fix(nimble): Preserve MAP spec in flat map column readers (facebookincubator#15803)

Summary:
X-link: facebookincubator/nimble#369

Pull Request resolved: facebookincubator#15803

A production issue S602695 VeloxRuntimeError blocking feature injection pipeline exposed an issue in the Nimble flat map reader.

For context, earlier this half ROO feature injection was enabled in production and supported by both Velox map and flat map vector encodings. A particular pipeline was blocked this past weekend due to flat map reader crashing. A temporary mitigation was to use regular map instead of the flat map encoding as it appeared flat map reader was attempting to set values that were uninitialized by the reader.

The culprit was how scan spec is constructed for flat map. Rather than appending all features to keys and values children, we should follow map scan spec construction by preserving the keys and values and create child columns using the preserved values child spec.

Reviewed By: Yuhta

Differential Revision: D89326920

fbshipit-source-id: e4b70115c8642b2cd6f4e8ee6bcf3e0ebae44d2a

Author: peterenescu

velox/dwio/common/SelectiveFlatMapColumnReader.cpp

@@ -57,42 +57,16 @@ void SelectiveFlatMapColumnReader::getValues(
   auto* resultFlatMap = prepareResult(*result, keysVector_, rows.size());
   setComplexNulls(rows, *result);
 
-  for (const auto& childSpec : scanSpec_->children()) {
-    VELOX_TRACE_HISTORY_PUSH("getValues %s", childSpec->fieldName().c_str());
-    if (!childSpec->keepValues()) {
-      continue;
-    }
-
-    VELOX_CHECK(
-        childSpec->readFromFile(),
-        "Flatmap children must always be read from file.");
-
-    if (childSpec->subscript() == kConstantChildSpecSubscript) {
-      continue;
-    }
-
-    const auto channel = childSpec->channel();
-    const auto index = childSpec->subscript();
-    auto& childResult = resultFlatMap->mapValuesAt(channel);
-
-    VELOX_CHECK(
-        !childSpec->deltaUpdate(),
-        "Delta update not supported in flat map yet");
-    VELOX_CHECK(
-        !childSpec->isConstant(),
-        "Flat map values cannot be constant in scanSpec.");
-    VELOX_CHECK_EQ(
-        childSpec->columnType(),
-        velox::common::ScanSpec::ColumnType::kRegular,
-        "Flat map only supports regular column types in scan spec.");
-
-    children_[index]->getValues(rows, &childResult);
-
-    for (size_t i = 0; i < children_.size(); ++i) {
-      const auto& inMap = inMapBuffer(i);
-      if (inMap) {
-        resultFlatMap->inMapsAt(i, true) = inMap;
-      }
+  // Loop over column readers
+  for (int i = 0; i < children_.size(); ++i) {
+    auto& child = children_[i];
+    VectorPtr values;
+    child->getValues(rows, &values);
+    resultFlatMap->mapValuesAt(i) = values;
+
+    const auto& inMap = inMapBuffer(i);
+    if (inMap) {
+      resultFlatMap->inMapsAt(i, true) = inMap;
     }
   }
 }