Backport ClickHouse#89680 to 25.8: Prevent false positive error on empty LowCardinality indexes

Author: robot-clickhouse

src/Columns/ColumnLowCardinality.cpp

@@ -240,6 +240,8 @@ static void checkPositionsAreLimited(const IColumn & positions, UInt64 limit)
 
         const auto & data = column_ptr->getData();
         size_t num_rows = data.size();
+        if (num_rows == 0)
+            return true;
         UInt64 max_position = 0;
         for (size_t i = 0; i < num_rows; ++i)
             max_position = std::max<UInt64>(max_position, data[i]);

src/Columns/tests/gtest_low_cardinality.cpp

@@ -62,3 +62,25 @@ TEST(ColumnLowCardinality, Clone)
     ASSERT_TRUE(assert_cast<const ColumnLowCardinality &>(*nullable_column).nestedIsNullable());
     ASSERT_FALSE(assert_cast<const ColumnLowCardinality &>(*column).nestedIsNullable());
 }
+
+TEST(ColumnLowCardinality, EmptyDictionaryEmptyIndexes)
+{
+    /// Test edge case: empty dictionary (size=0) with empty indexes (num_rows=0)
+    /// This should not throw an error, as empty indexes are always valid
+    /// Regression test for bug where check was: if (max_position >= limit)
+    /// When num_rows=0, max_position stays 0, and with limit=0, this incorrectly threw
+    
+    auto data_type = std::make_shared<DataTypeUInt32>();
+    auto low_cardinality_type = std::make_shared<DataTypeLowCardinality>(data_type);
+    auto column = low_cardinality_type->createColumn();
+    auto & lc_column = assert_cast<ColumnLowCardinality &>(*column);
+    
+    // Create empty keys and indexes columns
+    auto empty_keys = ColumnUInt32::create();
+    auto empty_indexes = ColumnUInt8::create();
+    
+    // This should NOT throw an exception
+    ASSERT_NO_THROW(lc_column.insertRangeFromDictionaryEncodedColumn(*empty_keys, *empty_indexes));
+    
+    ASSERT_EQ(column->size(), 0);
+}