fix(shredding): address shared-shredding review comments

Author: zjw1111

src/paimon/common/data/shredding/lru_map_shared_shredding_column_allocator.cpp

@@ -31,18 +31,11 @@ int32_t LruMapSharedShreddingColumnAllocator::SelectColumn(
     const std::vector<int32_t>& planned_col_to_field) const {
     int32_t selected_col = candidates.front();
     int64_t selected_last_used = std::numeric_limits<int64_t>::max();
-    bool selected_empty = false;
     for (int32_t col : candidates) {
-        bool is_empty = planned_col_to_field[col] == -1;
-        if (is_empty) {
-            if (!selected_empty || col < selected_col) {
-                selected_empty = true;
-                selected_col = col;
-            }
-            continue;
+        if (planned_col_to_field[col] == -1) {
+            return col;
         }
-        if (!selected_empty && (last_used_[col] < selected_last_used ||
-                                (last_used_[col] == selected_last_used && col < selected_col))) {
+        if (last_used_[col] < selected_last_used) {
             selected_col = col;
             selected_last_used = last_used_[col];
         }

src/paimon/common/data/shredding/lru_map_shared_shredding_column_allocator.h

@@ -23,7 +23,12 @@
 
 namespace paimon {
 
-/// Allocator that keeps cross-row column state and evicts least-recently-used columns.
+/// Allocator that keeps current column assignments across rows and evicts least-recently-used
+/// columns.
+///
+/// Keys that are still assigned to physical columns keep those columns when they appear again.
+/// New keys, including keys that were previously evicted, use empty columns first; if no empty
+/// column is available, the allocator replaces the least-recently-used physical column.
 class LruMapSharedShreddingColumnAllocator : public MapSharedShreddingColumnAllocator {
  public:
     /// @param num_columns Number of available physical columns.

src/paimon/common/data/shredding/map_shared_shredding_batch_converter_test.cpp

@@ -445,6 +445,13 @@ TEST_F(MapSharedShreddingBatchConverterTest, SequentialPlacementUsesSmallestColu
                         .ValueOrDie();
 
     AssertArrayEquals(expected, actual);
+
+    MapSharedShreddingFieldMeta expected_meta;
+    expected_meta.name_to_id = {{"a", 0}, {"b", 1}, {"c", 2}};
+    expected_meta.field_to_columns = {{0, {0}}, {1, {1}}, {2, {2}}};
+    expected_meta.num_columns = 3;
+    expected_meta.max_row_width = 3;
+    ASSERT_EQ(expected_meta, converter->BuildFieldMeta("tags").value());
 }
 
 TEST_F(MapSharedShreddingBatchConverterTest, LruPlacementPreservesResidentColumns) {

src/paimon/core/options/map_shared_shredding_column_placement_policy.h

@@ -20,11 +20,12 @@ namespace paimon {
 
 /// Specifies how shared-shredding MAP fields choose physical columns.
 enum class MapSharedShreddingColumnPlacementPolicy {
-    /// Keep input field order and place fields into columns 0..K-1.
+    /// Keep the key order from each input MAP row and place the first K keys into columns 0..K-1.
     PLAIN = 0,
-    /// Sort field IDs and place fields into columns 0..K-1.
+    /// Use a stable key order before placing the first K keys into columns 0..K-1.
     SEQUENTIAL = 1,
-    /// Choose empty columns first, then the least-recently-used physical column.
+    /// Reuse columns for recently seen keys when possible; otherwise choose an empty column first,
+    /// then the least-recently-used physical column.
     LRU = 2
 };