Flink: Document watermark generation feature (apache#9179)

docs/flink-queries.md

@@ -277,6 +277,75 @@ DataStream<Row> stream = env.fromSource(source, WatermarkStrategy.noWatermarks()
     "Iceberg Source as Avro GenericRecord", new GenericRecordAvroTypeInfo(avroSchema));
 ```
 
+### Emitting watermarks
+Emitting watermarks from the source itself could be beneficial for several purposes, like harnessing the
+[Flink Watermark Alignment](https://nightlies.apache.org/flink/flink-docs-stable/docs/dev/datastream/event-time/generating_watermarks/#watermark-alignment),
+or prevent triggering [windows](https://nightlies.apache.org/flink/flink-docs-stable/docs/dev/datastream/operators/windows/)
+too early when reading multiple data files concurrently.
+
+Enable watermark generation for an `IcebergSource` by setting the `watermarkColumn`.
+The supported column types are `timestamp`, `timestamptz` and `long`.
+Iceberg `timestamp` or `timestamptz` inherently contains the time precision. So there is no need
+to specify the time unit. But `long` type column doesn't contain time unit information. Use  
+`watermarkTimeUnit` to configure the conversion for long columns.
+
+The watermarks are generated based on column metrics stored for data files and emitted once per split.
+If multiple smaller files with different time ranges are combined into a single split, it can increase
+the out-of-orderliness and extra data buffering in the Flink state. The main purpose of watermark alignment
+is to reduce out-of-orderliness and excess data buffering in the Flink state. Hence it is recommended to
+set `read.split.open-file-cost` to a very large value to prevent combining multiple smaller files into a
+single split. The negative impact (of not combining small files into a single split) is on read throughput,
+especially if there are many small files. In typical stateful processing jobs, source read throughput is not
+the bottleneck. Hence this is probably a reasonable tradeoff.
+
+This feature requires column-level min-max stats. Make sure stats are generated for the watermark column
+during write phase. By default, the column metrics are collected for the first 100 columns of the table.
+If watermark column doesn't have stats enabled by default, use
+[write properties](configuration.md#write-properties) starting with `write.metadata.metrics` when needed.
+
+The following example could be useful if watermarks are used for windowing. The source reads Iceberg data files
+in order, using a timestamp column and emits watermarks:
+```java
+StreamExecutionEnvironment env = StreamExecutionEnvironment.createLocalEnvironment();
+TableLoader tableLoader = TableLoader.fromHadoopTable("hdfs://nn:8020/warehouse/path");
+
+DataStream<RowData> stream =
+    env.fromSource(
+        IcebergSource.forRowData()
+            .tableLoader(tableLoader)
+            // Watermark using timestamp column
+            .watermarkColumn("timestamp_column")
+            .build(),
+        // Watermarks are generated by the source, no need to generate it manually
+        WatermarkStrategy.<RowData>noWatermarks()
+            // Extract event timestamp from records
+            .withTimestampAssigner((record, eventTime) -> record.getTimestamp(pos, precision).getMillisecond()),
+        SOURCE_NAME,
+        TypeInformation.of(RowData.class));
+```
+
+Example for reading Iceberg table using a long event column for watermark alignment:
+```java
+StreamExecutionEnvironment env = StreamExecutionEnvironment.createLocalEnvironment();
+TableLoader tableLoader = TableLoader.fromHadoopTable("hdfs://nn:8020/warehouse/path");
+
+DataStream<RowData> stream =
+    env.fromSource(
+        IcebergSource source = IcebergSource.forRowData()
+            .tableLoader(tableLoader)
+            // Disable combining multiple files to a single split 
+            .set(FlinkReadOptions.SPLIT_FILE_OPEN_COST, String.valueOf(TableProperties.SPLIT_SIZE_DEFAULT))
+            // Watermark using long column
+            .watermarkColumn("long_column")
+            .watermarkTimeUnit(TimeUnit.MILLI_SCALE)
+            .build(),
+        // Watermarks are generated by the source, no need to generate it manually
+        WatermarkStrategy.<RowData>noWatermarks()
+            .withWatermarkAlignment(watermarkGroup, maxAllowedWatermarkDrift),
+        SOURCE_NAME,
+        TypeInformation.of(RowData.class));
+```
+
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