Change cost for REF to take into account cost for 1 extra key read_next

The main difference in code path between EQ_REF and REF is that for
REF we have to do an extra read_next on the index to check that there
is no more matching rows.

Before this patch we added a preference of EQ_REF by ensuring that REF
would always estimate to find at least 2 rows.

This patch adds the cost of the extra key read_next to REF access and
removes the code that limited REF to at least 2 rows. For some queries
this can have a big effect as the total estimated rows will be halved
for each REF table with 1 rows.

multi_range cost calculations are also changed to take into account
the difference between EQ_REF and REF.

The effect of the patch to the test suite:
- About 80 test case changed
- Almost all changes where for EXPLAIN where estimated rows for REF
  where changed from 2 to 1.
- A few test cases using explain extended had a change of 'filtered'.
  This is because of the estimated rows are now closer to the
  calculated selectivity.
- A very few test had a change of table order.
  This is because the change of estimated rows from 2 to 1 or the small
  cost change for REF
  (main.subselect_sj_jcl6, main.group_by, main.dervied_cond_pushdown,
  main.distinct, main.join_nested, main.order_by, main.join_cache)
- No key statistics and the estimated rows are now smaller which cased
  estimated filtering to be lower.
  (main.subselect_sj_mat)
- The number of total rows are halved.
  (main.derived_cond_pushdown)
- Plans with 1 row changed to use RANGE instead of REF.
  (main.group_min_max)
- ALL changed to REF
  (main.key_diff)
- Key changed from ref + index_only to PRIMARY key for InnoDB, as
  OPTIMIZER_ROW_LOOKUP_COST + OPTIMIZER_ROW_NEXT_FIND_COST is smaller than
  OPTIMIZER_KEY_LOOKUP_COST + OPTIMIZER_KEY_NEXT_FIND_COST.
  (main.join_outer_innodb)
- Cost changes printouts
  (main.opt_trace*)
- Result order change
  (innodb_gis.rtree)

mysql-test/main/costs.result

@@ -22,15 +22,15 @@ Last_query_cost	0.000000
 explain select count(*) from t1 where a > 0;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	PRIMARY	PRIMARY	4	NULL	12	Using where; Using index
-Last_query_cost	0.002795
+Last_query_cost	0.002877
 explain select count(*) from t1 where a > 1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	PRIMARY	PRIMARY	4	NULL	12	Using where; Using index
-Last_query_cost	0.002795
+Last_query_cost	0.002877
 explain select count(*) from t1 where a > 2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	PRIMARY	PRIMARY	4	NULL	11	Using where; Using index
-Last_query_cost	0.002665
+Last_query_cost	0.002747
 #
 # Shorter indexes are prefered over longer indexs
 #
@@ -41,20 +41,20 @@ Last_query_cost	0.007441
 explain select count(*) from t1 where b between 5 and 10;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	ba,bda	ba	5	NULL	6	Using where; Using index
-Last_query_cost	0.002015
+Last_query_cost	0.002097
 explain select sum(b+c) from t1 where b between 5 and 6 and c between 5 and 6;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	ba,bda,cba,cb	cba	10	NULL	2	Using where; Using index
-Last_query_cost	0.001494
+Last_query_cost	0.001577
 # Cost of 'd' should be slightly smaller as key 'ba' is longer than 'd'
 explain select count(*) from t1 where b > 6;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	ba,bda	ba	5	NULL	5	Using where; Using index
-Last_query_cost	0.001885
+Last_query_cost	0.001967
 explain select count(*) from t1 where d > 6;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	d	d	5	NULL	5	Using where; Using index
-Last_query_cost	0.001885
+Last_query_cost	0.001967
 #
 # Check covering index usage
 #
@@ -68,15 +68,15 @@ Last_query_cost	0.007441
 explain select count(*) from t1 where b=2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ref	ba,bda	ba	5	const	2	Using index
-Last_query_cost	0.001059
+Last_query_cost	0.001141
 explain select count(*) from t1 where b=2 and c=2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ref	ba,bda,cba,cb	cba	10	const,const	2	Using index
-Last_query_cost	0.001059
+Last_query_cost	0.001141
 explain select count(*) from t1 where b=3 and c between 3 and 4;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	ba,bda,cba,cb	cba	10	NULL	2	Using where; Using index
-Last_query_cost	0.001494
+Last_query_cost	0.001577
 #
 # Prefer eq keys over ref keys
 #
@@ -87,5 +87,5 @@ Last_query_cost	0.003126
 explain select a,b,e from t1 where d=10 or d=11;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	d	d	5	NULL	2	Using index condition
-Last_query_cost	0.003126
+Last_query_cost	0.003291
 drop table t1;