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3e0f63c18f
Back-ported from the mysql 5.6 code line the patch with
the following comment:
Fix for Bug#11757108 CHANGE IN EXECUTION PLAN FOR COUNT_DISTINCT_GROUP_ON_KEY
CAUSES PEFORMANCE REGRESSION
The cause for the performance regression is that the access strategy for the
GROUP BY query is changed form using "index scan" in mysql-5.1 to use "loose
index scan" in mysql-5.5. The index used for group by is unique and thus each
"loose scan" group will only contain one record. Since loose scan needs to
re-position on each "loose scan" group this query will do a re-position for
each index entry. Compared to just reading the next index entry as a normal
index scan does, the use of loose scan for this query becomes more expensive.
The cause for selecting to use loose scan for this query is that in the current
code when the size of the "loose scan" group is one, the formula for
calculating the cost estimates becomes almost identical to the cost of using
normal index scan. Differences in use of integer versus floating point arithmetic
can cause one or the other access strategy to be selected.
The main issue with the formula for estimating the cost of using loose scan is
that it does not take into account that it is more costly to do a re-position
for each "loose scan" group compared to just reading the next index entry.
Both index scan and loose scan estimates the cpu cost as:
"number of entries needed too read/scan" * ROW_EVALUATE_COST
The results from testing with the query in this bug indicates that the real
cost for doing re-position four to eight times higher than just reading the
next index entry. Thus, the cpu cost estimate for loose scan should be increased.
To account for the extra work to re-position in the index we increase the
cost for loose index scan to include the cost of navigating the index.
This is modelled as a function of the height of the b-tree:
navigation cost= ceil(log(records in table)/log(indexes per block))
* ROWID_COMPARE_COST;
This will avoid loose index scan being used for indexes where the "loose scan"
group contains very few index entries.
249 lines
6.5 KiB
Text
249 lines
6.5 KiB
Text
create table t1 (USR_ID integer not null, MAX_REQ integer not null, constraint PK_SEA_USER primary key (USR_ID)) engine=InnoDB;
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insert into t1 values (1, 3);
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select count(*) + MAX_REQ - MAX_REQ + MAX_REQ - MAX_REQ + MAX_REQ - MAX_REQ + MAX_REQ - MAX_REQ + MAX_REQ - MAX_REQ from t1 group by MAX_REQ;
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count(*) + MAX_REQ - MAX_REQ + MAX_REQ - MAX_REQ + MAX_REQ - MAX_REQ + MAX_REQ - MAX_REQ + MAX_REQ - MAX_REQ
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1
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select Case When Count(*) < MAX_REQ Then 1 Else 0 End from t1 where t1.USR_ID = 1 group by MAX_REQ;
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Case When Count(*) < MAX_REQ Then 1 Else 0 End
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1
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drop table t1;
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create table t1m (a int) engine=myisam;
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create table t1i (a int) engine=innodb;
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create table t2m (a int) engine=myisam;
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create table t2i (a int) engine=innodb;
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insert into t2m values (5);
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insert into t2i values (5);
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select min(a) from t1m;
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min(a)
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NULL
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select min(7) from t1m;
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min(7)
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NULL
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select min(7) from DUAL;
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min(7)
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7
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explain select min(7) from t2m join t1m;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Select tables optimized away
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select min(7) from t2m join t1m;
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min(7)
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NULL
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select max(a) from t1m;
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max(a)
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NULL
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select max(7) from t1m;
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max(7)
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NULL
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select max(7) from DUAL;
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max(7)
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7
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explain select max(7) from t2m join t1m;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Select tables optimized away
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select max(7) from t2m join t1m;
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max(7)
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NULL
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select 1, min(a) from t1m where a=99;
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1 min(a)
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1 NULL
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select 1, min(a) from t1m where 1=99;
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1 min(a)
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1 NULL
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select 1, min(1) from t1m where a=99;
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1 min(1)
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1 NULL
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select 1, min(1) from t1m where 1=99;
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1 min(1)
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1 NULL
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select 1, max(a) from t1m where a=99;
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1 max(a)
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1 NULL
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select 1, max(a) from t1m where 1=99;
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1 max(a)
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1 NULL
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select 1, max(1) from t1m where a=99;
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1 max(1)
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1 NULL
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select 1, max(1) from t1m where 1=99;
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1 max(1)
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1 NULL
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select min(a) from t1i;
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min(a)
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NULL
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select min(7) from t1i;
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min(7)
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NULL
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select min(7) from DUAL;
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min(7)
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7
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explain select min(7) from t2i join t1i;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t2i ALL NULL NULL NULL NULL 1
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1 SIMPLE t1i ALL NULL NULL NULL NULL 1 Using join buffer (flat, BNL join)
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select min(7) from t2i join t1i;
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min(7)
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NULL
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select max(a) from t1i;
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max(a)
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NULL
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select max(7) from t1i;
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max(7)
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NULL
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select max(7) from DUAL;
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max(7)
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7
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explain select max(7) from t2i join t1i;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t2i ALL NULL NULL NULL NULL 1
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1 SIMPLE t1i ALL NULL NULL NULL NULL 1 Using join buffer (flat, BNL join)
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select max(7) from t2i join t1i;
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max(7)
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NULL
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select 1, min(a) from t1i where a=99;
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1 min(a)
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1 NULL
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select 1, min(a) from t1i where 1=99;
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1 min(a)
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1 NULL
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select 1, min(1) from t1i where a=99;
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1 min(1)
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1 NULL
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select 1, min(1) from t1i where 1=99;
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1 min(1)
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1 NULL
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select 1, max(a) from t1i where a=99;
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1 max(a)
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1 NULL
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select 1, max(a) from t1i where 1=99;
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1 max(a)
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1 NULL
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select 1, max(1) from t1i where a=99;
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1 max(1)
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1 NULL
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select 1, max(1) from t1i where 1=99;
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1 max(1)
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1 NULL
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explain select count(*), min(7), max(7) from t1m, t1i;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1m system NULL NULL NULL NULL 0 const row not found
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1 SIMPLE t1i ALL NULL NULL NULL NULL 1
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select count(*), min(7), max(7) from t1m, t1i;
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count(*) min(7) max(7)
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0 NULL NULL
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explain select count(*), min(7), max(7) from t1m, t2i;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1m system NULL NULL NULL NULL 0 const row not found
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1 SIMPLE t2i ALL NULL NULL NULL NULL 1
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select count(*), min(7), max(7) from t1m, t2i;
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count(*) min(7) max(7)
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0 NULL NULL
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explain select count(*), min(7), max(7) from t2m, t1i;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t2m system NULL NULL NULL NULL 1
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1 SIMPLE t1i ALL NULL NULL NULL NULL 1
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select count(*), min(7), max(7) from t2m, t1i;
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count(*) min(7) max(7)
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0 NULL NULL
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drop table t1m, t1i, t2m, t2i;
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#
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# Bug #57954: BIT_AND function returns incorrect results when
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# semijoin=on
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CREATE TABLE c (
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pk INT,
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col_varchar_key VARCHAR(1),
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PRIMARY KEY (pk),
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KEY col_varchar_key (col_varchar_key)
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) ENGINE=InnoDB;
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INSERT INTO c VALUES (11,NULL);
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INSERT INTO c VALUES (16,'c');
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CREATE TABLE bb (
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pk INT,
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col_varchar_key VARCHAR(1),
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PRIMARY KEY (pk),
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KEY col_varchar_key (col_varchar_key)
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) ENGINE=InnoDB;
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INSERT INTO bb VALUES (10,NULL);
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SELECT straight_join BIT_AND(c.pk)
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FROM
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bb, c
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WHERE c.col_varchar_key='ABC'
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ORDER BY c.pk;
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BIT_AND(c.pk)
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18446744073709551615
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DROP TABLE c,bb;
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#
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# Bug #58050: BIT_OR and BIT_XOR return incorrect results when
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# semijoin=on
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#
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CREATE TABLE t1 (pk INT PRIMARY KEY, b INT, c INT) ENGINE=InnoDB;
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INSERT INTO t1 VALUES(1, 1, 1);
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CREATE TABLE t2 (pk INT PRIMARY KEY, b INT, c INT) ENGINE=InnoDB;
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INSERT INTO t2 VALUES (1, 1, NULL);
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SELECT t1.* FROM t1 JOIN t2 ON t1.c=t2.c WHERE t1.pk=1;
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pk b c
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SELECT BIT_OR(t1.b) FROM t1 JOIN t2 ON t1.c=t2.c WHERE t1.pk=1;
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BIT_OR(t1.b)
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0
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SELECT BIT_AND(t1.b) FROM t1 JOIN t2 ON t1.c=t2.c WHERE t1.pk=1;
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BIT_AND(t1.b)
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18446744073709551615
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SELECT BIT_XOR(t1.b) FROM t1 JOIN t2 ON t1.c=t2.c WHERE t1.pk=1;
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BIT_XOR(t1.b)
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0
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DROP TABLE t1, t2;
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#
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# Bug#12713907: STRANGE OPTIMIZE & WRONG RESULT UNDER ORDER BY
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# COUNT(*) LIMIT.
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#
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CREATE TABLE t1 (
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id BIGINT(20) ,
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member_id_to INT(11) ,
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r_date DATE ,
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PRIMARY KEY (id,r_date),
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KEY r_date_idx (r_date),
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KEY t1_idx01 (member_id_to)
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) ENGINE=InnoDB;
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INSERT INTO t1 VALUES
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(107924526,518491,'2011-05-01'),
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(107924527,518491,'2011-05-01'),
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(107924534,518491,'2011-06-21'),
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(107924535,518491,'2011-06-21'),
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(107924542,1601319,'2011-06-21'),
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(107924543,1601319,'2011-06-21'),
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(107924544,1601319,'2011-06-21'),
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(107924545,1601319,'2011-06-21');
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SELECT member_id_to, COUNT(*) FROM t1 WHERE r_date =
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'2011-06-21' GROUP BY member_id_to ORDER BY 2 LIMIT 1;
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member_id_to COUNT(*)
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518491 2
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DROP TABLE t1;
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#
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# MDEV-4269: crash when grouping by values()
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#
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SELECT @@storage_engine INTO @old_engine;
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set storage_engine=innodb;
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create table y select 1 b;
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select 1 from y group by b;
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1
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1
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select 1 from y group by values(b);
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1
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1
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drop table y;
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SET storage_engine=@old_engine;
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#
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# Bug#13723054 CRASH WITH MIN/MAX AFTER QUICK_GROUP_MIN_MAX_SELECT::NEXT_MIN
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#
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CREATE TABLE t1(a BLOB, b VARCHAR(255) CHARSET LATIN1, c INT,
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KEY(b, c, a(765))) ENGINE=INNODB;
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INSERT INTO t1(a, b, c) VALUES
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('', 'a', 0), ('', 'a', null), ('', 'a', 0), ('', 'a', null), ('', 'a', 0);
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ANALYZE TABLE t1;
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SELECT MIN(c) FROM t1 GROUP BY b;
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MIN(c)
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0
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EXPLAIN SELECT MIN(c) FROM t1 GROUP BY b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 range NULL b 263 NULL 3 Using index for group-by
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DROP TABLE t1;
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End of 5.5 tests
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