MDEV-22728: SIGFPE in Unique::get_cost_calc_buff_size from prepare_search_best_index_intersect on optimized builds

For low sort_buffer_size, in the cost calculation of using the Unique object the elements in the tree were evaluated to 0, make sure to have atleast 1 element in the Unique tree.

Also for the function Unique::get allocate memory for atleast MERGEBUFF2+1 keys.

2 files changed, 13 insertions, 1 deletions

diff --git a/sql/sql_class.h b/sql/sql_class.h
index 9071a2db516..7ca3896a69d 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -5216,6 +5216,9 @@ public:
   {
     ulonglong max_elems_in_tree=
       max_in_memory_size / ALIGN_SIZE(sizeof(TREE_ELEMENT)+key_size);
+
+    if (max_elems_in_tree == 0)
+      max_elems_in_tree= 1;
     return (int) (sizeof(uint)*(1 + nkeys/max_elems_in_tree));
   }
 
diff --git a/sql/uniques.cc b/sql/uniques.cc
index 03f25d31384..27da0fc54c6 100644
--- a/sql/uniques.cc
+++ b/sql/uniques.cc
@@ -317,6 +317,9 @@ double Unique::get_use_cost(uint *buffer, size_t nkeys, uint key_size,
   max_elements_in_tree= ((size_t) max_in_memory_size /
                          ALIGN_SIZE(sizeof(TREE_ELEMENT)+key_size));
 
+  if (max_elements_in_tree == 0)
+    max_elements_in_tree= 1;
+
   n_full_trees=    nkeys / max_elements_in_tree;
   last_tree_elems= nkeys % max_elements_in_tree;
 
@@ -781,7 +784,13 @@ bool Unique::get(TABLE *table)
   /* Not enough memory; Save the result to file && free memory used by tree */
   if (flush())
     return 1;
-  size_t buff_sz= (max_in_memory_size / full_size + 1) * full_size;
+
+  /*
+    merge_buffer must fit at least MERGEBUFF2 + 1 keys, because
+    merge_index() can merge that many BUFFPEKs at once. The extra space for
+    one key for Sort_param::unique_buff
+  */
+  size_t buff_sz= MY_MAX(MERGEBUFF2+1, max_in_memory_size/full_size+1) * full_size;
   if (!(sort_buffer= (uchar*) my_malloc(buff_sz, MYF(MY_THREAD_SPECIFIC|MY_WME))))
     return 1;