private void assertLowWatermark(SourceRecord lowWatermark) {
        checkState(
                isLowWatermarkEvent(lowWatermark),
                String.format(
                        "The first record should be low watermark signal event, but actual is %s",
                        lowWatermark));
    }
Fetcher to fetch data from table split, the split is the snapshot split {@link SnapshotSplit}.
Copy from com.ververica:flink-cdc-base:2.3.0.
inlong-sort/sort-flink/cdc-base/src/main/java/org/apache/inlong/sort/cdc/base/source/reader/external/IncrementalSourceScanFetcher.java