database/mariadb
1
0
Fork 0
mirror of https://github.com/MariaDB/server.git synced 2025-07-27 18:02:13 +03:00
Code Activity

MDEV-5926: EITS: Histogram estimates for column=least_possible_value are wrong

Browse Source 
[Attempt #2]
- Use a new selectivity calculation formula in Histogram::point_selectivity. 
  The formula is different from the old one because it was developed from scratch.
  it doesn't have any possible division-by-zero problems.
This commit is contained in:
Sergey Petrunya
2014-03-26 17:55:00 +04:00
parent e59dec0345
commit ad842b5f05
4 changed files with 191 additions and 20 deletions
Download Patch File Download Diff File Expand all files Collapse all files

92
sql/sql_statistics.h
View File

@ -113,7 +113,7 @@ class Histogram
private:
Histogram_type type;
uint8 size;
uint8 size; /* Size of values array, in bytes */
uchar *values;
uint prec_factor()
@ -142,6 +142,7 @@ public:
private:
uint get_value(uint i)
{
DBUG_ASSERT(i < get_width());
switch (type) {
case SINGLE_PREC_HB:
return (uint) (((uint8 *) values)[i]);
@ -150,7 +151,7 @@ private:
}
return 0;
}
/* Find the bucket which value 'pos' falls into. */
uint find_bucket(double pos, bool first)
{
uint val= (uint) (pos * prec_factor());
@ -169,6 +170,10 @@ private:
else
break;
}
if (val > get_value(i))
i++;
if (val == get_value(i))
{
if (first)
@ -234,24 +239,89 @@ public:
sel= bucket_sel * (max - min + 1);
return sel;
}
/*
Estimate selectivity of "col=const" using a histogram
@param pos Position of the "const" between column's min_value and
max_value. This is a number in [0..1] range.
@param avg_sel Average selectivity of condition "col=const" in this table.
It is calcuated as (#non_null_values / #distinct_values).
@return
Expected condition selectivity (a number between 0 and 1)
*/
double point_selectivity(double pos, double avg_sel)
{
double sel;
double bucket_sel= 1.0/(get_width() + 1);
/* Find the bucket that contains the value 'pos'. */
uint min= find_bucket(pos, TRUE);
uint pos_value= (uint) (pos * prec_factor());
/* Find how many buckets this value occupies */
uint max= min;
while (max + 1 < get_width() && get_value(max + 1) == get_value(max))
while (max + 1 < get_width() && get_value(max + 1) == pos_value)
max++;
double inv_prec_factor= (double) 1.0 / prec_factor();
double width= (max + 1 == get_width() ?
1.0 : get_value(max) * inv_prec_factor) -
(min == 0 ?
0.0 : get_value(min-1) * inv_prec_factor);
sel= avg_sel * (bucket_sel * (max + 1 - min)) / width;
if (max > min)
{
/*
The value occupies multiple buckets. Use start_bucket ... end_bucket as
selectivity.
*/
double bucket_sel= 1.0/(get_width() + 1);
sel= bucket_sel * (max - min + 1);
}
else
{
/*
The value 'pos' fits within one single histogram bucket.
Histogram buckets have the same numbers of rows, but they cover
different ranges of values.
We assume that values are uniformly distributed across the [0..1] value
range.
*/
/*
If all buckets covered value ranges of the same size, the width of
value range would be:
*/
double avg_bucket_width= 1.0 / (get_width() + 1);
/*
Let's see what is the width of value range that our bucket is covering.
(min==max currently. they are kept in the formula just in case we
will want to extend it to handle multi-bucket case)
*/
double inv_prec_factor= (double) 1.0 / prec_factor();
double current_bucket_width=
(max + 1 == get_width() ? 1.0 : (get_value(max) * inv_prec_factor)) -
(min == 0 ? 0.0 : (get_value(min-1) * inv_prec_factor));
/*
So:
- each bucket has the same #rows
- values are unformly distributed across the [min_value,max_value] domain.
If a bucket has value range that's N times bigger then average, than
each value will have to have N times fewer rows than average.
*/
DBUG_ASSERT(current_bucket_width);
sel= avg_sel * avg_bucket_width / current_bucket_width;
/*
(Q: if we just follow this proportion we may end up in a situation
where number of different values we expect to find in this bucket
exceeds the number of rows that this histogram has in a bucket. Are
we ok with this or we would want to have certain caps?)
*/
}
return sel;
}
};