MCOL-271 empty strings should not be NULLs (#2794)

This patch improves handling of NULLs in textual fields in ColumnStore.
Previously empty strings were considered NULLs and it could be a problem
if data scheme allows for empty strings. It was also one of major
reasons of behavior difference between ColumnStore and other engines in
MariaDB family.

Also, this patch fixes some other bugs and incorrect behavior, for
example, incorrect comparison for "column <= ''" which evaluates to
constant True for all purposes before this patch.

dbcon/joblist/groupconcat.cpp

@@ -339,11 +339,6 @@ void GroupConcatAgUM::merge(const rowgroup::Row& inRow, int64_t i)
   fConcator->merge(gccAg->concator().get());
 }
 
-void GroupConcatAgUM::getResult(uint8_t* buff)
-{
-  fConcator->getResultImpl(fGroupConcat->fSeparator);
-}
-
 uint8_t* GroupConcatAgUM::getResult()
 {
   return fConcator->getResult(fGroupConcat->fSeparator);
@@ -400,21 +395,22 @@ void GroupConcator::initialize(const rowgroup::SP_GroupConcat& gcc)
   // MCOL-901 This value comes from the Server and it is
   // too high(3MB) to allocate it for every instance.
   fGroupConcatLen = gcc->fSize;
-  fCurrentLength -= strlen(gcc->fSeparator.c_str());
+  size_t sepSize = gcc->fSeparator.size();
+  fCurrentLength -= sepSize; // XXX Yet I have to find out why spearator has c_str() as nullptr here.
   fTimeZone = gcc->fTimeZone;
 
   fConstCols = gcc->fConstCols;
-  fConstantLen = strlen(gcc->fSeparator.c_str());
+  fConstantLen = sepSize;
 
   for (uint64_t i = 0; i < fConstCols.size(); i++)
-    fConstantLen += strlen(fConstCols[i].first.c_str());
+    fConstantLen += strlen(fConstCols[i].first.str());
 }
 
 void GroupConcator::outputRow(std::ostringstream& oss, const rowgroup::Row& row)
 {
   const CalpontSystemCatalog::ColDataType* types = row.getColTypes();
   vector<uint32_t>::iterator i = fConcatColumns.begin();
-  vector<pair<string, uint32_t> >::iterator j = fConstCols.begin();
+  auto j = fConstCols.begin();
 
   uint64_t groupColCount = fConcatColumns.size() + fConstCols.size();
 
@@ -422,7 +418,7 @@ void GroupConcator::outputRow(std::ostringstream& oss, const rowgroup::Row& row)
   {
     if (j != fConstCols.end() && k == j->second)
     {
-      oss << j->first;
+      oss << j->first.safeString();
       j++;
       continue;
     }
@@ -476,7 +472,7 @@ void GroupConcator::outputRow(std::ostringstream& oss, const rowgroup::Row& row)
       case CalpontSystemCatalog::VARCHAR:
       case CalpontSystemCatalog::TEXT:
       {
-        oss << row.getStringField(*i).c_str();
+        oss << row.getStringField(*i).str();
         break;
       }
 
@@ -677,7 +673,7 @@ const string GroupConcator::toString() const
   oss << "GroupConcat size-" << fGroupConcatLen;
   oss << "Concat   cols: ";
   vector<uint32_t>::const_iterator i = fConcatColumns.begin();
-  vector<pair<string, uint32_t> >::const_iterator j = fConstCols.begin();
+  auto j = fConstCols.begin();
   uint64_t groupColCount = fConcatColumns.size() + fConstCols.size();
 
   for (uint64_t k = 0; k < groupColCount; k++)
@@ -725,9 +721,12 @@ void GroupConcatOrderBy::initialize(const rowgroup::SP_GroupConcat& gcc)
   fSessionMemLimit = gcc->fSessionMemLimit;
 
   vector<std::pair<uint32_t, uint32_t> >::iterator i = gcc->fGroupCols.begin();
-
   while (i != gcc->fGroupCols.end())
-    fConcatColumns.push_back((*(i++)).second);
+  {
+    auto x = (*i).second;
+    fConcatColumns.push_back(x);
+    i++;
+  }
 
   IdbOrderBy::initialize(gcc->fRowGroup);
 }
@@ -878,6 +877,7 @@ uint8_t* GroupConcatOrderBy::getResultImpl(const string& sep)
 
   size_t prevResultSize = 0;
   size_t rowsProcessed = 0;
+  bool isNull = true;
   while (rowStack.size() > 0)
   {
     if (addSep)
@@ -888,6 +888,7 @@ uint8_t* GroupConcatOrderBy::getResultImpl(const string& sep)
     const OrderByRow& topRow = rowStack.top();
     fRow0.setData(topRow.fData);
     outputRow(oss, fRow0);
+    isNull = false;
     rowStack.pop();
     if (rowsProcessed >= fRowsPerRG)
     {
@@ -903,11 +904,15 @@ uint8_t* GroupConcatOrderBy::getResultImpl(const string& sep)
     }
   }
 
-  return swapStreamWithStringAndReturnBuf(oss);
+  return swapStreamWithStringAndReturnBuf(oss, isNull);
 }
 
-uint8_t* GroupConcator::swapStreamWithStringAndReturnBuf(ostringstream& oss)
+uint8_t* GroupConcator::swapStreamWithStringAndReturnBuf(ostringstream& oss, bool isNull)
 {
+  if (isNull) {
+    outputBuf_.reset();
+    return nullptr;
+  }
   int64_t resultSize = oss.str().size();
   oss << '\0' << '\0';
   outputBuf_.reset(new std::string(std::move(*oss.rdbuf()).str()));
@@ -983,6 +988,7 @@ void GroupConcatNoOrder::initialize(const rowgroup::SP_GroupConcat& gcc)
     cerr << IDBErrorInfo::instance()->errorMsg(fErrorCode) << " @" << __FILE__ << ":" << __LINE__;
     throw IDBExcept(fErrorCode);
   }
+
   fMemSize += newSize;
 
   fData.reinit(fRowGroup, fRowsPerRG);
@@ -1046,9 +1052,11 @@ uint8_t* GroupConcatNoOrder::getResultImpl(const string& sep)
 {
   ostringstream oss;
   bool addSep = false;
+
   fDataQueue.push(fData);
   size_t prevResultSize = 0;
 
+  bool isNull = true;
   while (fDataQueue.size() > 0)
   {
     fRowGroup.setData(&fDataQueue.front());
@@ -1062,6 +1070,7 @@ uint8_t* GroupConcatNoOrder::getResultImpl(const string& sep)
         addSep = true;
 
       outputRow(oss, fRow);
+      isNull = false;
       fRow.nextRow();
     }
     size_t sizeDiff = oss.str().size() - prevResultSize;
@@ -1075,7 +1084,7 @@ uint8_t* GroupConcatNoOrder::getResultImpl(const string& sep)
     fDataQueue.pop();
   }
 
-  return swapStreamWithStringAndReturnBuf(oss);
+  return swapStreamWithStringAndReturnBuf(oss, isNull);
 }
 
 const string GroupConcatNoOrder::toString() const