MySQL 8.4.3
Source Code Documentation
merge_sort.h
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24
25#ifndef MERGE_SORT_INCLUDED
26#define MERGE_SORT_INCLUDED
27
28/**
29 @file
30
31 @brief
32 Merge sort and insert sort implementations. These sorting functions
33 are primarily intended for sorting of JOIN_TABs before the greedy
34 search algorithm is applied. Since the JOIN_TAB comparison functions
35 (Join_tab_compare*) are not transitive, the resulting order depends
36 on the sorting implementation to a certain degree.
37
38 Since the std::stable_sort and std::sort implementations differ
39 between platforms, the result of sorting JOIN_TABs may also differ.
40 In turn, the query execution plan would differ between platforms and
41 that is a problem with mtr tests (EXPLAIN output would vary).
42
43 If you intend to sort something transitive (which means almost
44 everything except JOIN_TABs) you should most likely use one of the
45 std sorting functions instead of this.
46*/
47
48#include <assert.h>
49#include <queue>
50
51/**
52 Sorts the elements in the range [first,last) into ascending order
53 using insertion sort.
54
55 @param first First element in an array of pointers to be sorted
56 @param last Element after the last element in an array of pointers
57 to be sorted
58 @param comp Comparison function object that, taking two pointers
59 of the same type as those contained in the range,
60 returns true if the first argument goes before the
61 second argument in the specific strict weak ordering
62 it defines, and false otherwise.
63
64 In our case comp should be a function object with an operator:
65
66 bool operator()(Element_type*, Element_type*)
67*/
68
69template <typename Element_type, typename Comp_func>
70void insert_sort(Element_type **first, Element_type **last, Comp_func comp) {
71 for (Element_type **high_water_mark = first + 1; high_water_mark < last;
72 high_water_mark++) {
73 for (Element_type **cur = high_water_mark; cur > first; cur--) {
74 if (comp(*(cur - 1), *cur)) break;
75
76 Element_type *tmp = *(cur - 1);
77 *(cur - 1) = *cur;
78 *cur = tmp;
79 }
80 }
81}
82
83/**
84 Sorts the elements in the range [first,last) into ascending order
85 using merge sort.
86
87 @param first First element in an array of pointers to be sorted
88 @param last Element after the last element in an array of pointers
89 to be sorted
90 @param comp Comparison function object that, taking two pointers
91 of the same type as those contained in the range,
92 returns true if the first argument goes before the
93 second argument in the specific strict weak ordering
94 it defines, and false otherwise.
95
96 In our case comp should be a function object with an operator:
97
98 bool operator()(Element_type*, Element_type*)
99*/
100
101template <typename Element_type, typename Comp_func>
102void merge_sort(Element_type **first, Element_type **last, Comp_func comp) {
103 const uint elements = static_cast<uint>(last - first);
104
105 /*
106 Tests showed that the value 5 was a good number for JOIN_TAB
107 ordering, which is the primary use case for this function
108 */
109 if (elements < 5) {
110 insert_sort(first, last, comp);
111 return;
112 }
113 Element_type **middle = first + (elements) / 2;
114
115 merge_sort(first, middle, comp);
116 merge_sort(middle, last, comp);
117
118 std::queue<Element_type *> merged;
119
120 Element_type **cur1 = first;
121 Element_type **cur2 = middle;
122
123 for (uint i = 0; i < elements; i++) {
124 assert(cur1 < middle || cur2 < last);
125
126 if (cur1 == middle)
127 merged.push(*cur2++);
128 else if (cur2 == last)
129 merged.push(*cur1++);
130 else if (comp(*cur1, *cur2))
131 merged.push(*cur1++);
132 else
133 merged.push(*cur2++);
134 }
135
136 Element_type **result = first;
137 while (!merged.empty()) {
138 *result++ = merged.front();
139 merged.pop();
140 }
141}
142
143#endif /* MERGE_SORT_INCLUDED */
void insert_sort(Element_type **first, Element_type **last, Comp_func comp)
Sorts the elements in the range [first,last) into ascending order using insertion sort.
Definition: merge_sort.h:70
void merge_sort(Element_type **first, Element_type **last, Comp_func comp)
Sorts the elements in the range [first,last) into ascending order using merge sort.
Definition: merge_sort.h:102
Definition: result.h:30