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Fixed bug#28404.

Browse Source 
This patch adds cost estimation for the queries with ORDER BY / GROUP BY
and LIMIT. 
If there was a ref/range access to the table whose rows were required
to be ordered in the result set the optimizer always employed this access
though a scan by a different index that was compatible with the required 
order could be cheaper to produce the first L rows of the result set.
Now for such queries the optimizer makes a choice between the cheapest
ref/range accesses not compatible with the given order and index scans
compatible with it.
This commit is contained in:
igor@olga.mysql.com
2007-08-02 12:45:56 -07:00
parent 642b0c00ed
commit cf39429295
14 changed files with 359 additions and 105 deletions
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8
mysql-test/r/distinct.result
View File

@ -209,16 +209,16 @@ id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL PRIMARY 4 NULL 4 Using index
explain SELECT distinct t1.a from t1 order by a desc limit 1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL PRIMARY 4 NULL 4 Using index
1 SIMPLE t1 index NULL PRIMARY 4 NULL 1 Using index
explain SELECT distinct a from t3 order by a desc limit 2;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t3 index NULL a 5 NULL 204 Using index
1 SIMPLE t3 index NULL a 5 NULL 40 Using index
explain SELECT distinct a,b from t3 order by a+1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t3 ALL NULL NULL NULL NULL 204 Using temporary; Using filesort
explain SELECT distinct a,b from t3 order by a limit 10;
explain SELECT distinct a,b from t3 order by a limit 2;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t3 index NULL a 5 NULL 204 Using temporary
1 SIMPLE t3 index NULL a 5 NULL 2 Using temporary
explain SELECT a,b from t3 group by a,b order by a+1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t3 ALL NULL NULL NULL NULL 204 Using temporary; Using filesort

2
mysql-test/r/endspace.result
View File

@ -154,7 +154,7 @@ teststring
teststring
explain select * from t1 order by text1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL key1 34 NULL 3
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using filesort
alter table t1 modify text1 char(32) binary not null;
select * from t1 order by text1;
text1

2
mysql-test/r/group_by.result
View File

@ -1144,7 +1144,7 @@ CREATE TABLE t2 (a INT, b INT, KEY(a));
INSERT INTO t2 VALUES (1, 1), (2, 2), (3,3), (4,4);
EXPLAIN SELECT a, SUM(b) FROM t2 GROUP BY a LIMIT 2;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t2 index NULL a 5 NULL 4
1 SIMPLE t2 index NULL a 5 NULL 2
EXPLAIN SELECT a, SUM(b) FROM t2 IGNORE INDEX (a) GROUP BY a LIMIT 2;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t2 ALL NULL NULL NULL NULL 4 Using temporary; Using filesort

10
mysql-test/r/group_min_max.result
View File

@ -1963,20 +1963,20 @@ id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 128 Using temporary; Using filesort
explain select a1,a2,count(a2) from t1 group by a1,a2,b;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL idx_t1_1 163 NULL 128 Using index
1 SIMPLE t1 index NULL idx_t1_2 147 NULL 128 Using index
explain extended select a1,a2,count(a2) from t1 where (a1 > 'a') group by a1,a2,b;
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t1 index idx_t1_0,idx_t1_1,idx_t1_2 idx_t1_1 163 NULL 128 75.00 Using where; Using index
1 SIMPLE t1 index idx_t1_0,idx_t1_1,idx_t1_2 idx_t1_2 147 NULL 128 75.00 Using where; Using index
Warnings:
Note 1003 select `test`.`t1`.`a1` AS `a1`,`test`.`t1`.`a2` AS `a2`,count(`test`.`t1`.`a2`) AS `count(a2)` from `test`.`t1` where (`test`.`t1`.`a1` > _latin1'a') group by `test`.`t1`.`a1`,`test`.`t1`.`a2`,`test`.`t1`.`b`
explain extended select sum(ord(a1)) from t1 where (a1 > 'a') group by a1,a2,b;
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t1 index idx_t1_0,idx_t1_1,idx_t1_2 idx_t1_1 163 NULL 128 75.00 Using where; Using index
1 SIMPLE t1 index idx_t1_0,idx_t1_1,idx_t1_2 idx_t1_2 147 NULL 128 75.00 Using where; Using index
Warnings:
Note 1003 select sum(ord(`test`.`t1`.`a1`)) AS `sum(ord(a1))` from `test`.`t1` where (`test`.`t1`.`a1` > _latin1'a') group by `test`.`t1`.`a1`,`test`.`t1`.`a2`,`test`.`t1`.`b`
explain select distinct(a1) from t1 where ord(a2) = 98;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL idx_t1_1 163 NULL 128 Using where; Using index
1 SIMPLE t1 index NULL idx_t1_2 147 NULL 128 Using where; Using index
select distinct(a1) from t1 where ord(a2) = 98;
a1
a
@ -2256,7 +2256,7 @@ EXPLAIN SELECT 1 FROM t1 AS t1_outer WHERE
a IN (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1_outer index NULL a 10 NULL 15 Using where; Using index
2 DEPENDENT SUBQUERY t1 index NULL a 10 NULL 15 Using index
2 DEPENDENT SUBQUERY t1 index NULL a 10 NULL 1 Using index
EXPLAIN SELECT 1 FROM t1 AS t1_outer GROUP BY a HAVING
a > (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2);
id select_type table type possible_keys key key_len ref rows Extra

2
mysql-test/r/innodb.result
View File

@ -947,7 +947,7 @@ id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL PRIMARY 4 NULL #
explain select * from t1 order by b;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL b 4 NULL #
1 SIMPLE t1 ALL NULL NULL NULL NULL # Using filesort
explain select * from t1 order by c;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL # Using filesort

22
mysql-test/r/innodb_mysql.result
View File

@ -851,13 +851,13 @@ EXPLAIN SELECT * FROM t1 WHERE b BETWEEN 1 AND 2 ORDER BY a;
id 1
select_type SIMPLE
table t1
type range
type index
possible_keys bkey
key bkey
key_len 5
key PRIMARY
key_len 4
ref NULL
rows 16
Extra Using where; Using index; Using filesort
rows 32
Extra Using where; Using index
SELECT * FROM t1 WHERE b BETWEEN 1 AND 2 ORDER BY a;
a b
1 2
@ -946,13 +946,13 @@ EXPLAIN SELECT * FROM t2 WHERE b=1 ORDER BY a;
id 1
select_type SIMPLE
table t2
type ref
type index
possible_keys bkey
key bkey
key_len 5
ref const
rows 8
Extra Using where; Using index; Using filesort
key PRIMARY
key_len 4
ref NULL
rows 16
Extra Using where; Using index
SELECT * FROM t2 WHERE b=1 ORDER BY a;
a b c
1 1 1

2
mysql-test/r/merge.result
View File

@ -86,7 +86,7 @@ a b
19 Testing
explain select a from t3 order by a desc limit 10;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t3 index NULL a 4 NULL 1131 Using index
1 SIMPLE t3 index NULL a 4 NULL 10 Using index
select a from t3 order by a desc limit 10;
a
699

55
mysql-test/r/order_by.result
View File

@ -1073,3 +1073,58 @@ id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 const PRIMARY,b b 5 const 1
1 SIMPLE t2 ref a a 5 const 2 Using where; Using index
DROP TABLE t1,t2;
CREATE TABLE t1(
id int auto_increment PRIMARY KEY, c2 int, c3 int, INDEX k2(c2), INDEX k3(c3));
INSERT INTO t1 (c2,c3) VALUES
(31,34),(35,38),(34,31),(32,35),(31,39),
(11,14),(15,18),(14,11),(12,15),(11,19);
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
SELECT COUNT(*) FROM t1;
COUNT(*)
40960
EXPLAIN SELECT id,c3 FROM t1 WHERE c2=11 ORDER BY c3 LIMIT 20;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index k2 k3 5 NULL 88 Using where
EXPLAIN SELECT id,c3 FROM t1 WHERE c2=11 ORDER BY c3 LIMIT 100;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ref k2 k2 5 const 9300 Using where; Using filesort
EXPLAIN SELECT id,c3 FROM t1 WHERE c2 BETWEEN 10 AND 12 ORDER BY c3 LIMIT 100;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index k2 k3 5 NULL 316 Using where
EXPLAIN SELECT id,c3 FROM t1 WHERE c2 BETWEEN 10 AND 12 ORDER BY c3 LIMIT 2000;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range k2 k2 5 NULL 12937 Using where; Using filesort
SELECT id,c3 FROM t1 WHERE c2=11 ORDER BY c3 LIMIT 20;
id c3
6 14
16 14
26 14
36 14
46 14
56 14
66 14
76 14
86 14
96 14
106 14
116 14
126 14
136 14
146 14
156 14
166 14
176 14
186 14
196 14
DROP TABLE t1;

2
mysql-test/r/select_found.result
View File

@ -84,7 +84,7 @@ UNIQUE KEY e_n (email,name)
EXPLAIN SELECT SQL_CALC_FOUND_ROWS DISTINCT email FROM t2 LEFT JOIN t1 ON kid = t2.id WHERE t1.id IS NULL LIMIT 10;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 system PRIMARY,kid NULL NULL NULL 0 const row not found
1 SIMPLE t2 index NULL e_n 104 NULL 200
1 SIMPLE t2 index NULL e_n 104 NULL 10
SELECT SQL_CALC_FOUND_ROWS DISTINCT email FROM t2 LEFT JOIN t1 ON kid = t2.id WHERE t1.id IS NULL LIMIT 10;
email
email1

2
mysql-test/r/subselect.result
View File

@ -3419,7 +3419,7 @@ EXPLAIN
SELECT * FROM t1 WHERE (a,b) = ANY (SELECT a, max(b) FROM t1 GROUP BY a);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 9 Using where
2 DEPENDENT SUBQUERY t1 index NULL a 8 NULL 9 Using filesort
2 DEPENDENT SUBQUERY t1 index NULL a 8 NULL 1 Using filesort
DROP TABLE t1;
create table t1( f1 int,f2 int);
insert into t1 values (1,1),(2,2);

2
mysql-test/t/distinct.test
View File

@ -97,7 +97,7 @@ explain SELECT t1.a from t1 group by a order by a desc;
explain SELECT distinct t1.a from t1 order by a desc limit 1;
explain SELECT distinct a from t3 order by a desc limit 2;
explain SELECT distinct a,b from t3 order by a+1;
explain SELECT distinct a,b from t3 order by a limit 10;
explain SELECT distinct a,b from t3 order by a limit 2;
explain SELECT a,b from t3 group by a,b order by a+1;
drop table t1,t2,t3,t4;

37
mysql-test/t/order_by.test
View File

@ -739,3 +739,40 @@ INSERT INTO t2 VALUES (1,1),(1,2),(2,1),(2,2);
EXPLAIN SELECT 1 FROM t1,t2 WHERE t1.b=2 AND t1.a=t2.a ORDER BY t2.b;
DROP TABLE t1,t2;
# End of 5.0
#
# Bug #28404: query with ORDER BY and ref access
#
CREATE TABLE t1(
id int auto_increment PRIMARY KEY, c2 int, c3 int, INDEX k2(c2), INDEX k3(c3));
INSERT INTO t1 (c2,c3) VALUES
(31,34),(35,38),(34,31),(32,35),(31,39),
(11,14),(15,18),(14,11),(12,15),(11,19);
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
INSERT INTO t1 (c2,c3) SELECT c2,c3 FROM t1;
SELECT COUNT(*) FROM t1;
EXPLAIN SELECT id,c3 FROM t1 WHERE c2=11 ORDER BY c3 LIMIT 20;
EXPLAIN SELECT id,c3 FROM t1 WHERE c2=11 ORDER BY c3 LIMIT 100;
EXPLAIN SELECT id,c3 FROM t1 WHERE c2 BETWEEN 10 AND 12 ORDER BY c3 LIMIT 100;
EXPLAIN SELECT id,c3 FROM t1 WHERE c2 BETWEEN 10 AND 12 ORDER BY c3 LIMIT 2000;
SELECT id,c3 FROM t1 WHERE c2=11 ORDER BY c3 LIMIT 20;
DROP TABLE t1;

310
sql/sql_select.cc
View File

@ -6450,6 +6450,7 @@ void JOIN_TAB::cleanup()
quick= 0;
x_free(cache.buff);
cache.buff= 0;
limit= 0;
if (table)
{
if (table->key_read)
@ -12588,9 +12589,12 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
{
int ref_key;
uint ref_key_parts;
int order_direction;
uint used_key_parts;
TABLE *table=tab->table;
SQL_SELECT *select=tab->select;
key_map usable_keys;
QUICK_SELECT_I *save_quick;
DBUG_ENTER("test_if_skip_sort_order");
LINT_INIT(ref_key_parts);
@ -12625,6 +12629,7 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
else if (select && select->quick) // Range found by opt_range
{
int quick_type= select->quick->get_type();
save_quick= select->quick;
/*
assume results are not ordered when index merge is used
TODO: sergeyp: Results of all index merge selects actually are ordered
@ -12644,8 +12649,6 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
/*
We come here when there is a REF key.
*/
int order_direction;
uint used_key_parts;
if (!usable_keys.is_set(ref_key))
{
/*
@ -12706,63 +12709,30 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
}
/* Check if we get the rows in requested sorted order by using the key */
if (usable_keys.is_set(ref_key) &&
(order_direction = test_if_order_by_key(order,table,ref_key,
&used_key_parts)))
{
if (order_direction == -1) // If ORDER BY ... DESC
{
if (select && select->quick)
{
/*
Don't reverse the sort order, if it's already done.
(In some cases test_if_order_by_key() can be called multiple times
*/
if (!select->quick->reverse_sorted())
{
QUICK_SELECT_DESC *tmp;
int quick_type= select->quick->get_type();
if (quick_type == QUICK_SELECT_I::QS_TYPE_INDEX_MERGE ||
quick_type == QUICK_SELECT_I::QS_TYPE_ROR_INTERSECT ||
quick_type == QUICK_SELECT_I::QS_TYPE_ROR_UNION ||
quick_type == QUICK_SELECT_I::QS_TYPE_GROUP_MIN_MAX)
DBUG_RETURN(0); // Use filesort
/* ORDER BY range_key DESC */
tmp= new QUICK_SELECT_DESC((QUICK_RANGE_SELECT*)(select->quick),
used_key_parts);
if (!tmp || tmp->error)
{
delete tmp;
DBUG_RETURN(0); // Reverse sort not supported
}
select->quick=tmp;
}
DBUG_RETURN(1);
}
if (tab->ref.key_parts < used_key_parts)
{
/*
SELECT * FROM t1 WHERE a=1 ORDER BY a DESC,b DESC
Use a traversal function that starts by reading the last row
with key part (A) and then traverse the index backwards.
*/
tab->read_first_record= join_read_last_key;
tab->read_record.read_record= join_read_prev_same;
/* fall through */
}
}
else if (select && select->quick)
select->quick->sorted= 1;
DBUG_RETURN(1); /* No need to sort */
}
(order_direction= test_if_order_by_key(order,table,ref_key,
&used_key_parts)))
goto check_reverse_order;
}
else
{
/* check if we can use a key to resolve the group */
/* Tables using JT_NEXT are handled here */
/*
Check whether there is an index compatible with the given order
usage of which is cheaper than usage of the ref_key index (ref_key>=0)
or a table scan.
It may be the case if ORDER/GROUP BY is used with LIMIT.
*/
uint nr;
key_map keys;
uint best_key_parts;
int best_key_direction;
ha_rows best_records;
double read_time;
int best_key= -1;
bool is_best_covering= FALSE;
double fanout= 1;
JOIN *join= tab->join;
uint tablenr= tab - join->join_tab;
ha_rows table_records= table->file->stats.records;
bool group= join->group && order == join->group_list;
/*
filesort() and join cache are usually faster than reading in
@ -12775,7 +12745,7 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
resolved with a key; This is because filesort() is usually faster than
retrieving all rows through an index.
*/
if (select_limit >= table->file->stats.records)
if (select_limit >= table_records)
{
keys= *table->file->keys_to_use_for_scanning();
keys.merge(table->covering_keys);
@ -12786,38 +12756,224 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
This is to allow users to use index in ORDER BY.
*/
if (table->force_index)
keys.merge(table->keys_in_use_for_query);
keys.merge(group ? table->keys_in_use_for_group_by :
table->keys_in_use_for_order_by);
keys.intersect(usable_keys);
}
else
keys= usable_keys;
read_time= join->best_positions[tablenr].read_time;
for (uint i= tablenr+1; i < join->tables; i++)
fanout*= join->best_positions[i].records_read; // fanout is always >= 1
for (nr=0; nr < table->s->keys ; nr++)
{
uint not_used;
if (keys.is_set(nr))
int direction;
if (keys.is_set(nr) &&
(direction= test_if_order_by_key(order, table, nr, &used_key_parts)))
{
int flag;
if ((flag= test_if_order_by_key(order, table, nr, &not_used)))
{
if (!no_changes)
{
tab->index=nr;
tab->read_first_record= (flag > 0 ? join_read_first:
join_read_last);
tab->type=JT_NEXT; // Read with index_first(), index_next()
if (table->covering_keys.is_set(nr))
{
table->key_read=1;
table->file->extra(HA_EXTRA_KEYREAD);
}
}
DBUG_RETURN(1);
bool is_covering= table->covering_keys.is_set(nr) ||
nr == table->s->primary_key &&
table->file->primary_key_is_clustered();
/*
Don't use an index scan with ORDER BY without limit.
For GROUP BY without limit always use index scan
if there is a suitable index.
Why we hold to this asymmetry hardly can be explained
rationally. It's easy to demonstrate that using
temporary table + filesort could be cheaper for grouping
queries too.
*/
if (is_covering ||
select_limit != HA_POS_ERROR ||
ref_key < 0 && (group || table->force_index))
{
double rec_per_key;
double index_scan_time;
KEY *keyinfo= tab->table->key_info+nr;
if (select_limit == HA_POS_ERROR)
select_limit= table_records;
if (group)
{
rec_per_key= keyinfo->rec_per_key[used_key_parts-1];
set_if_bigger(rec_per_key, 1);
/*
With a grouping query each group containing on average
rec_per_key records produces only one row that will
be included into the result set.
*/
if (select_limit > table_records/rec_per_key)
select_limit= table_records;
else
select_limit= (ha_rows) (select_limit*rec_per_key);
}
/*
If tab=tk is not the last joined table tn then to get first
L records from the result set we can expect to retrieve
only L/fanout(tk,tn) where fanout(tk,tn) says how many
rows in the record set on average will match each row tk.
Usually our estimates for fanouts are too pessimistic.
So the estimate for L/fanout(tk,tn) will be too optimistic
and as result we'll choose an index scan when using ref/range
access + filesort will be cheaper.
*/
select_limit= (ha_rows) (select_limit < fanout ?
1 : select_limit/fanout);
/*
We assume that each of the tested indexes is not correlated
with ref_key. Thus, to select first N records we have to scan
N/selectivity(ref_key) index entries.
selectivity(ref_key) = #scanned_records/#table_records =
table->quick_condition_rows/table_records.
In any case we can't select more than #table_records.
N/(table->quick_condition_rows/table_records) > table_records
<=> N > table->quick_condition_rows.
*/
if (select_limit > table->quick_condition_rows)
select_limit= table_records;
else
select_limit= (ha_rows) (select_limit *
(double) table_records /
table->quick_condition_rows);
rec_per_key= keyinfo->rec_per_key[keyinfo->key_parts-1];
set_if_bigger(rec_per_key, 1);
/*
Here we take into account the fact that rows are
accessed in sequences rec_per_key records in each.
Rows in such a sequence are supposed to be ordered
by rowid/primary key. When reading the data
in a sequence we'll touch not more pages than the
table file contains.
TODO. Use the formula for a disk sweep sequential access
to calculate the cost of accessing data rows for one
index entry.
*/
index_scan_time= select_limit/rec_per_key *
min(rec_per_key, table->file->scan_time());
if (is_covering ||
ref_key < 0 && (group || table->force_index) ||
index_scan_time < read_time)
{
ha_rows quick_records= table_records;
if (is_best_covering && !is_covering)
continue;
if (table->quick_keys.is_set(nr))
quick_records= table->quick_rows[nr];
if (best_key < 0 ||
(select_limit <= min(quick_records,best_records) ?
keyinfo->key_parts < best_key_parts :
quick_records < best_records))
{
best_key= nr;
best_key_parts= keyinfo->key_parts;
best_records= quick_records;
is_best_covering= is_covering;
best_key_direction= direction;
}
}
}
}
}
if (best_key >= 0)
{
bool quick_created= FALSE;
if (table->quick_keys.is_set(best_key) && best_key != ref_key)
{
key_map map;
map.clear_all(); // Force the creation of quick select
map.set_bit(best_key); // only best_key.
quick_created=
select->test_quick_select(join->thd, map, 0,
join->select_options & OPTION_FOUND_ROWS ?
HA_POS_ERROR :
join->unit->select_limit_cnt,
0) > 0;
}
if (!no_changes)
{
if (!quick_created)
{
tab->index= best_key;
tab->read_first_record= best_key_direction > 0 ?
join_read_first:join_read_last;
tab->type=JT_NEXT; // Read with index_first(), index_next()
if (table->covering_keys.is_set(best_key))
{
table->key_read=1;
table->file->extra(HA_EXTRA_KEYREAD);
}
table->file->ha_index_or_rnd_end();
if (join->select_options & SELECT_DESCRIBE)
{
tab->ref.key= -1;
tab->ref.key_parts= 0;
if (tab->select)
tab->select->quick= 0;
if (select_limit < table_records)
tab->limit= select_limit;
}
}
}
used_key_parts= best_key_parts;
order_direction= best_key_direction;
}
else
DBUG_RETURN(0);
}
DBUG_RETURN(0); // Can't use index.
check_reverse_order:
if (order_direction == -1) // If ORDER BY ... DESC
{
if (select && select->quick)
{
/*
Don't reverse the sort order, if it's already done.
(In some cases test_if_order_by_key() can be called multiple times
*/
if (!select->quick->reverse_sorted())
{
QUICK_SELECT_DESC *tmp;
int quick_type= select->quick->get_type();
if (quick_type == QUICK_SELECT_I::QS_TYPE_INDEX_MERGE ||
quick_type == QUICK_SELECT_I::QS_TYPE_ROR_INTERSECT ||
quick_type == QUICK_SELECT_I::QS_TYPE_ROR_UNION ||
quick_type == QUICK_SELECT_I::QS_TYPE_GROUP_MIN_MAX)
{
tab->limit= 0;
select->quick= save_quick;
DBUG_RETURN(0); // Use filesort
}
/* ORDER BY range_key DESC */
tmp= new QUICK_SELECT_DESC((QUICK_RANGE_SELECT*)(select->quick),
used_key_parts);
if (!tmp || tmp->error)
{
delete tmp;
select->quick= save_quick;
tab->limit= 0;
DBUG_RETURN(0); // Reverse sort not supported
}
select->quick=tmp;
}
}
else if (tab->ref.key >= 0 && tab->ref.key_parts < used_key_parts)
{
/*
SELECT * FROM t1 WHERE a=1 ORDER BY a DESC,b DESC
Use a traversal function that starts by reading the last row
with key part (A) and then traverse the index backwards.
*/
tab->read_first_record= join_read_last_key;
tab->read_record.read_record= join_read_prev_same;
}
}
else if (select && select->quick)
select->quick->sorted= 1;
DBUG_RETURN(1);
}
@ -15524,7 +15680,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
if (tab->select && tab->select->quick)
examined_rows= tab->select->quick->records;
else if (tab->type == JT_NEXT || tab->type == JT_ALL)
examined_rows= tab->table->file->records();
examined_rows= tab->limit ? tab->limit : tab->table->file->records();
else
examined_rows=(ha_rows)join->best_positions[i].records_read;

6
sql/sql_select.h
View File

@ -194,6 +194,12 @@ typedef struct st_join_table {
enum join_type type;
bool cached_eq_ref_table,eq_ref_table,not_used_in_distinct;
bool sorted;
/*
If it's not 0 the number stored this field indicates that the index
scan has been chosen to access the table data and we expect to scan
this number of rows for the table.
*/
ha_rows limit;
TABLE_REF ref;
JOIN_CACHE cache;
JOIN *join;