[Kernel] During Active-AddFile-Log-Replay do not pass the RemoveFile …

…to checkpoint reader

#### Which Delta project/connector is this regarding?

- [ ] Spark
- [ ] Standalone
- [ ] Flink
- [X] Kernel
- [ ] Other (fill in here)

## Description

Implemented a minor performance improvement to not read any RemoveFiles when we read checkpoint Parquet files during active-add-file-log-replay.
Fixes #4102

## How was this patch tested?

Existing unit test, manual test using
./run-tests.py --group kernel
and
delta/kernel/examples/run-kernel-examples.py --use-local

## Does this PR introduce _any_ user-facing changes?

No

kernel/kernel-api/src/main/java/io/delta/kernel/internal/replay/ActionsIterator.java

@@ -44,7 +44,11 @@
  * iterator of (ColumnarBatch, isFromCheckpoint) tuples, where the schema of the ColumnarBatch
  * semantically represents actions (or, a subset of action fields) parsed from the Delta Log.
  *
- * <p>Users must pass in a `readSchema` to select which actions and sub-fields they want to consume.
+ * <p>Users must pass in a `deltaReadSchema` to select which actions and sub-fields they want to
+ * consume.
+ *
+ * <p>Users can also pass in an optional `checkpointReadSchema` if it is different from
+ * `deltaReadSchema`.
  */
 public class ActionsIterator implements CloseableIterator<ActionWrapper> {
   private final Engine engine;
@@ -60,7 +64,14 @@ public class ActionsIterator implements CloseableIterator<ActionWrapper> {
    */
   private final LinkedList<DeltaLogFile> filesList;
 
-  private final StructType readSchema;
+  /** Schema used for reading delta files. */
+  private final StructType deltaReadSchema;
+
+  /**
+   * Schema to be used for reading checkpoint files. Checkpoint files can be Parquet or JSON in the
+   * case of v2 checkpoints.
+   */
+  private final StructType checkpointReadSchema;
 
   private final boolean schemaContainsAddOrRemoveFiles;
 
@@ -77,16 +88,26 @@ public class ActionsIterator implements CloseableIterator<ActionWrapper> {
   public ActionsIterator(
       Engine engine,
       List<FileStatus> files,
-      StructType readSchema,
+      StructType deltaReadSchema,
+      Optional<Predicate> checkpointPredicate) {
+    this(engine, files, deltaReadSchema, deltaReadSchema, checkpointPredicate);
+  }
+
+  public ActionsIterator(
+      Engine engine,
+      List<FileStatus> files,
+      StructType deltaReadSchema,
+      StructType checkpointReadSchema,
       Optional<Predicate> checkpointPredicate) {
     this.engine = engine;
     this.checkpointPredicate = checkpointPredicate;
     this.filesList = new LinkedList<>();
     this.filesList.addAll(
         files.stream().map(DeltaLogFile::forCommitOrCheckpoint).collect(Collectors.toList()));
-    this.readSchema = readSchema;
+    this.deltaReadSchema = deltaReadSchema;
+    this.checkpointReadSchema = checkpointReadSchema;
     this.actionsIter = Optional.empty();
-    this.schemaContainsAddOrRemoveFiles = LogReplay.containsAddOrRemoveFileActions(readSchema);
+    this.schemaContainsAddOrRemoveFiles = LogReplay.containsAddOrRemoveFileActions(deltaReadSchema);
   }
 
   @Override
@@ -105,7 +126,8 @@ public boolean hasNext() {
 
   /**
    * @return a tuple of (ColumnarBatch, isFromCheckpoint), where ColumnarBatch conforms to the
-   *     instance {@link #readSchema}.
+   *     instance {@link #deltaReadSchema} or {@link #checkpointReadSchema} (the latter when when
+   *     isFromCheckpoint=true).
    */
   @Override
   public ActionWrapper next() {
@@ -176,9 +198,9 @@ private CloseableIterator<ColumnarBatch> getActionsIterFromSinglePartOrV2Checkpo
       FileStatus file, String fileName) throws IOException {
     // If the sidecars may contain the current action, read sidecars from the top-level v2
     // checkpoint file(to be read later).
-    StructType modifiedReadSchema = readSchema;
+    StructType modifiedReadSchema = checkpointReadSchema;
     if (schemaContainsAddOrRemoveFiles) {
-      modifiedReadSchema = LogReplay.withSidecarFileSchema(readSchema);
+      modifiedReadSchema = LogReplay.withSidecarFileSchema(checkpointReadSchema);
     }
 
     long checkpointVersion = checkpointVersion(file.getPath());
@@ -195,7 +217,8 @@ private CloseableIterator<ColumnarBatch> getActionsIterFromSinglePartOrV2Checkpo
       checkpointPredicateIncludingSidecars = checkpointPredicate;
     }
     final CloseableIterator<ColumnarBatch> topLevelIter;
-    StructType finalModifiedReadSchema = modifiedReadSchema;
+    StructType finalReadSchema = modifiedReadSchema;
+
     if (fileName.endsWith(".parquet")) {
       topLevelIter =
           wrapEngineExceptionThrowsIO(
@@ -204,11 +227,11 @@ private CloseableIterator<ColumnarBatch> getActionsIterFromSinglePartOrV2Checkpo
                       .getParquetHandler()
                       .readParquetFiles(
                           singletonCloseableIterator(file),
-                          finalModifiedReadSchema,
+                          finalReadSchema,
                           checkpointPredicateIncludingSidecars),
               "Reading parquet log file `%s` with readSchema=%s and predicate=%s",
               file,
-              modifiedReadSchema,
+              finalReadSchema,
               checkpointPredicateIncludingSidecars);
     } else if (fileName.endsWith(".json")) {
       topLevelIter =
@@ -218,11 +241,11 @@ private CloseableIterator<ColumnarBatch> getActionsIterFromSinglePartOrV2Checkpo
                       .getJsonHandler()
                       .readJsonFiles(
                           singletonCloseableIterator(file),
-                          finalModifiedReadSchema,
+                          finalReadSchema,
                           checkpointPredicateIncludingSidecars),
               "Reading JSON log file `%s` with readSchema=%s and predicate=%s",
               file,
-              modifiedReadSchema,
+              finalReadSchema,
               checkpointPredicateIncludingSidecars);
     } else {
       throw new IOException("Unrecognized top level v2 checkpoint file format: " + fileName);
@@ -309,10 +332,12 @@ private CloseableIterator<ActionWrapper> getNextActionsIter() {
                         engine
                             .getJsonHandler()
                             .readJsonFiles(
-                                singletonCloseableIterator(nextFile), readSchema, Optional.empty()),
+                                singletonCloseableIterator(nextFile),
+                                deltaReadSchema,
+                                Optional.empty()),
                     "Reading JSON log file `%s` with readSchema=%s",
                     nextFile,
-                    readSchema);
+                    deltaReadSchema);
 
             return combine(
                 dataIter,
@@ -344,10 +369,11 @@ private CloseableIterator<ActionWrapper> getNextActionsIter() {
                     () ->
                         engine
                             .getParquetHandler()
-                            .readParquetFiles(checkpointFiles, readSchema, checkpointPredicate),
+                            .readParquetFiles(
+                                checkpointFiles, deltaReadSchema, checkpointPredicate),
                     "Reading checkpoint sidecars [%s] with readSchema=%s and predicate=%s",
                     checkpointFiles,
-                    readSchema,
+                    deltaReadSchema,
                     checkpointPredicate);
 
             long version = checkpointVersion(nextFilePath);

kernel/kernel-api/src/main/java/io/delta/kernel/internal/replay/ActiveAddFilesIterator.java

@@ -233,9 +233,13 @@ private void prepareNext() {
       }
     }
 
+    ColumnarBatch scanAddFiles = addRemoveColumnarBatch;
     // Step 3: Drop the RemoveFile column and use the selection vector to build a new
     //         FilteredColumnarBatch
-    ColumnarBatch scanAddFiles = addRemoveColumnarBatch.withDeletedColumnAt(1);
+    // For checkpoint files, we would only have read the adds, not the removes.
+    if (!isFromCheckpoint) {
+      scanAddFiles = scanAddFiles.withDeletedColumnAt(1);
+    }
 
     // Step 4: TODO: remove this step. This is a temporary requirement until the path
     //         in `add` is converted to absolute path.

kernel/kernel-api/src/main/java/io/delta/kernel/internal/replay/LogReplay.java

@@ -106,6 +106,11 @@ public static StructType getAddRemoveReadSchema(boolean shouldReadStats) {
         .add(REMOVEFILE_FIELD_NAME, REMOVE_FILE_SCHEMA);
   }
 
+  /** Read schema when searching only for AddFiles */
+  public static StructType getAddReadSchema(boolean shouldReadStats) {
+    return new StructType().add(ADDFILE_FIELD_NAME, getAddSchema(shouldReadStats));
+  }
+
   public static int ADD_FILE_ORDINAL = 0;
   public static int ADD_FILE_PATH_ORDINAL = AddFile.SCHEMA_WITHOUT_STATS.indexOf("path");
   public static int ADD_FILE_DV_ORDINAL = AddFile.SCHEMA_WITHOUT_STATS.indexOf("deletionVector");
@@ -196,11 +201,15 @@ public CloseableIterator<FilteredColumnarBatch> getAddFilesAsColumnarBatches(
       boolean shouldReadStats,
       Optional<Predicate> checkpointPredicate,
       ScanMetrics scanMetrics) {
+    // We do not need to look at any `remove` files from the checkpoints. Skip the column to save
+    // I/O. Note that we are still going to process the row groups. Adds and removes are randomly
+    // scattered through checkpoint part files, so row group push down is unlikely to be useful.
     final CloseableIterator<ActionWrapper> addRemoveIter =
         new ActionsIterator(
             engine,
             logSegment.allLogFilesReversed(),
             getAddRemoveReadSchema(shouldReadStats),
+            getAddReadSchema(shouldReadStats),
             checkpointPredicate);
     return new ActiveAddFilesIterator(engine, addRemoveIter, dataPath, scanMetrics);
   }