Bug#36805260: Unnecessary copying in StoreLinkedImmutableStringFromTableBuffers()

StoreLinkedImmutableStringFromTableBuffers() has a TableCollection
parameter which is passed by value. TableCollection is a relatively
big object, and copying it may also require allocating memory on the
heap, so it would be good to avoid the copying.

Pass the TableCollection as a constant reference instead.

Change-Id: Ib6a34aa4d7190c34249ff70555c87b0a82efe01a
(cherry picked from commit fbfa4e117f2aa88e686a661eb01a6c144098f176)

Author: kahatlen

sql/iterators/hash_join_buffer.cc

@@ -46,8 +46,9 @@
 using pack_rows::TableCollection;
 
 LinkedImmutableString StoreLinkedImmutableStringFromTableBuffers(
-    MEM_ROOT *mem_root, MEM_ROOT *overflow_mem_root, TableCollection tables,
-    LinkedImmutableString next_ptr, size_t row_size_upper_bound, bool *full) {
+    MEM_ROOT *mem_root, MEM_ROOT *overflow_mem_root,
+    const TableCollection &tables, LinkedImmutableString next_ptr,
+    size_t row_size_upper_bound, bool *full) {
   if (tables.has_blob_column()) {
     // The row size upper bound can have changed.
     row_size_upper_bound = ComputeRowSizeUpperBound(tables);

sql/iterators/hash_join_buffer.h

@@ -219,7 +219,7 @@ class HashJoinRowBuffer {
 /// External interface to the corresponding member in HashJoinRowBuffer
 LinkedImmutableString StoreLinkedImmutableStringFromTableBuffers(
     MEM_ROOT *mem_root, MEM_ROOT *overflow_mem_root,
-    pack_rows::TableCollection tables, LinkedImmutableString next_ptr,
+    const pack_rows::TableCollection &tables, LinkedImmutableString next_ptr,
     size_t row_size_upper_bound, bool *full);
 
 #endif  // SQL_ITERATORS_HASH_JOIN_BUFFER_H_