doc: Add documentation for pushing limit into plan (#20271)

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## Rationale for this change

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Besides pushing `LimitExec` down the query plan, there is another
optimization that allows plan nodes to *absorb* a limit, so it can
potentially stop early.

I’ve noticed that this form of limit absorption has not been implemented
by many operators. This suggests the optimization is non-obvious, so I’d
like to improve the documentation for it.

A recent PR that implements this optimization is:
- #20228

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Author: 2010YOUY01

datafusion/physical-optimizer/src/limit_pushdown.rs

@@ -17,6 +17,47 @@
 
 //! [`LimitPushdown`] pushes `LIMIT` down through `ExecutionPlan`s to reduce
 //! data transfer as much as possible.
+//!
+//! # Plan Limit Absorption
+//! In addition to pushing down [`LimitExec`] in the plan, some operators can
+//! "absorb" a limit and stop early during execution.
+//!
+//! ## Background: vectorized volcano execution model
+//! DataFusion uses a batched volcano model. For most operators, output is
+//! produced in batches of `datafusion.execution.batch_size` (default 8192), so
+//! the batch sizes typically look like:
+//! ```text
+//! 8192, 8192, ..., 8192, 100 (the final batch may be partial)
+//! ```
+//!
+//! ## Example
+//! For a join with an expensive, selective predicate:
+//! ```text
+//! LimitExec(fetch=10)
+//! -- NestedLoopJoinExec(on=expr_expensive_and_selective)
+//! --- DataSourceExec()
+//! --- DataSourceExec()
+//! ```
+//!
+//! Under this model, `NestedLoopJoinExec` would keep working until it can emit
+//! a full batch (8192 rows), even though the query only needs 10. If the limit
+//! cannot be pushed below the join, we can still embed it inside the join so it
+//! stops once the limit is satisfied. The transformed plan looks like:
+//!
+//! ```text
+//! NestedLoopJoinExec(on=expr_expensive_and_selective, fetch=10)
+//! --- DataSourceExec()
+//! --- DataSourceExec()
+//! ```
+//!
+//! ## Implementation
+//! The current optimizer rule optionally pushes `fetch` requirements into
+//! operators via [`ExecutionPlan::with_fetch`].
+//!
+//! To support early termination in operators, [`LimitedBatchCoalescer`](https://docs.rs/datafusion/latest/datafusion/physical_plan/coalesce/struct.LimitedBatchCoalescer.html)
+//! can help manage the output buffer.
+//!
+//! Reference implementation in Hash Join: <https://github.com/apache/datafusion/pull/20228>
 
 use std::fmt::Debug;
 use std::sync::Arc;

datafusion/physical-plan/src/execution_plan.rs

@@ -579,6 +579,10 @@ pub trait ExecutionPlan: Debug + DisplayAs + Send + Sync {
 
     /// Returns a fetching variant of this `ExecutionPlan` node, if it supports
     /// fetch limits. Returns `None` otherwise.
+    ///
+    /// See physical optimizer rule [`limit_pushdown`] for details.
+    ///
+    /// [`limit_pushdown`]: https://docs.rs/datafusion/latest/datafusion/physical_optimizer/limit_pushdown/index.html
     fn with_fetch(&self, _limit: Option<usize>) -> Option<Arc<dyn ExecutionPlan>> {
         None
     }