mysql中的trace用法及参数解释

trace工具用法：
1 mysql> set session optimizer_trace=“enabled=on”,end_markers_in_json=on; ‐‐开启trace
2 mysql> select * from employees where name > ‘a’ order by position;
3 mysql> SELECT * FROM information_schema.OPTIMIZER_TRACE;
4
5 查看trace字段：
6 {
7 “steps”: [
8 {
9 “join_preparation”: { ‐‐第一阶段：SQL准备阶段
10 “select#”: 1,
11 “steps”: [
12 {
13 “expanded_query”: "/* select#1 / select employees.id AS id,employees.name AS name,empl oyees.age AS age,employees.position AS position,employees.hire_time AS hire_time from employees where (employees.name > ‘a’) order by employees.position"
14 }
15 ] / steps /
16 } / join_preparation /
17 },
18 {
19 “join_optimization”: { ‐‐第二阶段：SQL优化阶段
20 “select#”: 1,
21 “steps”: [
22 {
23 “condition_processing”: { ‐‐条件处理
24 “condition”: “WHERE”,
25 “original_condition”: “(employees.name > ‘a’)”,
26 “steps”: [
27 {
28 “transformation”: “equality_propagation”,
29 “resulting_condition”: “(employees.name > ‘a’)”
30 },
31 {
32 “transformation”: “constant_propagation”,
33 “resulting_condition”: “(employees.name > ‘a’)”
34 },
35 {
36 “transformation”: “trivial_condition_removal”,
37 “resulting_condition”: “(employees.name > ‘a’)”
38 }
39 ] / steps /
40 } / condition_processing /
41 },
42 {
43 “substitute_generated_columns”: {
44 } / substitute_generated_columns /
45 },
46 {
47 “table_dependencies”: [ ‐‐表依赖详情
48 {
49 “table”: “employees”,
50 “row_may_be_null”: false,
51 “map_bit”: 0,
52 “depends_on_map_bits”: [
53 ] / depends_on_map_bits /
54 }
55 ] / table_dependencies /
56 },
57 {
58 “ref_optimizer_key_uses”: [
59 ] / ref_optimizer_key_uses /
60 },
61 {
62 “rows_estimation”: [ ‐‐预估表的访问成本
63 {
64 “table”: “employees”,
65 “range_analysis”: {
66 “table_scan”: { ‐‐全表扫描情况
67 “rows”: 10123, ‐‐扫描行数
68 “cost”: 2054.7 ‐‐查询成本
69 } / table_scan /,
70 “potential_range_indexes”: [ ‐‐查询可能使用的索引
71 {
72 “index”: “PRIMARY”, ‐‐主键索引
73 “usable”: false,
74 “cause”: “not_applicable”
75 },
76 {
77 “index”: “idx_name_age_position”, ‐‐辅助索引
78 “usable”: true,
79 “key_parts”: [
80 “name”,
81 “age”,
82 “position”,
83 “id”
84 ] / key_parts /
85 }
86 ] / potential_range_indexes /,
87 “setup_range_conditions”: [
88 ] / setup_range_conditions /,
89 “group_index_range”: {
90 “chosen”: false,
91 “cause”: “not_group_by_or_distinct”
92 } / group_index_range /,
93 “analyzing_range_alternatives”: { ‐‐分析各个索引使用成本
94 “range_scan_alternatives”: [
95 {
96 “index”: “idx_name_age_position”,
97 “ranges”: [
98 “a < name” ‐‐索引使用范围
99 ] / ranges /,
100 “index_dives_for_eq_ranges”: true,
101 “rowid_ordered”: false, ‐‐使用该索引获取的记录是否按照主键排序
102 “using_mrr”: false,
103 “index_only”: false, ‐‐是否使用覆盖索引
104 “rows”: 5061, ‐‐索引扫描行数
105 “cost”: 6074.2, ‐‐索引使用成本
106 “chosen”: false, ‐‐是否选择该索引
107 “cause”: “cost”
108 }
109 ] / range_scan_alternatives /,
110 “analyzing_roworder_intersect”: {
111 “usable”: false,
112 “cause”: “too_few_roworder_scans”
113 } / analyzing_roworder_intersect /
114 } / analyzing_range_alternatives /
115 } / range_analysis /
116 }
117 ] / rows_estimation /
118 },
119 {
120 “considered_execution_plans”: [
121 {
122 “plan_prefix”: [
123 ] / plan_prefix /,
124 “table”: “employees”,
125 “best_access_path”: { ‐‐最优访问路径
126 “considered_access_paths”: [ ‐‐最终选择的访问路径
127 {
128 “rows_to_scan”: 10123,
129 “access_type”: “scan”, ‐‐访问类型：为scan，全表扫描
130 “resulting_rows”: 10123,
131 “cost”: 2052.6,
132 “chosen”: true, ‐‐确定选择
133 “use_tmp_table”: true
134 }
135 ] / considered_access_paths /
136 } / best_access_path /,
137 “condition_filtering_pct”: 100,
138 “rows_for_plan”: 10123,
139 “cost_for_plan”: 2052.6,
140 “sort_cost”: 10123,
141 “new_cost_for_plan”: 12176,
142 “chosen”: true
143 }
144 ] / considered_execution_plans /
145 },
146 {
147 “attaching_conditions_to_tables”: {
148 “original_condition”: “(employees.name > ‘a’)”,
149 “attached_conditions_computation”: [
150 ] / attached_conditions_computation /,
151 “attached_conditions_summary”: [
152 {
153 “table”: “employees”,
154 “attached”: “(employees.name > ‘a’)”
155 }
156 ] / attached_conditions_summary /
157 } / attaching_conditions_to_tables /
158 },
159 {
160 “clause_processing”: {
161 “clause”: “ORDER BY”,
162 “original_clause”: “employees.position”,
163 “items”: [
164 {
165 “item”: “employees.position”
166 }
167 ] / items /,
168 “resulting_clause_is_simple”: true,
169 “resulting_clause”: “employees.position”
170 } / clause_processing /
171 },
172 {
173 “reconsidering_access_paths_for_index_ordering”: {
174 “clause”: “ORDER BY”,
175 “steps”: [
176 ] / steps /,
177 “index_order_summary”: {
178 “table”: “employees”,
179 “index_provides_order”: false,
180 “order_direction”: “undefined”,
181 “index”: “unknown”,
182 “plan_changed”: false
183 } / index_order_summary /
184 } / reconsidering_access_paths_for_index_ordering /
185 },
186 {
187 “refine_plan”: [
188 {
189 “table”: “employees”
190 }
191 ] / refine_plan /
192 }
193 ] / steps /
194 } / join_optimization /
195 },
196 {
197 “join_execution”: { ‐‐第三阶段：SQL执行阶段
198 “select#”: 1,
199 “steps”: [
200 ] / steps /
201 } / join_execution /
202 }
203 ] / steps */
204 }
205
206 结论：全表扫描的成本低于索引扫描，所以mysql最终选择全表扫描
207
208 mysql> select * from employees where name > ‘zzz’ order by position;
209 mysql> SELECT * FROM information_schema.OPTIMIZER_TRACE;
210
211 查看trace字段可知索引扫描的成本低于全表扫描，所以mysql最终选择索引扫描
212
213 mysql> set session optimizer_trace=“enabled=off”; ‐‐关闭trace