viirya commented on code in PR #13469:
URL: https://github.com/apache/datafusion/pull/13469#discussion_r1847131367
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datafusion/physical-plan/src/joins/sort_merge_join.rs:
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@@ -68,8 +68,46 @@ use crate::{
RecordBatchStream, SendableRecordBatchStream, Statistics,
};
-/// join execution plan executes partitions in parallel and combines them into
a set of
-/// partitions.
+/// Join execution plan that executes equi-join predicates on multiple
partitions using Sort-Merge
+/// join algorithm and applies an optional filter post join. Can be used to
join arbitrarily large
+/// inputs where one or both of the inputs don't fit in the available memory.
+///
+/// # Join Expressions
+///
+/// Equi-join predicate (e.g. `<col1> = <col2>`) expressions are represented
by [`Self::on`].
+///
+/// Non-equality predicates, which can not be pushed down to join inputs (e.g.
+/// `<col1> != <col2>`) are known as "filter expressions" and are evaluated
+/// after the equijoin predicates. They are represented by [`Self::filter`].
These are optional
+/// expressions.
+///
+/// # Sorting
+///
+/// Assumes that both the left and right input to the join are pre-sorted. It
is not the
+/// responisibility of this execution plan to sort the inputs.
+///
+/// # "Streamed" vs "Buffered"
+///
+/// Buffered input is buffered for all record batches having the same value of
join key.
+/// If the memory limit increases beyond the specified value and spilling is
enabled,
+/// buffered batches could be spilled to disk. If spilling is disabled, the
execution
Review Comment:
Is there a config for spilling? Shall we mention it here too?
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