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.

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The Power of MySQL EXPLAIN
Manyi Lu

2. EXPLAIN: The Query Execution Plan
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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.