rionmonster commented on code in PR #27225:
URL: https://github.com/apache/flink/pull/27225#discussion_r2514390541
##########
docs/content/docs/dev/table/tuning.md:
##########
@@ -368,3 +368,57 @@ FROM TenantKafka t
LEFT JOIN InventoryKafka i ON t.tenant_id = i.tenant_id AND ...;
```
+## Delta Joins
+
+In streaming jobs, regular joins keep all historical data from both inputs to
ensure accuracy. Over time, this causes the state to grow continuously,
increasing resource usage and impacting stability.
+
+To mitigate these challenges, Flink introduces the delta join operator. The
key idea is to replace the large state maintained by regular joins with a
bidirectional lookup-based join that directly reuses data from the source
tables. Compared to traditional regular joins, delta joins substantially reduce
state size, enhances job stability, and lowers overall resource consumption.
+
+This feature is enabled by default. A regular join will be automatically
optimized into a delta join when all the following conditions are met:
+
+1. The sql pattern satisfies the optimization criteria. For details, please
refer to [Supported Features and Limitations]({{< ref "docs/dev/table/tuning"
>}}#supported-features-and-limitations)
Review Comment:
Minor nit: we may want to capitalize instances of “sql” to “SQL” for
consistency with the existing documentation, where “SQL” appears to be the
predominant usage.
Otherwise, looks good.
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