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cleanup: test_if_subpart documented to highlight side effect

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Also document (through function parameters names) which ORDER must be passed
to the function: first GROUP BY, then ORDER BY.
This commit is contained in:
Vicențiu Ciorbaru
2021-01-19 10:12:37 +02:00
parent d028fb9ffa
commit 7e7ca09ce1
1 changed files with 20 additions and 8 deletions
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28
sql/sql_select.cc
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@ -254,7 +254,7 @@ static ORDER *create_distinct_group(THD *thd, Ref_ptr_array ref_pointer_array,
ORDER *order, List<Item> &fields,
List<Item> &all_fields,
bool *all_order_by_fields_used);
static bool test_if_subpart(ORDER *a,ORDER *b);
static bool test_if_subpart(ORDER *group_by, ORDER *order_by);
static TABLE *get_sort_by_table(ORDER *a,ORDER *b,List<TABLE_LIST> &tables,
table_map const_tables);
static void calc_group_buffer(JOIN *join, ORDER *group);
@ -2793,6 +2793,12 @@ int JOIN::optimize_stage2()
calc_group_buffer(this, group_list);
}
/*
Remove ORDER BY in the following cases:
- GROUP BY is more specific. Example GROUP BY a, b ORDER BY a
- If there are aggregate functions and no GROUP BY, this always leads
to one row result, no point in sorting.
*/
if (test_if_subpart(group_list, order) ||
(!group_list && tmp_table_param.sum_func_count))
{
@ -25018,21 +25024,27 @@ count_field_types(SELECT_LEX *select_lex, TMP_TABLE_PARAM *param,
/**
Return 1 if second is a subpart of first argument.
If first parts has different direction, change it to second part
(group is sorted like order)
SIDE EFFECT:
For all the first items in the group by list that match, the sort
direction of the GROUP BY items are set to the same as those given by the
ORDER BY.
The direction of the group does not matter if the ORDER BY clause overrides
it anyway.
*/
static bool
test_if_subpart(ORDER *a,ORDER *b)
test_if_subpart(ORDER *group_by, ORDER *order_by)
{
for (; a && b; a=a->next,b=b->next)
while (group_by && order_by)
{
if ((*a->item)->eq(*b->item,1))
a->direction=b->direction;
if ((*group_by->item)->eq(*order_by->item, 1))
group_by->direction= order_by->direction;
else
return 0;
group_by= group_by->next;
order_by= order_by->next;
}
return MY_TEST(!b);
return MY_TEST(!order_by);
}
/**