diff options
Diffstat (limited to 'sql/ha_myisammrg.cc')
| -rw-r--r-- | sql/ha_myisammrg.cc | 22 |
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 |