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MDEV-28882: Assertion `tmp >= 0' failed in best_access_path

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Histogram_json_hb::range_selectivity() may return small negative
numbers due to rounding errors in the histogram.

Make sure the returned value is non-negative.
Add an assert to catch negative values that are not small.

(attempt #2)
This commit is contained in:
Sergei Petrunia
2022-06-22 11:39:53 +03:00
parent 54ac356dea
commit 51bce3c59a
3 changed files with 67 additions and 1 deletions
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31
mysql-test/main/statistics_json.result
View File

@ -8322,3 +8322,34 @@ histogram
] ]
} }
drop table t1; drop table t1;
#
# MDEV-28882: Assertion `tmp >= 0' failed in best_access_path
#
CREATE TABLE t1 (a varchar(1));
INSERT INTO t1 VALUES ('o'),('s'),('j'),('s'),('y'),('s'),('l'),
('q'),('x'),('m'),('t'),('d'),('v'),('j'),('p'),('t'),('b'),('q');
set histogram_type=json_hb;
analyze table t1 persistent for all;
Table Op Msg_type Msg_text
test.t1 analyze status Engine-independent statistics collected
test.t1 analyze status OK
# filtered must not be negative:
explain format=json select * from t1 where a > 'y';
EXPLAIN
{
"query_block": {
"select_id": 1,
"nested_loop": [
{
"table": {
"table_name": "t1",
"access_type": "ALL",
"rows": 18,
"filtered": 5.555555344,
"attached_condition": "t1.a > 'y'"
}
}
]
}
}
drop table t1;

16
mysql-test/main/statistics_json.test
View File

@ -460,3 +460,19 @@ from mysql.column_stats where table_name='t1' and db_name=database();
drop table t1; drop table t1;
--echo #
--echo # MDEV-28882: Assertion `tmp >= 0' failed in best_access_path
--echo #
CREATE TABLE t1 (a varchar(1));
INSERT INTO t1 VALUES ('o'),('s'),('j'),('s'),('y'),('s'),('l'),
('q'),('x'),('m'),('t'),('d'),('v'),('j'),('p'),('t'),('b'),('q');
set histogram_type=json_hb;
analyze table t1 persistent for all;
--echo # filtered must not be negative:
explain format=json select * from t1 where a > 'y';
drop table t1;

21
sql/opt_histogram_json.cc
View File

@ -959,9 +959,16 @@ std::string& Histogram_json_hb::get_end_value(int idx)
@param field The table field histogram is for. We don't care about the @param field The table field histogram is for. We don't care about the
field's current value, we only need its virtual functions to field's current value, we only need its virtual functions to
perform various operations perform various operations
@param min_endp Left endpoint, or NULL if there is none @param min_endp Left endpoint, or NULL if there is none
@param max_endp Right endpoint, or NULL if there is none @param max_endp Right endpoint, or NULL if there is none
@param avg_sel Average selectivity of "field=const" equality for this field
@return
Range selectivity: a number between 0.0 and 1.0.
@note
This may return 0.0. Adjustments to avoid multiply-by-zero meltdown are
made elsewhere.
*/ */
double Histogram_json_hb::range_selectivity(Field *field, key_range *min_endp, double Histogram_json_hb::range_selectivity(Field *field, key_range *min_endp,
@ -1062,6 +1069,18 @@ double Histogram_json_hb::range_selectivity(Field *field, key_range *min_endp,
else else
max= 1.0; max= 1.0;
if (min > max)
{
/*
This can happen due to rounding errors.
What is the acceptable error size? Json_writer::add_double() uses
%.11lg format. This gives 9 digits after the dot. A histogram may have
hundreds of buckets, let's multiply the error by 1000. 9-3=6
*/
DBUG_ASSERT(max < min + 1e-6);
max= min;
}
return max - min; return max - min;
} }