Revert "Reader iterator unification (ydb-platform#12387)"

This reverts commit ed0816f.

ydb/core/tx/columnshard/engines/column_engine.cpp

@@ -51,16 +51,13 @@ TSelectInfo::TStats TSelectInfo::Stats() const {
     return out;
 }
 
-TString TSelectInfo::DebugString() const {
-    TStringBuilder result;
-    result << "count:" << PortionsOrderedPK.size() << ";";
+void TSelectInfo::DebugStream(IOutputStream& out) {
     if (PortionsOrderedPK.size()) {
-        result << "portions:";
+        out << "portions:";
         for (auto& portionInfo : PortionsOrderedPK) {
-            result << portionInfo->DebugString();
+            out << portionInfo->DebugString();
         }
     }
-    return result;
 }
 
 }   // namespace NKikimr::NOlap

ydb/core/tx/columnshard/engines/column_engine.h

@@ -50,7 +50,7 @@ struct TSelectInfo {
 
     TStats Stats() const;
 
-    TString DebugString() const;
+    void DebugStream(IOutputStream& out);
 };
 
 class TColumnEngineStats {

ydb/core/tx/columnshard/engines/reader/abstract/read_metadata.h

@@ -30,7 +30,7 @@ class TDataStorageAccessor {
 };
 
 // Holds all metadata that is needed to perform read/scan
-class TReadMetadataBase {
+struct TReadMetadataBase {
 public:
     enum class ESorting {
         NONE = 0 /* "not_sorted" */,
@@ -149,8 +149,8 @@ class TReadMetadataBase {
 
     ui64 Limit = 0;
 
-    virtual TString DebugString() const {
-        return TStringBuilder() << " predicate{" << (PKRangesFilter ? PKRangesFilter->DebugString() : "no_initialized") << "}"
+    virtual void Dump(IOutputStream& out) const {
+        out << " predicate{" << (PKRangesFilter ? PKRangesFilter->DebugString() : "no_initialized") << "}"
             << " " << Sorting << " sorted";
     }
 
@@ -175,6 +175,12 @@ class TReadMetadataBase {
     virtual std::unique_ptr<TScanIteratorBase> StartScan(const std::shared_ptr<TReadContext>& readContext) const = 0;
     virtual std::vector<TNameTypeInfo> GetKeyYqlSchema() const = 0;
 
+    // TODO:  can this only be done for base class?
+    friend IOutputStream& operator<<(IOutputStream& out, const TReadMetadataBase& meta) {
+        meta.Dump(out);
+        return out;
+    }
+
     const TProgramContainer& GetProgram() const {
         return Program;
     }