Fixed bug #21727.

This is a performance issue for queries with subqueries evaluation
of which requires filesort.
Allocation of memory for the sort buffer at each evaluation of a
subquery may take a significant amount of time if the buffer is rather big.
With the fix we allocate the buffer at the first evaluation of the
subquery and reuse it at each subsequent evaluation.

mysql-test/r/subselect.result

@@ -3545,3 +3545,19 @@ FROM t1 GROUP BY t1.a LIMIT 1)
 2
 2
 DROP TABLE t1,t2;
+CREATE TABLE t1 (a int, b int auto_increment, PRIMARY KEY (b));
+CREATE TABLE t2 (x int auto_increment, y int, z int,
+PRIMARY KEY (x), FOREIGN KEY (y) REFERENCES t1 (b));
+SET SESSION sort_buffer_size = 32 * 1024;
+SELECT SQL_NO_CACHE COUNT(*) 
+FROM (SELECT  a, b, (SELECT x FROM t2 WHERE y=b ORDER BY z DESC LIMIT 1) c
+FROM t1) t;
+COUNT(*)
+3000
+SET SESSION sort_buffer_size = 8 * 1024 * 1024;
+SELECT SQL_NO_CACHE COUNT(*) 
+FROM (SELECT  a, b, (SELECT x FROM t2 WHERE y=b ORDER BY z DESC LIMIT 1) c
+FROM t1) t;
+COUNT(*)
+3000
+DROP TABLE t1,t2;

mysql-test/t/subselect.test

@@ -2426,3 +2426,40 @@ SELECT (
 FROM t1 t2
 GROUP BY t2.a;
 DROP TABLE t1,t2;  
+
+#
+# Bug #21727: Correlated subquery that requires filesort:
+#             slow with big sort_buffer_size 
+#
+
+CREATE TABLE t1 (a int, b int auto_increment, PRIMARY KEY (b));
+CREATE TABLE t2 (x int auto_increment, y int, z int,
+                 PRIMARY KEY (x), FOREIGN KEY (y) REFERENCES t1 (b));
+
+disable_query_log;
+let $1=3000;
+while ($1)
+{
+  eval INSERT INTO t1(a) VALUES(RAND()*1000);
+  eval SELECT MAX(b) FROM t1 INTO @id;
+  let $2=10;
+  while ($2)
+  {
+    eval INSERT INTO t2(y,z) VALUES(@id,RAND()*1000);
+    dec $2;
+  } 
+  dec $1;
+}
+enable_query_log;
+
+SET SESSION sort_buffer_size = 32 * 1024;
+SELECT SQL_NO_CACHE COUNT(*) 
+  FROM (SELECT  a, b, (SELECT x FROM t2 WHERE y=b ORDER BY z DESC LIMIT 1) c
+          FROM t1) t;
+
+SET SESSION sort_buffer_size = 8 * 1024 * 1024;
+SELECT SQL_NO_CACHE COUNT(*) 
+  FROM (SELECT  a, b, (SELECT x FROM t2 WHERE y=b ORDER BY z DESC LIMIT 1) c
+          FROM t1) t;
+
+DROP TABLE t1,t2;

sql/filesort.cc

@@ -109,6 +109,8 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
   DBUG_PUSH("");		/* No DBUG here */
 #endif
   FILESORT_INFO table_sort;
+  TABLE_LIST *tab= table->pos_in_table_list;
+  Item_subselect *subselect= tab ? tab->containing_subselect() : 0;
   /* 
     Don't use table->sort in filesort as it is also used by 
     QUICK_INDEX_MERGE_SELECT. Work with a copy and put it back at the end 
@@ -121,7 +123,6 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
   my_b_clear(&tempfile);
   my_b_clear(&buffpek_pointers);
   buffpek=0;
-  sort_keys= (uchar **) NULL;
   error= 1;
   bzero((char*) &param,sizeof(param));
   param.sort_length= sortlength(thd, sortorder, s_length, &multi_byte_charset);
@@ -202,13 +203,15 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
     ulong old_memavl;
     ulong keys= memavl/(param.rec_length+sizeof(char*));
     param.keys=(uint) min(records+1, keys);
-    if ((sort_keys= (uchar **) make_char_array(param.keys, param.rec_length,
-					       MYF(0))))
+    if (table_sort.sort_keys ||
+        (table_sort.sort_keys= (uchar **) make_char_array(param.keys, param.rec_length,
+                                               MYF(0))))
       break;
     old_memavl=memavl;
     if ((memavl=memavl/4*3) < min_sort_memory && old_memavl > min_sort_memory)
       memavl= min_sort_memory;
   }
+  sort_keys= table_sort.sort_keys;
   if (memavl < min_sort_memory)
   {
     my_error(ER_OUTOFMEMORY,MYF(ME_ERROR+ME_WAITTANG),
@@ -235,8 +238,12 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
   }
   else
   {
-    if (!(buffpek=read_buffpek_from_file(&buffpek_pointers, maxbuffer)))
+    if (!table_sort.buffpek && table_sort.buffpek_len < maxbuffer &&
+        !(table_sort.buffpek=
+          (byte *) read_buffpek_from_file(&buffpek_pointers, maxbuffer)))
       goto err;
+    buffpek= (BUFFPEK *) table_sort.buffpek;
+    table_sort.buffpek_len= maxbuffer;
     close_cached_file(&buffpek_pointers);
 	/* Open cached file if it isn't open */
     if (! my_b_inited(outfile) &&
@@ -269,8 +276,14 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
  err:
   if (param.tmp_buffer)
     x_free(param.tmp_buffer);
-  x_free((gptr) sort_keys);
-  x_free((gptr) buffpek);
+  if (!subselect || !subselect->is_uncacheable())
+  {
+    x_free((gptr) sort_keys);
+    table_sort.sort_keys= 0;
+    x_free((gptr) buffpek);
+    table_sort.buffpek= 0;
+    table_sort.buffpek_len= 0;
+  }
   close_cached_file(&tempfile);
   close_cached_file(&buffpek_pointers);
   if (my_b_inited(outfile))
@@ -301,13 +314,27 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
 } /* filesort */
 
 
-void filesort_free_buffers(TABLE *table)
+void filesort_free_buffers(TABLE *table, bool full)
 {
   if (table->sort.record_pointers)
   {
     my_free((gptr) table->sort.record_pointers,MYF(0));
     table->sort.record_pointers=0;
   }
+  if (full)
+  {
+    if (table->sort.sort_keys )
+    {
+      x_free((gptr) table->sort.sort_keys);
+      table->sort.sort_keys= 0;
+    }
+    if (table->sort.buffpek)
+    {
+      x_free((gptr) table->sort.buffpek);
+      table->sort.buffpek= 0;
+      table->sort.buffpek_len= 0;
+    }
+  }
   if (table->sort.addon_buf)
   {
     my_free((char *) table->sort.addon_buf, MYF(0));

sql/item_subselect.h

@@ -114,6 +114,7 @@ public:
     single select and union subqueries only.
   */
   bool is_evaluated() const;
+  bool is_uncacheable() const;
 
   /*
     Used by max/min subquery to initialize value presence registration
@@ -428,3 +429,9 @@ inline bool Item_subselect::is_evaluated() const
   return engine->is_executed();
 }
 
+inline bool Item_subselect::is_uncacheable() const
+{
+  return engine->uncacheable();
+}
+
+

sql/mysql_priv.h

@@ -1465,7 +1465,7 @@ void end_read_record(READ_RECORD *info);
 ha_rows filesort(THD *thd, TABLE *form,struct st_sort_field *sortorder,
 		 uint s_length, SQL_SELECT *select,
 		 ha_rows max_rows, ha_rows *examined_rows);
-void filesort_free_buffers(TABLE *table);
+void filesort_free_buffers(TABLE *table, bool full);
 void change_double_for_sort(double nr,byte *to);
 double my_double_round(double value, int dec, bool truncate);
 int get_quick_record(SQL_SELECT *select);

sql/records.cc

@@ -193,7 +193,7 @@ void end_read_record(READ_RECORD *info)
   }
   if (info->table)
   {
-    filesort_free_buffers(info->table);
+    filesort_free_buffers(info->table,0);
     (void) info->file->extra(HA_EXTRA_NO_CACHE);
     if (info->read_record != rr_quick) // otherwise quick_range does it
       (void) info->file->ha_index_or_rnd_end();

sql/sql_base.cc

@@ -1478,6 +1478,7 @@ TABLE *open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
   table->file->ft_handler= 0;
   if (table->timestamp_field)
     table->timestamp_field_type= table->timestamp_field->get_auto_set_type();
+  table->pos_in_table_list= table_list;
   table_list->updatable= 1; // It is not derived table nor non-updatable VIEW
   DBUG_ASSERT(table->key_read == 0);
   DBUG_RETURN(table);
@@ -2762,6 +2763,7 @@ TABLE *open_temporary_table(THD *thd, const char *path, const char *db,
     if (thd->slave_thread)
       slave_open_temp_tables++;
   }
+  tmp_table->pos_in_table_list= 0;
   DBUG_RETURN(tmp_table);
 }
 

sql/sql_select.cc

@@ -1260,14 +1260,14 @@ JOIN::reinit()
     exec_tmp_table1->file->extra(HA_EXTRA_RESET_STATE);
     exec_tmp_table1->file->delete_all_rows();
     free_io_cache(exec_tmp_table1);
-    filesort_free_buffers(exec_tmp_table1);
+    filesort_free_buffers(exec_tmp_table1,0);
   }
   if (exec_tmp_table2)
   {
     exec_tmp_table2->file->extra(HA_EXTRA_RESET_STATE);
     exec_tmp_table2->file->delete_all_rows();
     free_io_cache(exec_tmp_table2);
-    filesort_free_buffers(exec_tmp_table2);
+    filesort_free_buffers(exec_tmp_table2,0);
   }
   if (items0)
     set_items_ref_array(items0);
@@ -6066,7 +6066,7 @@ void JOIN::cleanup(bool full)
     if (tables > const_tables) // Test for not-const tables
     {
       free_io_cache(table[const_tables]);
-      filesort_free_buffers(table[const_tables]);
+      filesort_free_buffers(table[const_tables],full);
     }
 
     if (full)

sql/sql_show.cc

@@ -3977,7 +3977,7 @@ bool get_schema_tables_result(JOIN *join)
         table_list->table->file->extra(HA_EXTRA_RESET_STATE);
         table_list->table->file->delete_all_rows();
         free_io_cache(table_list->table);
-        filesort_free_buffers(table_list->table);
+        filesort_free_buffers(table_list->table,1);
         table_list->table->null_row= 0;
       }
       else

sql/sql_table.cc

@@ -2266,7 +2266,6 @@ static bool mysql_admin_table(THD* thd, TABLE_LIST* tables,
       goto send_result;
     }
 
-    table->table->pos_in_table_list= table;
     if ((table->table->db_stat & HA_READ_ONLY) && open_for_modify)
     {
       char buff[FN_REFLEN + MYSQL_ERRMSG_SIZE];
@@ -4256,8 +4255,6 @@ bool mysql_checksum_table(THD *thd, TABLE_LIST *tables, HA_CHECK_OPT *check_opt)
     }
     else
     {
-      t->pos_in_table_list= table;
-
       if (t->file->table_flags() & HA_HAS_CHECKSUM &&
 	  !(check_opt->flags & T_EXTEND))
 	protocol->store((ulonglong)t->file->checksum());

sql/table.cc

@@ -3018,6 +3018,23 @@ void st_table_list::reinit_before_use(THD *thd)
          embedding->nested_join->join_list.head() == embedded);
 }
 
+/*
+  Return subselect that contains the FROM list this table is taken from
+
+  SYNOPSIS
+    st_table_list::containing_subselect()
+ 
+  RETURN
+    Subselect item for the subquery that contains the FROM list
+    this table is taken from if there is any
+    0 - otherwise
+
+*/
+
+Item_subselect *st_table_list::containing_subselect()
+{    
+  return (select_lex ? select_lex->master_unit()->item : 0);
+}
 
 /*****************************************************************************
 ** Instansiate templates

sql/table.h

@@ -18,6 +18,7 @@
 /* Structs that defines the TABLE */
 
 class Item;				/* Needed by ORDER */
+class Item_subselect;
 class GRANT_TABLE;
 class st_select_lex_unit;
 class st_select_lex;
@@ -68,6 +69,9 @@ enum frm_type_enum
 typedef struct st_filesort_info
 {
   IO_CACHE *io_cache;           /* If sorted through filebyte                */
+  uchar   **sort_keys;          /* Buffer for sorting keys                   */
+  byte     *buffpek;            /* Buffer for buffpek structures             */
+  uint      buffpek_len;        /* Max number of buffpeks in the buffer      */
   byte     *addon_buf;          /* Pointer to a buffer if sorted with fields */
   uint      addon_length;       /* Length of the buffer                      */
   struct st_sort_addon_field *addon_field;     /* Pointer to the fields info */
@@ -678,6 +682,7 @@ typedef struct st_table_list
     procedure.
   */
   void reinit_before_use(THD *thd);
+  Item_subselect *containing_subselect();
 
 private:
   bool prep_check_option(THD *thd, uint8 check_opt_type);