MDEV-30786 SIGFPE in cost_group_min_max on EXP

The problem was that for merge tables without any underlaying tables the
block size was 0, which the code could not handle.

Fixed by ensuring that MERGE tables never have a block size of 0.
Fixed also a wrong assumption of number of seeks needed to scan
merge tables.

mysql-test/main/merge.result

@@ -3965,5 +3965,19 @@ SELECT DISTINCT f FROM tm WHERE f IN (47, 126, 97, 48, 73, 0);
 f
 DROP TABLE tm, t1, t2;
 #
+# MDEV-30786 SIGFPE in cost_group_min_max on EXP
+#
+SET use_stat_tables='preferably';
+CREATE TABLE t1 (a INT,b INT,KEY i1 (a),KEY i2 (b)) ENGINE=MRG_MyISAM;
+ANALYZE LOCAL TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	note	The storage engine for the table doesn't support analyze
+EXPLAIN SELECT DISTINCT a FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	NULL	i1	5	NULL	1	Using index for group-by
+drop table t1;
+set use_stat_tables=default;
+#
 # End of 11.0 tests
 #

mysql-test/main/merge.test

@@ -2922,6 +2922,16 @@ CREATE TABLE tm (f INT, KEY(f)) ENGINE=MERGE UNION = (t1, t2);
 SELECT DISTINCT f FROM tm WHERE f IN (47, 126, 97, 48, 73, 0);
 DROP TABLE tm, t1, t2;
 
+--echo #
+--echo # MDEV-30786 SIGFPE in cost_group_min_max on EXP
+--echo #
+SET use_stat_tables='preferably';
+CREATE TABLE t1 (a INT,b INT,KEY i1 (a),KEY i2 (b)) ENGINE=MRG_MyISAM;
+ANALYZE LOCAL TABLE t1;
+EXPLAIN SELECT DISTINCT a FROM t1;
+drop table t1;
+set use_stat_tables=default;
+
 --echo #
 --echo # End of 11.0 tests
 --echo #

storage/myisammrg/ha_myisammrg.cc

@@ -343,7 +343,7 @@ IO_AND_CPU_COST ha_myisammrg::rnd_pos_time(ha_rows rows)
 {
   IO_AND_CPU_COST cost= handler::rnd_pos_time(rows);
   /*
-    Row data is notcached. costs.row_lookup_cost includes the cost of
+    Row data is not cached. costs.row_lookup_cost includes the cost of
     the reading the row from system (probably cached by the OS).
   */
   cost.io= 0;
@@ -1300,26 +1300,17 @@ int ha_myisammrg::info(uint flag)
   table->s->keys_in_use.set_prefix(table->s->keys);
   stats.mean_rec_length= mrg_info.reclength;
 
-  /* 
+  /*
     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.
+    If the merge table has N underlying tables, there will be
+    N more disk seeks compared to a scanning a normal MyISAM table.
+    The number of bytes read is the rougly the same for a normal MyISAM
+    and a MyISAM merge tables.
   */
-  stats.block_size= 0;
-  if (file->tables)
-    stats.block_size= myisam_block_size / file->tables;
-
+  stats.block_size= myisam_block_size;
+
   stats.update_time= 0;
 #if SIZEOF_OFF_T > 4
   ref_length=6;					// Should be big enough