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MDEV-5846 MySQL Bug #18144 - Cost with FORCE/USE index seems incorrect in some cases.

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This commit is contained in:
Sergei Golubchik
2014-03-20 23:27:08 +01:00
parent dee0fd4233
commit 3bb249612f
4 changed files with 69 additions and 2 deletions
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27
mysql-test/r/key.result
View File

@ -612,3 +612,30 @@ SELECT 1 as RES FROM t1 AS t1_outer WHERE
(SELECT max(b) FROM t1 GROUP BY a HAVING a < 2) > 12;
RES
DROP TABLE t1;
#
# Bug#18144: Cost with FORCE/USE index seems incorrect in some cases.
#
# We are interested in showing that the cost for the last plan is higher
# than for the preceding two plans.
#
CREATE TABLE t1( a INT, b INT, KEY( a ) );
INSERT INTO t1 values (1, 2), (1, 3), (2, 3), (2, 4), (3, 4), (3, 5);
EXPLAIN SELECT a, SUM( b ) FROM t1 GROUP BY a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 6 Using temporary; Using filesort
SHOW STATUS LIKE 'Last_query_cost';
Variable_name Value
Last_query_cost 9.212184
EXPLAIN SELECT a, SUM( b ) FROM t1 USE INDEX( a ) GROUP BY a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 6 Using temporary; Using filesort
SHOW STATUS LIKE 'Last_query_cost';
Variable_name Value
Last_query_cost 9.212184
EXPLAIN SELECT a, SUM( b ) FROM t1 FORCE INDEX( a ) GROUP BY a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL a 5 NULL 6
SHOW STATUS LIKE 'Last_query_cost';
Variable_name Value
Last_query_cost 14.199000
DROP TABLE t1;

20
mysql-test/t/key.test
View File

@ -561,3 +561,23 @@ SELECT 1 as RES FROM t1 AS t1_outer WHERE
(SELECT max(b) FROM t1 GROUP BY a HAVING a < 2) > 12;
DROP TABLE t1;
--echo #
--echo # Bug#18144: Cost with FORCE/USE index seems incorrect in some cases.
--echo #
--echo # We are interested in showing that the cost for the last plan is higher
--echo # than for the preceding two plans.
--echo #
CREATE TABLE t1( a INT, b INT, KEY( a ) );
INSERT INTO t1 values (1, 2), (1, 3), (2, 3), (2, 4), (3, 4), (3, 5);
EXPLAIN SELECT a, SUM( b ) FROM t1 GROUP BY a;
SHOW STATUS LIKE 'Last_query_cost';
EXPLAIN SELECT a, SUM( b ) FROM t1 USE INDEX( a ) GROUP BY a;
SHOW STATUS LIKE 'Last_query_cost';
EXPLAIN SELECT a, SUM( b ) FROM t1 FORCE INDEX( a ) GROUP BY a;
SHOW STATUS LIKE 'Last_query_cost';
DROP TABLE t1;

12
sql/handler.h
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@ -2721,6 +2721,18 @@ public:
}
virtual double scan_time()
{ return ulonglong2double(stats.data_file_length) / IO_SIZE + 2; }
/**
The cost of reading a set of ranges from the table using an index
to access it.
@param index The index number.
@param ranges The number of ranges to be read.
@param rows Total number of rows to be read.
This method can be used to calculate the total cost of scanning a table
using an index by calling it using read_time(index, 1, table_size).
*/
virtual double read_time(uint index, uint ranges, ha_rows rows)
{ return rows2double(ranges+rows); }

10
sql/sql_select.cc
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@ -6158,7 +6158,11 @@ best_access_path(JOIN *join,
else
{
/* Estimate cost of reading table. */
if (s->table->force_index && !best_key) // index scan
tmp= s->table->file->read_time(s->ref.key, 1, s->records);
else // table scan
tmp= s->scan_time();
if ((s->table->map & join->outer_join) || disable_jbuf) // Can't use join cache
{
/*
@ -7611,7 +7615,11 @@ best_extension_by_limited_search(JOIN *join,
if (join->sort_by_table &&
join->sort_by_table !=
join->positions[join->const_tables].table->table)
/* We have to make a temp table */
/*
We may have to make a temp table, note that this is only a
heuristic since we cannot know for sure at this point.
Hence it may be wrong.
*/
current_read_time+= current_record_count;
if (current_read_time < join->best_read)
{