Discovered this in TODO-4919.
Check TODO-4919 for table DDL and imitation-stats.diff .
Consider a query (constants replaced)
explain
|
SELECT
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task0.sys_id
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FROM
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task task0 ignore index(number)
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WHERE
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task0.sys_class_name = 'const' AND
|
task0.assignment_group = 'uuid-const' AND
|
task0.state IN (1,2,3)
|
ORDER BY
|
task0.number DESC limit 0,20
|
and two indexes:
-- assignment_group = 'uuid-const' matches 1% of the table
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INDEX assignment_group(assignment_group)
|
|
-- sys_class_name = 'const' matches 50% of the table
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INDEX sys_class_name_2(sys_class_name, number)
|
Range optimizer produces range costs.
The costs originate from handler::multi_range_read_info_const() and
do include TIME_FOR_COMPARE:
cost->cpu_cost+= (rows2double(total_rows) / TIME_FOR_COMPARE +
|
MULTI_RANGE_READ_SETUP_COST);
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Then, he code in best_access_path() calls
tmp= adjust_quick_cost(table->opt_range[key].cost,
|
table->opt_range[key].rows);
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to remove TIME_FOR_COMPARE from quick select's cost before comparing
it with other costs. Makes sense.
Then, in get_range_limit_read_cost(), we take the quick select cost:
double best_cost= (double) table->opt_range[keynr].cost;
|
and compare it with the cost we got from cost_for_index_read:
if (ref_rows > 0)
|
{
|
double tmp= cost_for_index_read(tab->join->thd, table, keynr,
|
ref_rows,
|
(ha_rows) tab->worst_seeks);
|
if (tmp < best_cost)
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Looking inside cost_for_index_read(), I see the cost is computed as follows:
cost= ((file->keyread_time(key, 0, records) +
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file->read_time(key, 1, MY_MIN(records, worst_seeks))));
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That is, it doesn't include TIME_FOR_COMPARE.
Should this be fixed?
There are no comments yet on this issue.
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