Showcase dereference pushdown limited effectiveness in Iceberg

Dereference pushdown is effective on filtering
the data being read from the source so that only relevant
nested data is being provided by the connector to the engine.
However, the whole content corresponding to a nested column
is currently read from the data files.

plugin/trino-iceberg/src/test/java/io/trino/plugin/iceberg/BaseIcebergConnectorTest.java

@@ -4930,11 +4930,16 @@ public void testProjectionPushdownReadsLessData()
                 getSession(),
                 selectQuery,
                 statsWithPushdown -> {
+                    DataSize physicalDataSizeWithPushdown = statsWithPushdown.getPhysicalInputDataSize();
                     DataSize processedDataSizeWithPushdown = statsWithPushdown.getProcessedInputDataSize();
                     assertQueryStats(
                             sessionWithoutPushdown,
                             selectQuery,
-                            statsWithoutPushdown -> assertThat(statsWithoutPushdown.getProcessedInputDataSize()).isGreaterThan(processedDataSizeWithPushdown),
+                            statsWithoutPushdown -> {
+                                //TODO (https://github.com/trinodb/trino/issues/17156) add dereference pushdown on the physical layer
+                                assertThat(statsWithoutPushdown.getPhysicalInputDataSize()).isEqualTo(physicalDataSizeWithPushdown);
+                                assertThat(statsWithoutPushdown.getProcessedInputDataSize()).isGreaterThan(processedDataSizeWithPushdown);
+                            },
                             results -> assertEquals(results.getOnlyColumnAsSet(), expected));
                 },
                 results -> assertEquals(results.getOnlyColumnAsSet(), expected));