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Bug#50939: Loose Index Scan unduly relies on engine to

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remember range endpoints

The Loose Index Scan optimization keeps track of a sequence
of intervals. For the current interval it maintains the
current interval's endpoints. But the maximum endpoint was
not stored in the SQL layer; rather, it relied on the
storage engine to retain this value in-between reads. By
coincidence this holds for MyISAM and InnoDB. Not for the
partitioning engine, however.

Fixed by making the key values iterator 
(QUICK_RANGE_SELECT) keep track of the current maximum endpoint.
This is also more efficient as we save a call through the
handler API in case of open-ended intervals.

The code to calculate endpoints was extracted into 
separate methods in QUICK_RANGE_SELECT, and it was possible to
get rid of some code duplication as part of fix.
This commit is contained in:
Martin Hansson 2010-05-10 09:23:23 +02:00
parent bea068326f
commit 1c5200f67d
4 changed files with 199 additions and 62 deletions
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44
mysql-test/r/range.result
View file

@ -1653,4 +1653,48 @@ a b
0 0
1 1
DROP TABLE t1;
#
# Bug#50939: Loose Index Scan unduly relies on engine to remember range
# endpoints
#
CREATE TABLE t1 (
a INT,
b INT,
KEY ( a, b )
) PARTITION BY HASH (a) PARTITIONS 1;
CREATE TABLE t2 (
a INT,
b INT,
KEY ( a, b )
);
INSERT INTO t1 VALUES (1, 1), (2, 2), (3, 3), (4, 4), (5, 5);
INSERT INTO t1 SELECT a + 5, b + 5 FROM t1;
INSERT INTO t1 SELECT a + 10, b + 10 FROM t1;
INSERT INTO t1 SELECT a + 20, b + 20 FROM t1;
INSERT INTO t1 SELECT a + 40, b + 40 FROM t1;
INSERT INTO t2 SELECT * FROM t1;
# plans should be identical
EXPLAIN SELECT a, MAX(b) FROM t1 WHERE a IN (10,100) GROUP BY a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range a a 5 NULL 1 Using where; Using index for group-by
EXPLAIN SELECT a, MAX(b) FROM t2 WHERE a IN (10,100) GROUP BY a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t2 range a a 5 NULL 2 Using where; Using index for group-by
FLUSH status;
SELECT a, MAX(b) FROM t1 WHERE a IN (10, 100) GROUP BY a;
a MAX(b)
10 10
# Should be no more than 4 reads.
SHOW status LIKE 'handler_read_key';
Variable_name Value
Handler_read_key 4
FLUSH status;
SELECT a, MAX(b) FROM t2 WHERE a IN (10, 100) GROUP BY a;
a MAX(b)
10 10
# Should be no more than 4 reads.
SHOW status LIKE 'handler_read_key';
Variable_name Value
Handler_read_key 4
DROP TABLE t1, t2;
End of 5.1 tests

41
mysql-test/t/range.test
View file

@ -1313,4 +1313,45 @@ SELECT * FROM t1 FORCE INDEX (PRIMARY)
DROP TABLE t1;
--echo #
--echo # Bug#50939: Loose Index Scan unduly relies on engine to remember range
--echo # endpoints
--echo #
CREATE TABLE t1 (
a INT,
b INT,
KEY ( a, b )
) PARTITION BY HASH (a) PARTITIONS 1;
CREATE TABLE t2 (
a INT,
b INT,
KEY ( a, b )
);
INSERT INTO t1 VALUES (1, 1), (2, 2), (3, 3), (4, 4), (5, 5);
INSERT INTO t1 SELECT a + 5, b + 5 FROM t1;
INSERT INTO t1 SELECT a + 10, b + 10 FROM t1;
INSERT INTO t1 SELECT a + 20, b + 20 FROM t1;
INSERT INTO t1 SELECT a + 40, b + 40 FROM t1;
INSERT INTO t2 SELECT * FROM t1;
--echo # plans should be identical
EXPLAIN SELECT a, MAX(b) FROM t1 WHERE a IN (10,100) GROUP BY a;
EXPLAIN SELECT a, MAX(b) FROM t2 WHERE a IN (10,100) GROUP BY a;
FLUSH status;
SELECT a, MAX(b) FROM t1 WHERE a IN (10, 100) GROUP BY a;
--echo # Should be no more than 4 reads.
SHOW status LIKE 'handler_read_key';
FLUSH status;
SELECT a, MAX(b) FROM t2 WHERE a IN (10, 100) GROUP BY a;
--echo # Should be no more than 4 reads.
SHOW status LIKE 'handler_read_key';
DROP TABLE t1, t2;
--echo End of 5.1 tests

93
sql/opt_range.cc
View file

@ -8532,8 +8532,6 @@ int QUICK_RANGE_SELECT::get_next()
{
int result;
KEY_MULTI_RANGE *mrange;
key_range *start_key;
key_range *end_key;
DBUG_ENTER("QUICK_RANGE_SELECT::get_next");
DBUG_ASSERT(multi_range_length && multi_range &&
(cur_range >= (QUICK_RANGE**) ranges.buffer) &&
@ -8573,26 +8571,9 @@ int QUICK_RANGE_SELECT::get_next()
mrange_slot < mrange_end;
mrange_slot++)
{
start_key= &mrange_slot->start_key;
end_key= &mrange_slot->end_key;
last_range= *(cur_range++);
start_key->key= (const uchar*) last_range->min_key;
start_key->length= last_range->min_length;
start_key->flag= ((last_range->flag & NEAR_MIN) ? HA_READ_AFTER_KEY :
(last_range->flag & EQ_RANGE) ?
HA_READ_KEY_EXACT : HA_READ_KEY_OR_NEXT);
start_key->keypart_map= last_range->min_keypart_map;
end_key->key= (const uchar*) last_range->max_key;
end_key->length= last_range->max_length;
/*
We use HA_READ_AFTER_KEY here because if we are reading on a key
prefix. We want to find all keys with this prefix.
*/
end_key->flag= (last_range->flag & NEAR_MAX ? HA_READ_BEFORE_KEY :
HA_READ_AFTER_KEY);
end_key->keypart_map= last_range->max_keypart_map;
last_range->make_min_endpoint(&mrange_slot->start_key);
last_range->make_max_endpoint(&mrange_slot->end_key);
mrange_slot->range_flag= last_range->flag;
}
@ -8616,49 +8597,52 @@ end:
/*
Get the next record with a different prefix.
SYNOPSIS
QUICK_RANGE_SELECT::get_next_prefix()
prefix_length length of cur_prefix
cur_prefix prefix of a key to be searched for
@param prefix_length length of cur_prefix
@param group_key_parts The number of key parts in the group prefix
@param cur_prefix prefix of a key to be searched for
DESCRIPTION
Each subsequent call to the method retrieves the first record that has a
prefix with length prefix_length different from cur_prefix, such that the
record with the new prefix is within the ranges described by
this->ranges. The record found is stored into the buffer pointed by
this->record.
The method is useful for GROUP-BY queries with range conditions to
discover the prefix of the next group that satisfies the range conditions.
Each subsequent call to the method retrieves the first record that has a
prefix with length prefix_length and which is different from cur_prefix,
such that the record with the new prefix is within the ranges described by
this->ranges. The record found is stored into the buffer pointed by
this->record. The method is useful for GROUP-BY queries with range
conditions to discover the prefix of the next group that satisfies the range
conditions.
@todo
TODO
This method is a modified copy of QUICK_RANGE_SELECT::get_next(), so both
methods should be unified into a more general one to reduce code
duplication.
RETURN
0 on success
HA_ERR_END_OF_FILE if returned all keys
other if some error occurred
@retval 0 on success
@retval HA_ERR_END_OF_FILE if returned all keys
@retval other if some error occurred
*/
int QUICK_RANGE_SELECT::get_next_prefix(uint prefix_length,
key_part_map keypart_map,
uint group_key_parts,
uchar *cur_prefix)
{
DBUG_ENTER("QUICK_RANGE_SELECT::get_next_prefix");
const key_part_map keypart_map= make_prev_keypart_map(group_key_parts);
for (;;)
{
int result;
key_range start_key, end_key;
if (last_range)
{
/* Read the next record in the same range with prefix after cur_prefix. */
DBUG_ASSERT(cur_prefix != 0);
DBUG_ASSERT(cur_prefix != NULL);
result= file->index_read_map(record, cur_prefix, keypart_map,
HA_READ_AFTER_KEY);
if (result || (file->compare_key(file->end_range) <= 0))
if (result || last_range->max_keypart_map == 0)
DBUG_RETURN(result);
key_range previous_endpoint;
last_range->make_max_endpoint(&previous_endpoint, prefix_length, keypart_map);
if (file->compare_key(&previous_endpoint) <= 0)
DBUG_RETURN(0);
}
uint count= ranges.elements - (cur_range - (QUICK_RANGE**) ranges.buffer);
@ -8670,21 +8654,9 @@ int QUICK_RANGE_SELECT::get_next_prefix(uint prefix_length,
}
last_range= *(cur_range++);
start_key.key= (const uchar*) last_range->min_key;
start_key.length= min(last_range->min_length, prefix_length);
start_key.keypart_map= last_range->min_keypart_map & keypart_map;
start_key.flag= ((last_range->flag & NEAR_MIN) ? HA_READ_AFTER_KEY :
(last_range->flag & EQ_RANGE) ?
HA_READ_KEY_EXACT : HA_READ_KEY_OR_NEXT);
end_key.key= (const uchar*) last_range->max_key;
end_key.length= min(last_range->max_length, prefix_length);
end_key.keypart_map= last_range->max_keypart_map & keypart_map;
/*
We use READ_AFTER_KEY here because if we are reading on a key
prefix we want to find all keys with this prefix
*/
end_key.flag= (last_range->flag & NEAR_MAX ? HA_READ_BEFORE_KEY :
HA_READ_AFTER_KEY);
key_range start_key, end_key;
last_range->make_min_endpoint(&start_key, prefix_length, keypart_map);
last_range->make_max_endpoint(&end_key, prefix_length, keypart_map);
result= file->read_range_first(last_range->min_keypart_map ? &start_key : 0,
last_range->max_keypart_map ? &end_key : 0,
@ -8779,9 +8751,9 @@ bool QUICK_RANGE_SELECT::row_in_ranges()
}
/*
This is a hack: we inherit from QUICK_SELECT so that we can use the
This is a hack: we inherit from QUICK_RANGE_SELECT so that we can use the
get_next() interface, but we have to hold a pointer to the original
QUICK_SELECT because its data are used all over the place. What
QUICK_RANGE_SELECT because its data are used all over the place. What
should be done is to factor out the data that is needed into a base
class (QUICK_SELECT), and then have two subclasses (_ASC and _DESC)
which handle the ranges and implement the get_next() function. But
@ -10903,7 +10875,8 @@ int QUICK_GROUP_MIN_MAX_SELECT::next_prefix()
{
uchar *cur_prefix= seen_first_key ? group_prefix : NULL;
if ((result= quick_prefix_select->get_next_prefix(group_prefix_len,
make_prev_keypart_map(group_key_parts), cur_prefix)))
group_key_parts,
cur_prefix)))
DBUG_RETURN(result);
seen_first_key= TRUE;
}

83
sql/opt_range.h
View file

@ -65,6 +65,85 @@ class QUICK_RANGE :public Sql_alloc {
dummy=0;
#endif
}
/**
Initalizes a key_range object for communication with storage engine.
This function facilitates communication with the Storage Engine API by
translating the minimum endpoint of the interval represented by this
QUICK_RANGE into an index range endpoint specifier for the engine.
@param Pointer to an uninitialized key_range C struct.
@param prefix_length The length of the search key prefix to be used for
lookup.
@param keypart_map A set (bitmap) of keyparts to be used.
*/
void make_min_endpoint(key_range *kr, uint prefix_length,
key_part_map keypart_map) {
make_min_endpoint(kr);
kr->length= min(kr->length, prefix_length);
kr->keypart_map&= keypart_map;
}
/**
Initalizes a key_range object for communication with storage engine.
This function facilitates communication with the Storage Engine API by
translating the minimum endpoint of the interval represented by this
QUICK_RANGE into an index range endpoint specifier for the engine.
@param Pointer to an uninitialized key_range C struct.
*/
void make_min_endpoint(key_range *kr) {
kr->key= (const uchar*)min_key;
kr->length= min_length;
kr->keypart_map= min_keypart_map;
kr->flag= ((flag & NEAR_MIN) ? HA_READ_AFTER_KEY :
(flag & EQ_RANGE) ? HA_READ_KEY_EXACT : HA_READ_KEY_OR_NEXT);
}
/**
Initalizes a key_range object for communication with storage engine.
This function facilitates communication with the Storage Engine API by
translating the maximum endpoint of the interval represented by this
QUICK_RANGE into an index range endpoint specifier for the engine.
@param Pointer to an uninitialized key_range C struct.
@param prefix_length The length of the search key prefix to be used for
lookup.
@param keypart_map A set (bitmap) of keyparts to be used.
*/
void make_max_endpoint(key_range *kr, uint prefix_length,
key_part_map keypart_map) {
make_max_endpoint(kr);
kr->length= min(kr->length, prefix_length);
kr->keypart_map&= keypart_map;
}
/**
Initalizes a key_range object for communication with storage engine.
This function facilitates communication with the Storage Engine API by
translating the maximum endpoint of the interval represented by this
QUICK_RANGE into an index range endpoint specifier for the engine.
@param Pointer to an uninitialized key_range C struct.
*/
void make_max_endpoint(key_range *kr) {
kr->key= (const uchar*)max_key;
kr->length= max_length;
kr->keypart_map= max_keypart_map;
/*
We use READ_AFTER_KEY here because if we are reading on a key
prefix we want to find all keys with this prefix
*/
kr->flag= (flag & NEAR_MAX ? HA_READ_BEFORE_KEY : HA_READ_AFTER_KEY);
}
};
@ -331,7 +410,7 @@ public:
int reset(void);
int get_next();
void range_end();
int get_next_prefix(uint prefix_length, key_part_map keypart_map,
int get_next_prefix(uint prefix_length, uint group_key_parts,
uchar *cur_prefix);
bool reverse_sorted() { return 0; }
bool unique_key_range();
@ -611,7 +690,7 @@ private:
uchar *record; /* Buffer where the next record is returned. */
uchar *tmp_record; /* Temporary storage for next_min(), next_max(). */
uchar *group_prefix; /* Key prefix consisting of the GROUP fields. */
uint group_prefix_len; /* Length of the group prefix. */
const uint group_prefix_len; /* Length of the group prefix. */
uint group_key_parts; /* A number of keyparts in the group prefix */
uchar *last_prefix; /* Prefix of the last group for detecting EOF. */
bool have_min; /* Specify whether we are computing */