Refactor Sort_param::init_for_filesort

No logic changes.
Extract some of init_for_filesort logic into a separate function:
* Sort_param::setup_lengths_and_limit can be used to fill in the various
  xxx_length members of Sort_param, without having to allocate any of the
  other buffers.

Reviewer: Monty

sql/filesort.cc

@@ -87,43 +87,62 @@ static uint32 read_keypart_length(const uchar *from, uint bytes)
 }
 
 
-// @param sortlen  [Maximum] length of the sort key
-void Sort_param::init_for_filesort(uint sortlen, TABLE *table,
-                                   ha_rows maxrows, Filesort *filesort)
+/*
+  Initialize Sort_param for doing a filesort
+
+  @param table       Table to sort
+  @param Filesort    Filesort parameter to filesort()
+  @param sortlen     [Maximum] length of the sort key
+  @param limit_rows  Number of rows to return (may be less than rows to sort)
+*/
+
+void Sort_param::init_for_filesort(TABLE *table, Filesort *filesort,
+                                   uint sortlen, ha_rows limit_rows_arg)
 {
   DBUG_ASSERT(addon_fields == NULL);
 
-  sort_length= sortlen;
-  ref_length= table->file->ref_length;
-  accepted_rows= filesort->accepted_rows;
-
   if (!(table->file->ha_table_flags() & HA_FAST_KEY_READ) &&
       !table->fulltext_searched && !filesort->sort_positions)
   {
-    /* 
-      Get the descriptors of all fields whose values are appended 
+    /*
+      Get the descriptors of all fields whose values are appended
       to sorted fields and get its total length in addon_buf.length
     */
-    addon_fields= get_addon_fields(table, sort_length, &addon_length,
+    addon_fields= get_addon_fields(table, sortlen, &addon_length,
                                    &m_packable_length);
   }
-  if (using_addon_fields())
+  DBUG_ASSERT((using_addon_fields() == 0 || addon_length != 0));
+
+  setup_lengths_and_limit(table, sortlen, addon_length, limit_rows_arg);
+  accepted_rows= filesort->accepted_rows;
+}
+
+
+void Sort_param::setup_lengths_and_limit(TABLE *table,
+                                         uint sort_len_arg,
+                                         uint addon_length_arg,
+                                         ha_rows limit_rows_arg)
+{
+  sort_form= table;
+  sort_length= sort_len_arg;
+  limit_rows= limit_rows_arg;
+  ref_length= table->file->ref_length;
+
+  if (addon_length_arg)
   {
-    DBUG_ASSERT(addon_length < UINT_MAX32);
-    res_length= addon_length;
+    DBUG_ASSERT(addon_length_arg < UINT_MAX32);
+    res_length= addon_length_arg;
   }
   else
   {
     res_length= ref_length;
     /* 
-      The reference to the record is considered 
-      as an additional sorted field
+      The reference (rowid) to the record is considered as an additional
+      sorted field as we want to access rows in rowid order if possible.
     */
     sort_length+= ref_length;
   }
-  rec_length= sort_length + addon_length;
-  limit_rows= maxrows;
-  sort_form= table;
+  rec_length= sort_length + addon_length_arg;
 }
 
 

sql/sql_sort.h

@@ -590,10 +590,14 @@ class Sort_param {
     */
     tmp_buffer.set_charset(&my_charset_bin);
   }
-  void init_for_filesort(uint sortlen, TABLE *table,
-                         ha_rows maxrows, Filesort *filesort);
 
-   void  (*unpack)(TABLE *);
+  void init_for_filesort(TABLE *table, Filesort *filesort,
+                         uint sortlen, ha_rows limit_rows_arg);
+  void setup_lengths_and_limit(TABLE *table,
+                               uint sortlen,
+                               uint addon_length,
+                               ha_rows limit_rows_arg);
+  void  (*unpack)(TABLE *);
   /// Enables the packing of addons if possible.
   void try_to_pack_addons(ulong max_length_for_sort_data);