The Power of MySQL Explain
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The document discusses improvements to the EXPLAIN statement in MySQL. Some key points discussed include: - EXPLAIN was expanded in MySQL 5.6 to provide information for data modifying statements like INSERT, UPDATE, DELETE in addition to SELECT statements. - A structured JSON output for EXPLAIN was introduced to make the query execution plan more easily readable. - Optimizer traces were added to provide insight into how the query planner selected a particular execution plan.
![MySQL 5.6: Structured EXPLAIN
EXPLAIN FORMAT=JSON
SELECT * FROM t2 WHERE i > 1 AND j < 3;
{
"query_block": {
"select_id": 1,
"table": {
"table_name": "t2",
"access_type": "range",
"possible_keys": [
"PRIMARY"
] /* possible_keys */,
...
Copyright © 2012 Oracle and/or its affiliates. All rights reserved.](https://image.slidesharecdn.com/mysqlinnovationdaymysqlexplain-130322100359-phpapp01/85/The-Power-of-MySQL-Explain-6-320.jpg)
![Structured EXPLAIN:
View with JSON Plugin in Browser for Expand/Collapse
{
- query_block: {
select_id: 1,
- nested_loop: [
- {
+ table: { … }
},
- {
+ table: { … }
}
]
}
}
Copyright © 2012 Oracle and/or its affiliates. All rights reserved.](https://image.slidesharecdn.com/mysqlinnovationdaymysqlexplain-130322100359-phpapp01/85/The-Power-of-MySQL-Explain-8-320.jpg)
![Optimizer Traces: Query Plan Debugging
SET SESSION.OPTIMIZER_TRACE=‘enabled=on’; "rows_estimation": [
{
SELECT v FROM t1 WHERE i1 = 1 AND v = 'a'; "table": "`t1`",
SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE; "range_analysis": {
"table_scan": {
"rows": 5,
"cost": 4.1
},
"potential_range_indices": [
{
"index": "v_idx",
• EXPLAIN shows the generated plan "usable": true,
"key_parts": [
• TRACE shows how the plan was generated, "v",
"i1"
decision points etc. ]
}
• JSON format ],
"best_covering_index_scan": {
• Developers, support, advanced customers "index": "v_idx",
"cost": 2.0063,
"chosen": true
} ,
Copyright © 2012 Oracle and/or its affiliates. All rights reserved.](https://image.slidesharecdn.com/mysqlinnovationdaymysqlexplain-130322100359-phpapp01/85/The-Power-of-MySQL-Explain-11-320.jpg)
The Power of MySQL Explain
- 1. <Insert Picture Here> The Power of MySQL EXPLAIN Manyi Lu
- 2. EXPLAIN: The Query Execution Plan category JOIN film JOIN category film EXPLAIN returns a row of information for each "table" used in the SELECT statement. The "table" can be a real table, a derived or temporary table, a subquery, or a union result. Copyright © 2012 Oracle and/or its affiliates. All rights reserved.
- 3. EXPLAIN in MySQL 5.5: Room for Improvement SELECT statements only What about INSERT, UPDATE, DELETE ? Tabular output Difficult to see the structure of the query plan More information would be useful E.g., When are the WHERE conditions evaluated? Shows the chosen plan, but does not tell you why this plan was chosen. Copyright © 2012 Oracle and/or its affiliates. All rights reserved.
- 4. MySQL 5.6: EXPLAIN for Data-Modifying Statements EXPLAIN UPDATE t1 SET b = 'b' WHERE a > 2 G id: 1 select_type: SIMPLE table: t1 type: range possible_keys: a,a_2 key: a key_len: 16 ref: null rows: 2 Extra: Using where; Using temporary Copyright © 2012 Oracle and/or its affiliates. All rights reserved.
- 5. MySQL 5.6: EXPLAIN for Data-Modifying Statements, cont. EXPLAIN INSERT INTO t1 SELECT * FROM t2 WHERE a IN (1, 3, 5) G id: 1 select_type: SIMPLE table: t2 type: range possible_keys: t2i1 key: t2i1 key_len: 4 ref: null rows: 3 Extra: Using index condition Copyright © 2012 Oracle and/or its affiliates. All rights reserved.
- 6. MySQL 5.6: Structured EXPLAIN EXPLAIN FORMAT=JSON SELECT * FROM t2 WHERE i > 1 AND j < 3; { "query_block": { "select_id": 1, "table": { "table_name": "t2", "access_type": "range", "possible_keys": [ "PRIMARY" ] /* possible_keys */, ... Copyright © 2012 Oracle and/or its affiliates. All rights reserved.
- 7. MySQL 5.6: Structured EXPLAIN, cont. ... "key": "PRIMARY", "key_length": "4", "rows": 2, "filtered": 100, "index_condition": "(`test`.`t2`.`i` > 1)", "attached_condition": "(`test`.`t2`.`j` < 3)" } /* table */ } /* query_block */ } Copyright © 2012 Oracle and/or its affiliates. All rights reserved.
- 8. Structured EXPLAIN: View with JSON Plugin in Browser for Expand/Collapse { - query_block: { select_id: 1, - nested_loop: [ - { + table: { … } }, - { + table: { … } } ] } } Copyright © 2012 Oracle and/or its affiliates. All rights reserved.
- 9. Structured EXPLAIN: View with JSON Plugin in Browser for Expand/Collapse, cont. { - query_block: { select_id: 1, - nested_loop: [ - { - table: { table_name: "t1", access_type: "ALL", rows: 3, filtered: 100, attached_condition: "(`test`.`t1`.`j` > 1)" } }, ... Copyright © 2012 Oracle and/or its affiliates. All rights reserved.
- 10. Structured EXPLAIN: Subqueries EXPLAIN FORMAT=JSON SELECT * FROM (SELECT * FROM t1) t; { - query_block: { select_id: 1, - table: { table_name: "t", access_type: "ALL", rows: 3, filtered: 100, + materialized_from_subquery: { … } } } } Copyright © 2012 Oracle and/or its affiliates. All rights reserved.
- 11. Optimizer Traces: Query Plan Debugging SET SESSION.OPTIMIZER_TRACE=‘enabled=on’; "rows_estimation": [ { SELECT v FROM t1 WHERE i1 = 1 AND v = 'a'; "table": "`t1`", SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE; "range_analysis": { "table_scan": { "rows": 5, "cost": 4.1 }, "potential_range_indices": [ { "index": "v_idx", • EXPLAIN shows the generated plan "usable": true, "key_parts": [ • TRACE shows how the plan was generated, "v", "i1" decision points etc. ] } • JSON format ], "best_covering_index_scan": { • Developers, support, advanced customers "index": "v_idx", "cost": 2.0063, "chosen": true } , Copyright © 2012 Oracle and/or its affiliates. All rights reserved.