent — the result can be in **any encoding**, not just
+canonical, enabling encoding-preserving execution (e.g., FSST returning a filtered FSST array
+to a DuckDB exporter).
+
+### Constant handling
+
+When an encoding matches on a specific child type (e.g., `as_opt::<PrimitiveVTable>()`), a
+`ConstantArray` child won't match — it's columnar but not Primitive. Encodings should either
+check for constants explicitly, or — preferably — register a reduce rule that handles the
+constant case before execution runs. For example:
+`Dict(Constant(code), values)` → `Constant(values.scalar_at(code))`.
+
+Note: an encoding that blindly returns `ExecuteChild(i)` when its child is already columnar
+(but doesn't match the expected concrete type) will loop forever. This is trivial for the
+scheduler to detect — if `ExecuteChild(i)` is returned and child `i` is already columnar, the
+scheduler can abort with an error.
+
+### Scheduler
+
+The scheduler uses an explicit work stack, bounding stack depth regardless of encoding tree
+depth.
+
+```rust
+fn execute_to_columnar(root: ArrayRef, ctx: &mut ExecutionCtx) -> VortexResult<Columnar> {
+    let mut current = optimize_recursive(root, ctx)?;
+    let mut stack: Vec<(ArrayRef, usize)> = vec![];  // (parent, child_idx)
+
+    loop {
+        // Check if current is columnar — return to parent or finish
+        if let Some(c) = current.as_columnar() {
+            match stack.pop() {
+                None => return Ok(c),
+                Some((parent, child_idx)) => {
+                    current = parent.with_child(child_idx, c.into_array());
+                    current = optimize_recursive(current, ctx)?;
+                    continue;
+                }
+            }
+        }
+
+        // Try execute_parent
+        if let Some(rewritten) = try_execute_parent(&current, ctx)? {
+            current = optimize_recursive(rewritten, ctx)?;
+            continue;
+        }
+
+        // Execute
+        match current.vtable().execute(&current, ctx)? {
+            ExecutionStep::ExecuteChild(i) => {
+                let child = current.child(i);
+                stack.push((current, i));
+                current = optimize_recursive(child, ctx)?;
+            }
+            ExecutionStep::Done(result) => {
+                current = result.into_array();
+            }
+        }
+    }
+}
+```
+
+### Execution cache
+
+If the same `Arc<dyn Array>` appears as a child of multiple parents (e.g., `a < 10 & a > 5`),
+the scheduler executes it independently each time. The execution cache deduplicates this work.
+
+The cache lives in `ExecutionCtx`, keyed by `ByPtr<ArrayRef>` — a newtype that implements
+`Hash` and `Eq` via `Arc::as_ptr()`. Because the key holds a clone of the `Arc`, the source
+array cannot be deallocated while cached, so the pointer cannot be reused for a different array.
+
+```rust
+struct ByPtr(ArrayRef);
+
+impl Hash for ByPtr {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        Arc::as_ptr(&self.0).hash(state);
+    }
+}
+
+impl PartialEq for ByPtr {
+    fn eq(&self, other: &Self) -> bool {
+        Arc::as_ptr(&self.0) == Arc::as_ptr(&other.0)
+    }
+}
+
+pub struct ExecutionCtx {
+    cache: HashMap<ByPtr, ArrayRef>,  // source → one-step result
+    // ...
+}
+```
+
+**When to cache.** Three options:
+
+1. **Always cache.** Memory explodes — most arrays are executed once and never revisited.
+
+2. **Pre-scan the tree.** Walk the tree before execution, count pointer occurrences, cache only
+   shared nodes. Accurate for a single tree, but doesn't work across multiple trees executed
+   with the same `ExecutionCtx` (e.g., scan columns sharing a dictionary or filter mask).
+
+3. **`Arc::strong_count > 1` heuristic.** O(1) to check. Over-caches when external references
+   exist (layout reader, scan builder) but never under-caches for a genuinely shared
+   sub-expression. Works across independent tree executions within the same `ExecutionCtx`.
+   False positives cost memory, not correctness.
+
+We use option 3. The cache is dropped when the `ExecutionCtx` is dropped.
+
+### Examples
+
+**DictArray** — execute codes into Primitive, then gather:
+
+```rust
+fn execute(dict: &DictArray, ctx: &mut ExecutionCtx) -> VortexResult<ExecutionStep> {
+    let Some(codes) = dict.codes().as_opt::<PrimitiveVTable>() else {
+        return Ok(ExecutionStep::ExecuteChild(0));
+    };
+    let gathered = gather(dict.values(), codes, ctx)?;
+    Ok(ExecutionStep::Done(gathered))
+}
+```
+
+Note: if codes is a `ConstantArray`, the scheduler returns it as columnar. A reduce rule
+`Dict(Constant(code), values) → Constant(values.scalar_at(code))` handles this before execute
+runs.
+
+**ScalarFnArray** — columnarize children left-to-right, then evaluate:
+
+```rust
+fn execute(sfn: &ScalarFnArray, ctx: &mut ExecutionCtx) -> VortexResult<ExecutionStep> {
+    for (i, child) in sfn.children().iter().enumerate() {
+        if child.as_opt::<AnyColumnar>().is_none() {
+            return Ok(ExecutionStep::ExecuteChild(i));
+        }
+    }
+    let result = sfn.scalar_fn().execute(sfn.columnar_children(), ctx)?;
+    Ok(ExecutionStep::Done(result))
+}
+```
+
+Left-to-right ordering is deterministic and simple. ScalarFn designers should consider this
+when choosing input order — the first input is executed first.
+
+**FilterArray** — columnarize child, then apply mask:
+
+```rust
+fn execute(filter: &FilterArray, ctx: &mut ExecutionCtx) -> VortexResult<ExecutionStep> {
+    let Some(child) = filter.child().as_opt::<AnyCanonical>() else {
+        return Ok(ExecutionStep::ExecuteChild(0));
+    };
+    let filtered = filter.mask().apply_to(child.into())?;
+    Ok(ExecutionStep::Done(Columnar::Canonical(filtered)))
+}
+```
+
+**BitPacked** — leaf, decompresses directly:
+
+```rust
+fn execute(bp: &BitPackedArray, ctx: &mut ExecutionCtx) -> VortexResult<ExecutionStep> {
+    Ok(ExecutionStep::Done(Columnar::Canonical(Canonical::Primitive(unpack(bp)?))))
+}
+```
+
+### Cross-step optimization
+
+`optimize_recursive` runs when the scheduler pops from the stack (after a child reaches
+columnar and is replaced into its parent). Reduce rules fire on the new tree shape.
+
+Example: `ScalarFn(upper, [Dict(BitPacked(codes), values)])`. Pre-execution optimization fires
+a reduce_parent rule on Dict, pushing `upper` into values. The tree becomes
+`Dict(BitPacked(codes), upper(values))`. Dict then executes its codes into Primitive and
+gathers.
+
+### Encoding-preserving execution
+
+`execute_parent` returns results in any encoding. Exporters drive the scheduler loop and
+inspect the tree after each step. If an encoding the exporter cares about becomes visible
+(DictArray for DuckDB dictionary vectors, FSST for DuckDB FSST vectors), the exporter
+intercepts it without decompressing.
+
+### Removing ExecutionCtx from VTable methods
+
+The `execute` and `execute_parent` signatures shown above accept `&mut ExecutionCtx`. This gives
+encodings the ability to recursively execute children, bypassing the scheduler's caching and
+cross-step optimization. Nothing in the type system prevents it.
+
+A stronger design: remove `ExecutionCtx` from the VTable method signatures entirely. The
+scheduler owns the execution state (cache, tracing). `execute` receives no context. The method signature itself
+communicates "return a step, don't execute anything."
+
+This also eliminates the current ergonomic friction of
+`let ctx = session.create_execution_ctx(); array.execute(&mut ctx)` — callers just call the
+scheduler directly.
+
+If `execute_parent` also yields `ExecutionStep` (see unresolved questions), the same argument
+applies: it gets resource access but not execution power. The scheduler is the only code that
+drives execution.
+
+### Decompress-into-buffer
+
+This model does not support caller-owned output buffers. Each `Done(Columnar)` allocates its
+own output. ChunkedArray cannot share a builder across chunks, and FSST cannot push views
+directly into a caller's VarBinViewBuilder. This is a trade-off we accept in exchange for
+encoding-preserving execution, cross-step optimization, and bounded stack depth.
+
+## Alternatives
+
+### Canonical builder model
+
+`execute` pushes results into a caller-owned `CanonicalBuilder` (closed enum mirroring
+`Canonical` in mutable builder form). Single recursive descent, no iteration loop. Natively
+supports decompress-into-buffer: ChunkedArray writes all chunks into one builder, FSST pushes
+views directly into VarBinViewBuilder.
+
+However, always producing canonical output has structural limitations:
+
+- **No encoding-preserving execution.** `execute_parent` pushes into a builder, so results are
+  always canonical. FSST can handle a FilterArray parent (fused filter + decompress into the
+  builder), but it can't return a filtered FSST array for a DuckDB exporter. Exporters that
+  want to preserve an encoding must use encoding-specific logic outside the framework.
+- **No cross-step optimization.** Reduce runs once before the single descent.
+- **Stack overflow.** Recurses into children; stack depth equals encoding tree depth.
+
+The scheduler model's wins (encoding preservation, cross-step optimization, bounded stack) are
+structurally unrecoverable in the builder model. The builder model's win (zero-copy
+decompress-into-buffer) is a real cost of the scheduler model, but one we accept given the
+asymmetry.
+
+## Unresolved Questions
+
+- **Explicit work stack details.** The scheduler sketch shows the concept. The exact data
+  structure (e.g., handling multiple `ExecuteChild` calls from the same parent for different
+  children) needs design work.
+
+- **Iterative execute_parent.** The current design has `execute_parent` return
+  `Option<ArrayRef>`. An alternative is to return `Option<ExecutionStep>`, allowing it to
+  request child execution before handling the parent. The execution cache may be sufficient for
+  cases where `execute_parent` needs data access to children (e.g., RunEnd binary-searching
+  compressed ends — the cache ensures decompression happens once). We have no compelling
+  example for or against iterative `execute_parent` yet.
+
+- **Targeted child execution.** `ExecuteChild(i)` currently executes the child to columnar.
+  An alternative is to allow the encoding to specify a matcher for when it should be re-entered
+  — e.g., `ExecuteChildInto(i, ArrayId)` would execute child `i` until it matches a specific
+  encoding or reaches columnar, whichever comes first. This enables early re-entry before full
+  columnarization — for example, an encoding could request its child be executed until it becomes
+  a DictArray, then operate on the dictionary directly. The trade-off is additional complexity in
+  the scheduler and ExecutionStep enum.
+
+- **FilterArray.** FilterArray continues to exist as a lazy wrapper. It is not subsumed by the
+  execution method. Whether to unify Filter/Slice/Take wrappers is orthogonal.