feat: Populate footerBufferOverread for Nimble speculative IO

dwio/nimble/tablet/TabletReader.cpp

@@ -373,6 +373,9 @@ void TabletReader::loadFooter(
           footerOffset,
           footerBuf->size(),
           footerBuf->asMutable<char>());
+    } else {
+      stats_.footerBufferOverread =
+          static_cast<int64_t>(footerIoSize - requiredSize);
     }
 
     const uint64_t footerOffset =

dwio/nimble/tablet/TabletReader.h

@@ -303,6 +303,17 @@ class TabletReader {
     return ps_.footerSize();
   }
 
+  /// TabletReader-level statistics collected during file open.
+  struct Stats {
+    /// Bytes read beyond the footer + postscript in speculative mode.
+    /// Zero in adaptive mode (exact-size read).
+    int64_t footerBufferOverread{0};
+  };
+
+  const Stats& stats() const {
+    return stats_;
+  }
+
   CompressionType footerCompressionType() const {
     return ps_.footerCompressionType();
   }
@@ -525,6 +536,7 @@ class TabletReader {
   const std::unique_ptr<MetadataInput> metadataInput_;
 
   uint64_t fileSize_{0};
+  Stats stats_;
   Postscript ps_;
   std::unique_ptr<MetadataBuffer> footer_;
   std::unique_ptr<MetadataBuffer> stripes_;

dwio/nimble/tablet/tests/TabletTest.cpp

@@ -4406,6 +4406,8 @@ TEST_P(TabletTest, readerOptionsAdaptiveMode) {
 
   EXPECT_EQ(tablet->stripeCount(), 1);
   EXPECT_EQ(tablet->stripeRowCount(0), 500);
+  // Adaptive mode reads the exact footer size, so no bytes are wasted.
+  EXPECT_EQ(tablet->stats().footerBufferOverread, 0);
 }
 
 TEST_P(TabletTest, readerOptionsSpeculativeMode) {
@@ -4438,6 +4440,11 @@ TEST_P(TabletTest, readerOptionsSpeculativeMode) {
 
   EXPECT_EQ(tablet->stripeCount(), 1);
   EXPECT_EQ(tablet->stripeRowCount(0), 600);
+  // Speculative mode reads more than needed; overread is the wasted bytes.
+  const auto footerRequired = tablet->footerSize() + nimble::kPostscriptSize;
+  EXPECT_EQ(
+      tablet->stats().footerBufferOverread,
+      std::min<uint64_t>(1024, file.size()) - footerRequired);
 }
 
 TEST_P(TabletTest, configureOptionsIndexFlags) {

dwio/nimble/velox/selective/SelectiveNimbleReader.cpp

@@ -328,6 +328,8 @@ uint64_t SelectiveNimbleRowReader::next(
 void SelectiveNimbleRowReader::updateRuntimeStats(
     dwio::common::RuntimeStatistics& stats) const {
   stats.skippedStrides += skippedStripes_;
+  stats.footerBufferOverread +=
+      readerBase_->tablet().stats().footerBufferOverread;
   stats.columnReaderStats.mergeFrom(columnReaderStatistics_);
 }