[refactoring](multi-catalog)data_lake_reader_refactoring.

be/src/exec/scan/file_scanner.h

@@ -90,7 +90,9 @@ class FileScanner : public Scanner {
             : Scanner(state, profile),
               _params(params),
               _col_name_to_slot_id(colname_to_slot_id),
-              _real_tuple_desc(tuple_desc) {};
+              _real_tuple_desc(tuple_desc) {
+        _configure_file_scan_handlers();
+    };
 
     Status read_lines_from_range(const TFileRangeDesc& range, const std::list<int64_t>& row_ids,
                                  Block* result_block, const ExternalFileMappingInfo& external_info,
@@ -107,6 +109,9 @@ class FileScanner : public Scanner {
 
     Status _get_next_reader();
 
+    // Build a ReaderInitContext with shared fields from FileScanner members.
+    void _fill_base_init_context(ReaderInitContext* ctx);
+
     // TODO: cast input block columns type to string.
     Status _cast_src_block(Block* block) { return Status::OK(); }
 
@@ -128,10 +133,10 @@ class FileScanner : public Scanner {
     std::vector<SlotDescriptor*> _file_slot_descs;
     // col names from _file_slot_descs
     std::vector<std::string> _file_col_names;
+    // Unified column descriptors for init_reader (includes file, partition, missing, synthesized cols)
+    std::vector<ColumnDescriptor> _column_descs;
 
-    // Partition source slot descriptors
-    std::vector<SlotDescriptor*> _partition_slot_descs;
-    // Partition slot id to index in _partition_slot_descs
+    // Partition slot id to partition key index (for matching columns_from_path)
     std::unordered_map<SlotId, int> _partition_slot_index_map;
     // created from param.expr_of_dest_slot
     // For query, it saves default value expr of all dest columns, or nullptr for NULL.
@@ -152,8 +157,6 @@ class FileScanner : public Scanner {
     // Get from GenericReader, save the existing columns in file to their type.
     std::unordered_map<std::string, DataTypePtr> _slot_lower_name_to_col_type;
     // Get from GenericReader, save columns that required by scan but not exist in file.
-    // These columns will be filled by default value or null.
-    std::unordered_set<std::string> _missing_cols;
 
     // The col lowercase name of source file to type of source file.
     std::map<std::string, DataTypePtr> _source_file_col_name_types;
@@ -192,7 +195,6 @@ class FileScanner : public Scanner {
     std::unordered_map<std::string, std::tuple<std::string, const SlotDescriptor*>>
             _partition_col_descs;
     std::unordered_map<std::string, bool> _partition_value_is_null;
-    std::unordered_map<std::string, VExprContextSPtr> _missing_col_descs;
 
     // idx of skip_bitmap_col in _input_tuple_desc
     int32_t _skip_bitmap_col_idx {-1};
@@ -230,34 +232,39 @@ class FileScanner : public Scanner {
     // otherwise, point to _output_tuple_desc
     const TupleDescriptor* _real_tuple_desc = nullptr;
 
-    std::pair<std::shared_ptr<RowIdColumnIteratorV2>, int> _row_id_column_iterator_pair = {nullptr,
-                                                                                           -1};
-    bool _need_iceberg_rowid_column = false;
-    int _iceberg_rowid_column_pos = -1;
-    // for iceberg row lineage
-    RowLineageColumns _row_lineage_columns;
     int64_t _last_bytes_read_from_local = 0;
     int64_t _last_bytes_read_from_remote = 0;
 
+    Status (FileScanner::*_init_src_block_handler)(Block* block) = nullptr;
+    Status (FileScanner::*_process_src_block_after_read_handler)(Block* block) = nullptr;
+    bool (FileScanner::*_should_push_down_predicates_handler)(
+            TFileFormatType::type format_type) const = nullptr;
+    bool (FileScanner::*_should_enable_condition_cache_handler)() const = nullptr;
+
     // Condition cache for external tables
     uint64_t _condition_cache_digest = 0;
     segment_v2::ConditionCache::ExternalCacheKey _condition_cache_key;
     std::shared_ptr<std::vector<bool>> _condition_cache;
     std::shared_ptr<ConditionCacheContext> _condition_cache_ctx;
     int64_t _condition_cache_hit_count = 0;
 
+    void _configure_file_scan_handlers();
+
     Status _init_expr_ctxes();
     Status _init_src_block(Block* block);
+    Status _init_src_block_for_load(Block* block);
+    Status _init_src_block_for_query(Block* block);
+    Status _process_src_block_after_read(Block* block);
+    Status _process_src_block_after_read_for_load(Block* block);
+    Status _process_src_block_after_read_for_query(Block* block);
     Status _check_output_block_types();
     Status _cast_to_input_block(Block* block);
-    Status _fill_columns_from_path(size_t rows);
-    Status _fill_missing_columns(size_t rows);
     Status _pre_filter_src_block();
     Status _convert_to_output_block(Block* block);
     Status _truncate_char_or_varchar_columns(Block* block);
     void _truncate_char_or_varchar_column(Block* block, int idx, int len);
     Status _generate_partition_columns();
-    Status _generate_missing_columns();
+
     bool _check_partition_prune_expr(const VExprSPtr& expr);
     void _init_runtime_filter_partition_prune_ctxs();
     void _init_runtime_filter_partition_prune_block();
@@ -267,11 +274,11 @@ class FileScanner : public Scanner {
     void _get_slot_ids(VExpr* expr, std::vector<int>* slot_ids);
     Status _generate_truncate_columns(bool need_to_get_parsed_schema);
     Status _set_fill_or_truncate_columns(bool need_to_get_parsed_schema);
-    Status _init_orc_reader(std::unique_ptr<OrcReader>&& orc_reader,
-                            FileMetaCache* file_meta_cache_ptr);
-    Status _init_parquet_reader(std::unique_ptr<ParquetReader>&& parquet_reader,
-                                FileMetaCache* file_meta_cache_ptr);
-    Status _create_row_id_column_iterator();
+    Status _init_orc_reader(FileMetaCache* file_meta_cache_ptr,
+                            std::unique_ptr<OrcReader> orc_reader = nullptr);
+    Status _init_parquet_reader(FileMetaCache* file_meta_cache_ptr,
+                                std::unique_ptr<ParquetReader> parquet_reader = nullptr);
+    std::shared_ptr<segment_v2::RowIdColumnIteratorV2> _create_row_id_column_iterator();
 
     TFileFormatType::type _get_current_format_type() {
         // for compatibility, if format_type is not set in range, use the format type of params
@@ -291,6 +298,11 @@ class FileScanner : public Scanner {
     }
 
     bool _should_enable_condition_cache();
+    bool _should_enable_condition_cache_for_load() const;
+    bool _should_enable_condition_cache_for_query() const;
+    bool _should_push_down_predicates(TFileFormatType::type format_type) const;
+    bool _should_push_down_predicates_for_load(TFileFormatType::type format_type) const;
+    bool _should_push_down_predicates_for_query(TFileFormatType::type format_type) const;
     void _init_reader_condition_cache();
     void _finalize_reader_condition_cache();
 

be/src/format/arrow/arrow_stream_reader.cpp

@@ -64,7 +64,7 @@ Status ArrowStreamReader::init_reader() {
     return Status::OK();
 }
 
-Status ArrowStreamReader::get_next_block(Block* block, size_t* read_rows, bool* eof) {
+Status ArrowStreamReader::_do_get_next_block(Block* block, size_t* read_rows, bool* eof) {
     bool has_next = false;
     RETURN_IF_ERROR(_pip_stream->HasNext(&has_next));
     if (!has_next) {
@@ -125,8 +125,8 @@ Status ArrowStreamReader::get_next_block(Block* block, size_t* read_rows, bool*
     return Status::OK();
 }
 
-Status ArrowStreamReader::get_columns(std::unordered_map<std::string, DataTypePtr>* name_to_type,
-                                      std::unordered_set<std::string>* missing_cols) {
+Status ArrowStreamReader::_get_columns_impl(
+        std::unordered_map<std::string, DataTypePtr>* name_to_type) {
     for (const auto& slot : _file_slot_descs) {
         name_to_type->emplace(slot->col_name(), slot->type());
     }

be/src/format/arrow/arrow_stream_reader.h

@@ -53,10 +53,12 @@ class ArrowStreamReader : public GenericReader {
 
     Status init_reader();
 
-    Status get_next_block(Block* block, size_t* read_rows, bool* eof) override;
+    Status _do_get_next_block(Block* block, size_t* read_rows, bool* eof) override;
 
-    Status get_columns(std::unordered_map<std::string, DataTypePtr>* name_to_type,
-                       std::unordered_set<std::string>* missing_cols) override;
+    Status _get_columns_impl(std::unordered_map<std::string, DataTypePtr>* name_to_type) override;
+
+protected:
+    Status _do_init_reader(ReaderInitContext* /*ctx*/) override { return init_reader(); }
 
 private:
     RuntimeState* _state;

be/src/format/column_descriptor.h

@@ -0,0 +1,56 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#pragma once
+
+#include <string>
+
+#include "exprs/vexpr_fwd.h"
+
+namespace doris {
+class SlotDescriptor;
+
+/// Column categories for table format reading.
+///
+/// Each column requested by the query is classified into one of these categories.
+/// The category determines how the column's value is obtained:
+///   - REGULAR:       Read directly from the data file (Parquet/ORC).
+///                    If the column is absent from a file (schema evolution),
+///                    its default_expr is used to produce a default value.
+///   - PARTITION_KEY:  Filled from partition metadata (e.g. Hive path partitions).
+///   - SYNTHESIZED:    Never in the data file; fully computed at runtime
+///                     (e.g. Doris V2 __DORIS_ICEBERG_ROWID_COL__).
+///   - GENERATED:      May or may not exist in the data file. If present but null,
+///                     the value is backfilled at runtime (e.g. Iceberg V3 _row_id).
+enum class ColumnCategory {
+    REGULAR,
+    PARTITION_KEY,
+    SYNTHESIZED,
+    GENERATED,
+};
+
+/// Describes a column requested by the query, along with its category.
+struct ColumnDescriptor {
+    std::string name;
+    const SlotDescriptor* slot_desc = nullptr;
+    ColumnCategory category = ColumnCategory::REGULAR;
+    /// Default value expression when this column is missing from the data file.
+    /// nullptr means fill with NULL. Built once per table scan in FileScanner.
+    VExprContextSPtr default_expr;
+};
+
+} // namespace doris

be/src/format/count_reader.h

@@ -0,0 +1,104 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+
+#include "common/status.h"
+#include "core/block/block.h"
+#include "format/generic_reader.h"
+
+namespace doris {
+
+/// A lightweight reader that emits row counts without reading any actual data.
+/// Used as a decorator to replace the real reader when COUNT(*) push down is active.
+///
+/// Instead of duplicating the COUNT short-circuit logic in every format reader
+/// (ORC, Parquet, etc.), FileScanner creates a CountReader after the real reader
+/// is initialized and the total row count is known. The CountReader then serves
+/// all subsequent get_next_block calls by simply resizing columns.
+///
+/// This cleanly separates the "how many rows" concern from the actual data reading,
+/// eliminating duplicated COUNT blocks across format readers.
+class CountReader : public GenericReader {
+public:
+    /// @param total_rows   Total number of rows to emit (post-filter).
+    /// @param batch_size   Maximum rows per batch.
+    /// @param inner_reader The original reader, kept alive for profile collection
+    ///                     and lifecycle management. Ownership is transferred.
+    CountReader(int64_t total_rows, size_t batch_size,
+                std::unique_ptr<GenericReader> inner_reader = nullptr)
+            : _remaining_rows(total_rows),
+              _batch_size(batch_size),
+              _inner_reader(std::move(inner_reader)) {
+        set_push_down_agg_type(TPushAggOp::type::COUNT);
+    }
+
+    ~CountReader() override = default;
+
+    Status _do_get_next_block(Block* block, size_t* read_rows, bool* eof) override {
+        auto rows = std::min(_remaining_rows, static_cast<int64_t>(_batch_size));
+        _remaining_rows -= rows;
+
+        auto mutate_columns = block->mutate_columns();
+        for (auto& col : mutate_columns) {
+            col->resize(rows);
+        }
+        block->set_columns(std::move(mutate_columns));
+
+        *read_rows = rows;
+        *eof = (_remaining_rows == 0);
+        return Status::OK();
+    }
+
+    /// CountReader counts rows by definition.
+    bool count_read_rows() override { return true; }
+
+    /// Delegate to inner reader if available, otherwise return our total.
+    int64_t get_total_rows() const override {
+        return _inner_reader ? _inner_reader->get_total_rows() : _initial_total_rows();
+    }
+
+    Status close() override {
+        if (_inner_reader) {
+            return _inner_reader->close();
+        }
+        return Status::OK();
+    }
+
+    /// Access the inner reader for profile collection or other lifecycle needs.
+    GenericReader* inner_reader() const { return _inner_reader.get(); }
+
+protected:
+    void _collect_profile_before_close() override {
+        if (_inner_reader) {
+            _inner_reader->collect_profile_before_close();
+        }
+    }
+
+private:
+    int64_t _initial_total_rows() const { return _remaining_rows; }
+
+    int64_t _remaining_rows;
+    size_t _batch_size;
+    std::unique_ptr<GenericReader> _inner_reader;
+};
+
+} // namespace doris