BUG#17314: Can't use index_merge/intersection for MERGE tables

1. Fix index access costs for MERGE tables, set block_size=myisam_block_size/#underlying_tables 
   instead of 0 which it was before.
2. Make index scans on MERGE table to return records in (key_tuple, merge_table_rowid) order, 
   instead of just (key_tuple) order. This makes an index scan on MERGE table to be truly a ROR-scan
   which is a requirement for index_merge union/intersection.


myisammrg/myrg_queue.c:
  BUG#17314: Make index scans on MERGE table return records ordered by (keytuple, merge_table_rowid).
mysql-test/r/index_merge.result:
  Testcase for BUG#17314
mysql-test/r/merge.result:
  BUG#17314: update testcase result
mysql-test/t/index_merge.test:
  Testcase for BUG#17314
sql/ha_myisammrg.cc:
  BUG#17314: For MERGE tables, set handler::block_size to myisam_block_size/#underlying_tables, and not to 0.

1 files changed, 21 insertions, 1 deletions

diff --git a/sql/ha_myisammrg.cc b/sql/ha_myisammrg.cc
index da4136def68..9780f163634 100644
--- a/sql/ha_myisammrg.cc
+++ b/sql/ha_myisammrg.cc
@@ -288,7 +288,27 @@ void ha_myisammrg::info(uint flag)
   table->s->db_options_in_use= info.options;
   table->s->is_view= 1;
   mean_rec_length= info.reclength;
-  block_size=0;
+  
+  /* 
+    The handler::block_size is used all over the code in index scan cost
+    calculations. It is used to get number of disk seeks required to
+    retrieve a number of index tuples.
+    If the merge table has N underlying tables, then (assuming underlying
+    tables have equal size, the only "simple" approach we can use)
+    retrieving X index records from a merge table will require N times more
+    disk seeks compared to doing the same on a MyISAM table with equal
+    number of records.
+    In the edge case (file_tables > myisam_block_size) we'll get
+    block_size==0, and index calculation code will act as if we need one
+    disk seek to retrieve one index tuple.
+
+    TODO: In 5.2 index scan cost calculation will be factored out into a
+    virtual function in class handler and we'll be able to remove this hack.
+  */
+  block_size= 0;
+  if (file->tables)
+    block_size= myisam_block_size / file->tables;
+  
   update_time=0;
 #if SIZEOF_OFF_T > 4
   ref_length=6;					// Should be big enough