kangpinghuang commented on a change in pull request #764: Convert next_row 
interface to next_block
URL: https://github.com/apache/incubator-doris/pull/764#discussion_r267604235
 
 

 ##########
 File path: be/src/olap/rowset/alpha_rowset_reader.cpp
 ##########
 @@ -54,66 +48,57 @@ OLAPStatus AlphaRowsetReader::init(RowsetReaderContext* 
read_context) {
     _dst_cursor = new(std::nothrow)RowCursor();
     _dst_cursor->init(*(_current_read_context->tablet_schema));
     OLAPStatus status = _init_column_datas(read_context);
-    return status;
-}
 
-bool AlphaRowsetReader::has_next() {
-    bool next_flag = false;
-    for (int i = 0; i < _column_datas.size(); ++i) {
-        auto& row_block = _row_blocks[i];
-        if (row_block != nullptr) {
-            if (row_block->has_remaining()) {
-                next_flag = true;
+    Version version = _alpha_rowset_meta->version();
+    _is_singleton_rowset = (version.first == version.second);
+    bool merge = false;
+    /*
+     * For singleton rowset, there exists three situations.
+     *   1. DUP_KEYS tablet has no necessities to merge row in advance.
+     *   2. COMPACTION/CHECKSUM/ALTER_TABLET task has no necessities to
+     *      merge row in advance.
+     *   3. QUERY task will set preaggregation. If preaggregation is
+     *      set to be false, it is necessary to merge row in advance.
+     * For cumulative rowset, there is no necessities to merge row in advance.
+     */
+    if (_is_singleton_rowset) {
+        if (_current_read_context->tablet_schema->keys_type() == DUP_KEYS) {
+            // DUP_KEYS tablet
+            _next_block = 
&AlphaRowsetReader::_next_block_for_singleton_without_merge;
+        } else {
+            if (_current_read_context->reader_type == READER_QUERY
+                    && !_current_read_context->preaggregation) {
+                // QUERY task which set preaggregation to be true.
+                _next_block = 
&AlphaRowsetReader::_next_block_for_singleton_with_merge;
+                merge = true;
             } else {
-                OLAPStatus status = _get_next_block(i, &_row_blocks[i]);
-                if (status == OLAP_ERR_DATA_EOF) {
-                    _row_blocks[i] = nullptr;
-                    continue;
-                } else if (status != OLAP_SUCCESS) {
-                    LOG(WARNING) << "_get_next_block failed, status:" << 
status;
-                    return false;
-                } else {
-                    if (_row_blocks[i] != nullptr && 
_row_blocks[i]->has_remaining()) {
-                        next_flag = true;
-                    }
-                }
+                // COMPACTION/CHECKSUM/ALTER_TABLET task
+                _next_block = 
&AlphaRowsetReader::_next_block_for_singleton_without_merge;
             }
         }
+    } else {
+        // query task to scan cumulative rowset
+        _next_block = &AlphaRowsetReader::_next_block_for_cumulative_rowset;
     }
-    return next_flag;
-}
 
-OLAPStatus AlphaRowsetReader::next(RowCursor** row) {
-    OLAPStatus status = OLAP_SUCCESS;
-    if (_is_cumulative_rowset) {
-        status = _get_next_row_for_cumulative_rowset(row);
-    } else {
-        status = _get_next_row_for_singleton_rowset(row);
+    if (_is_singleton_rowset && merge) {
+        _read_block.reset(new RowBlock(_current_read_context->tablet_schema));
+        if (_read_block == nullptr) {
+            LOG(WARNING) << "new row block failed in reader";
+            return OLAP_ERR_MALLOC_ERROR;
+        }
+        RowBlockInfo block_info;
+        block_info.row_num = 
_current_read_context->tablet_schema->num_rows_per_row_block();
+        block_info.null_supported = true;
+        _read_block->init(block_info);
+        _dst_cursor = new(std::nothrow)RowCursor();
 
 Review comment:
   ```suggestion
           _dst_cursor = new (std::nothrow)RowCursor();
   ```

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