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https://issues.apache.org/jira/browse/SPARK-58611?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ASF GitHub Bot updated SPARK-58611:
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Labels: pull-request-available (was: )
> Left anti stream-stream join
> ----------------------------
>
> Key: SPARK-58611
> URL: https://issues.apache.org/jira/browse/SPARK-58611
> Project: Spark
> Issue Type: Improvement
> Components: Structured Streaming
> Affects Versions: 4.3.0
> Reporter: Ganesha S
> Priority: Major
> Labels: pull-request-available
>
> Stream-stream join currently supports Inner, LeftOuter, RightOuter, FullOuter
> and LeftSemi. LeftAnti is rejected at analysis time:
> {noformat}
> LeftAnti joins with a streaming DataFrame/Dataset on the right are not
> supported
> {noformat}
> This leaves the common "find left rows with no match on the right" pattern
> without a native streaming implementation: impressions without clicks, orders
> without shipments, sessions without conversion, alerts without
> acknowledgement. Users work around it with NOT IN / NOT EXISTS rewrites or
> hand-written transformWithState logic, both more expensive and easy to get
> subtly wrong.
> LeftSemi was added under this umbrella in SPARK-32862; its complement was
> never done. Note stream-static LEFT ANTI already works when only the left
> side is streaming -- only a streaming right side is rejected -- so the gap is
> specifically stream-stream.
> h2. Approach
> Unlike left semi, left anti cannot emit while joining. A semi match is
> positively determined: the moment a left row matches, it can be emitted. "No
> match exists" is only decidable once the watermark guarantees no future right
> row can match. Left anti is therefore implemented on the eviction path,
> reusing the existing left outer plumbing in
> \{{StreamingSymmetricHashJoinExec}}:
> * nothing is emitted when a left row matches;
> * at left-side state eviction, rows whose \{{matched}} flag is false are
> emitted as bare left rows (left outer emits them joined with nulls instead);
> * a matched left row stays in state carrying \{{matched = true}} so that it
> is suppressed at eviction time, rather than being dropped from state early
> the way left semi does;
> * a left row that fails the pre-join filter can never match, so it is emitted
> immediately without being added to state.
> h2. Requirements
> * *Watermark on the right side + time constraints are mandatory*, same
> contract as left outer. Without them the engine can never conclude "no match".
> * *Append output mode only.* Update mode would have to emit rows before the
> watermark can rule out a future match, and such a row could be invalidated by
> a later batch. This is the opposite of SPARK-56384, which allowed Update for
> Inner/LeftSemi precisely because those have no early-firing concern.
> * *No new state format version.* The \{{matched}} flag already persisted by
> state format v2/v3/v4 is exactly the signal left anti needs, so existing
> checkpoints need no migration. Format v1 is already rejected for non-inner
> joins.
> * *RightAnti is out of scope* -- it is not among the join types planned onto
> this operator.
> h2. Trade-off
> Left anti buffers every left row until eviction, whereas left semi drops
> matched left rows from state eagerly. This is inherent: a matched row must be
> retained so that it can be _suppressed_ at eviction rather than emitted.
> State size for left anti is therefore comparable to left outer, not to left
> semi.
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