Moved window function computation code from JOIN::exec_inner() into

a separate function, JOIN::process_window_functions().

Author: spetrunia

sql/sql_select.cc

@@ -3235,6 +3235,7 @@ void JOIN::exec_inner()
     error= thd->is_error();
     DBUG_VOID_RETURN;
   }
+  process_window_functions(curr_fields_list);
 
   THD_STAGE_INFO(thd, stage_sending_data);
   DBUG_PRINT("info", ("%s", thd->proc_info));

sql/sql_select.h

@@ -1518,6 +1518,9 @@ class JOIN :public Sql_alloc
   int reinit();
   int init_execution();
   void exec();
+
+  void process_window_functions(List<Item> *curr_fields_list);
+
   void exec_inner();
   bool prepare_result(List<Item> **columns_list);
   int destroy();
@@ -2291,4 +2294,5 @@ class Pushdown_query: public Sql_alloc
   int execute(JOIN *join);
 };
 
+bool test_if_order_compatible(SQL_I_List<ORDER> &a, SQL_I_List<ORDER> &b);
 #endif /* SQL_SELECT_INCLUDED */

sql/sql_window.cc

@@ -1,4 +1,6 @@
 #include "sql_select.h"
+#include "item_windowfunc.h"
+#include "filesort.h"
 #include "sql_window.h"
 
 
@@ -77,3 +79,140 @@ setup_windows(THD *thd, Ref_ptr_array ref_pointer_array, TABLE_LIST *tables,
   }
   DBUG_RETURN(0);
 }
+
+
+/*
+  @brief
+    This function is called by JOIN::exec to compute window function values
+  
+  @detail
+    JOIN::exec calls this after it has filled the temporary table with query
+    output. The temporary table has fields to store window function values.
+*/
+
+void JOIN::process_window_functions(List<Item> *curr_fields_list)
+{
+  /*
+   TODO Get this code to set can_compute_window_function during preparation,
+   not during execution.
+
+   The reason for this is the following:
+   Our single scan optimization for window functions without tmp table,
+   is valid, if and only if, we only need to perform one sorting operation,
+   via filesort. The cases where we need to perform one sorting operation only:
+
+   * A select with only one window function.
+   * A select with multiple window functions, but they must have their
+     partition and order by clauses compatible. This means that one ordering
+     is acceptable for both window functions.
+
+       For example:
+       partition by a, b, c; order by d, e    results in sorting by a b c d e.
+       partition by a; order by d             results in sorting by a d.
+
+       This kind of sorting is compatible. The less specific partition does
+       not care for the order of b and c columns so it is valid if we sort
+       by those in case of equality over a.
+
+       partition by a, b; order by d, e      results in sorting by a b d e
+       partition by a; order by e            results in sorting by a e
+
+      This sorting is incompatible due to the order by clause. The partition by
+      clause is compatible, (partition by a) is a prefix for (partition by a, b)
+      However, order by e is not a prefix for order by d, e, thus it is not
+      compatible.
+
+    The rule for having compatible sorting is thus:
+      Each partition order must contain the other window functions partitions
+      prefixes, or be a prefix itself. This must hold true for all partitions.
+      Analog for the order by clause.
+
+  */
+
+  List<Item_window_func> window_functions;
+  SQL_I_List<ORDER> largest_partition;
+  SQL_I_List<ORDER> largest_order_by;
+  List_iterator_fast<Item> it(*curr_fields_list);
+  bool can_compute_window_live = !need_tmp;
+
+  Item *item;
+  // Construct the window_functions item list and check if they can be
+  // computed using only one sorting.
+  //
+  // TODO: Perhaps group functions into compatible sorting bins
+  // to minimize the number of sorting passes required to compute all of them.
+  while ((item= it++))
+  {
+    if (item->type() == Item::WINDOW_FUNC_ITEM)
+    {
+      Item_window_func *item_win = (Item_window_func *) item;
+      window_functions.push_back(item_win);
+      if (!can_compute_window_live)
+        continue;  // No point checking  since we have to perform multiple sorts.
+      Window_spec *spec = item_win->window_spec;
+      // Having an empty partition list on one window function and a
+      // not empty list on a separate window function causes the sorting
+      // to be incompatible.
+      //
+      // Example:
+      // over (partition by a, order by x) && over (order by x).
+      //
+      // The first function requires an ordering by a first and then by x,
+      // while the seond function requires an ordering by x first.
+      // The same restriction is not required for the order by clause.
+      if (largest_partition.elements && !spec->partition_list.elements)
+      {
+        can_compute_window_live= FALSE;
+        continue;
+      }
+      can_compute_window_live= test_if_order_compatible(largest_partition,
+                                                        spec->partition_list);
+      if (!can_compute_window_live)
+        continue;
+
+      can_compute_window_live= test_if_order_compatible(largest_order_by,
+                                                        spec->order_list);
+      if (!can_compute_window_live)
+        continue;
+
+      if (largest_partition.elements < spec->partition_list.elements)
+        largest_partition = spec->partition_list;
+      if (largest_order_by.elements < spec->order_list.elements)
+        largest_order_by = spec->order_list;
+    }
+  }
+
+  if (can_compute_window_live && window_functions.elements && table_count == 1)
+  {
+    ha_rows examined_rows = 0;
+    ha_rows found_rows = 0;
+    ha_rows filesort_retval;
+    SORT_FIELD *s_order= (SORT_FIELD *) my_malloc(sizeof(SORT_FIELD) *
+        (largest_partition.elements + largest_order_by.elements) + 1,
+        MYF(MY_WME | MY_ZEROFILL | MY_THREAD_SPECIFIC));
+
+    size_t pos= 0;
+    for (ORDER* curr = largest_partition.first; curr; curr=curr->next, pos++)
+      s_order[pos].item = *curr->item;
+
+    for (ORDER* curr = largest_order_by.first; curr; curr=curr->next, pos++)
+      s_order[pos].item = *curr->item;
+
+    table[0]->sort.io_cache=(IO_CACHE*) my_malloc(sizeof(IO_CACHE),
+                                               MYF(MY_WME | MY_ZEROFILL|
+                                                   MY_THREAD_SPECIFIC));
+
+
+    filesort_retval= filesort(thd, table[0], s_order,
+                              (largest_partition.elements + largest_order_by.elements),
+                              this->select, HA_POS_ERROR, FALSE,
+                              &examined_rows, &found_rows,
+                              this->explain->ops_tracker.report_sorting(thd));
+    table[0]->sort.found_records= filesort_retval;
+
+    join_tab->read_first_record = join_init_read_record;
+    join_tab->records= found_rows;
+
+    my_free(s_order);
+  }
+}

sql/sql_window.h

@@ -5,6 +5,13 @@
 #include "my_global.h"
 #include "item.h"
 
+/*
+  Window functions module. 
+  
+  Each instance of window function has its own element in SELECT_LEX::window_specs.
+*/
+
+
 class Window_frame_bound : public Sql_alloc
 {