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MDEV-6442: Assertion `join->best_read < double(...)' failed with optimizer_use_condition_selectivity >=3

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- Fix the crash by making get_column_range_cardinality() 
  to handle the special case where Column_stats objects 
  is an all-zeros object (the question of what is the point
  of having Field::read_stats point to such object remains a 
  mystery)

- Added a few comments. Learning the code still.
This commit is contained in:
Sergey Petrunya
2014-10-06 15:29:22 +04:00
parent 689ffe3559
commit fd4c9af398
4 changed files with 70 additions and 3 deletions
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16
sql/sql_statistics.cc
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@ -3502,7 +3502,7 @@ double get_column_range_cardinality(Field *field,
!(range_flag & NEAR_MIN);
if (col_non_nulls < 1)
res= 0;
res= 0; /* this is likely wrong, see MDEV-6843 */
else if (min_endp && max_endp && min_endp->length == max_endp->length &&
!memcmp(min_endp->key, max_endp->key, min_endp->length))
{
@ -3515,6 +3515,15 @@ double get_column_range_cardinality(Field *field,
{
double avg_frequency= col_stats->get_avg_frequency();
res= avg_frequency;
/*
psergey-todo: what does check for min_value, max_value mean?
min/max_value are set to NULL in alloc_statistics_for_table() and
alloc_statistics_for_table_share(). Both functions will immediately
call create_min_max_statistical_fields_for_table and
create_min_max_statistical_fields_for_table_share() respectively,
which will set min/max_value to be valid pointers, unless OOM
occurs.
*/
if (avg_frequency > 1.0 + 0.000001 &&
col_stats->min_value && col_stats->max_value)
{
@ -3530,6 +3539,11 @@ double get_column_range_cardinality(Field *field,
avg_frequency / col_non_nulls);
}
}
else if (avg_frequency == 0.0)
{
/* This actually means there is no statistics data */
res= tab_records;
}
}
}
else