The document discusses the EXPLAIN statement in MySQL. It provides examples of using the traditional EXPLAIN output and the new JSON format for EXPLAIN. The JSON format provides more detailed information about the query plan and execution in a structured format. It allows seeing things like how conditions are split and when subqueries are evaluated.

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2. Powerful EXPLAIN in
MySQL 5.6
Evgeny Potemkin
MySQL Optimizer team
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3. Program Agenda
 Introduction
 Optimizer trace
 EXPLAIN in JSON format
 Explaining INSERT, UPDATE, DELETE
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4. Graphic Section Divider
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5. Optimizer Trace
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6. Optimizer trace: quick start
mysql> SET optimizer_trace= “enabled=on“,
end_markers_in_json=on;
mysql> SELECT * FROM orders WHERE customer_id <> 6;
mysql> SELECT trace FROM
information_schema.OPTIMIZER_TRACE;
mysql> SET optimizer_trace="enabled=off";
QUERY SELECT * FROM orders WHERE customer_id <> 6
TRACE “steps”: [ { "join_preparation": { "select#": 1,… } … } …]
MISSING_BYTES_BEYOND_MAX_MEM_SIZE 0
INSUFFICIENT_PRIVILEGES 0
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7. Optimizer trace: what’s inside
 Steps
– Join preparation
 Modifications to subquery
 Expanded query
– Join optimization
 Range optimizer info
 Plan generation process
 Condition optimization
 Processing of ORDER/GROUP BY
– Join execution
 Tmp tables related info
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8. Optimizer trace: example
mysql> SET optimizer_trace="enabled=on";
mysql> SELECT * FROM t1,t2 WHERE f1=1 AND f1=f2 AND f2>0;
mysql> SELECT trace FROM information_schema.optimizer_trace;
mysql> SET optimizer_trace="enabled=off";
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9. Optimizer trace: example
join_optimization / steps / condition_processing
"condition": "WHERE",
"original_condition": "((`t1`.`f1` = 1) and (`t1`.`f1` = `t2`.`f2`) and (`t2`.`f2` > 0))",
"steps": [
{
"transformation": "equality_propagation",
"resulting_condition": "((`t2`.`f2` > 0) and multiple equal(1, `t1`.`f1`, `t2`.`f2`))"
}, {
"transformation": "constant_propagation",
"resulting_condition": "((1 > 0) and multiple equal(1, `t1`.`f1`, `t2`.`f2`))"
}, {
"transformation": "trivial_condition_removal",
"resulting_condition": "multiple equal(1, `t1`.`f1`, `t2`.`f2`)"
}
] /* steps */
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10. Optimizer trace: example
join_optimization / row_estimation / table : t1, range_analysis
"table_scan": { "analyzing_range_alternatives": {
"rows": 5, "range_scan_alternatives": [ {
"cost": 5.1085 "index": "f1",
} /* table_scan */, "ranges": [ "1 <= f1 <= 1“ ] /* ranges */,
"potential_range_indices": [ { "index_dives_for_eq_ranges": true,
"index": "f1", "usable": true, "rows": 2, "cost": 3.41,
"key_parts": [ "f1“ ] /* key_parts */ } "chosen": false,
] /* potential_range_indices */, "cause": "cost"
"best_covering_index_scan": { }
"index": "f1", "cost": 2.093, "chosen": true ] /* range_scan_alternatives */,
} /* best_covering_index_scan */, "analyzing_roworder_intersect": {
"group_index_range": { "usable": false, "cause": "too_few_roworder_scans"
"chosen": false, "cause": "not_single_table" } /* analyzing_roworder_intersect */
} /* group_index_range */, } /* analyzing_range_alternatives */
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11. Optimizer trace: example
join_optimization / considered_execution_plan
"plan_prefix": [ ] /* plan_prefix */, "plan_prefix": [ "`t2`“ ] /* plan_prefix */,
"table": "`t2`", "table": "`t1`",
"best_access_path": { "best_access_path": {
"considered_access_paths": [ { "considered_access_paths": [ {
"access_type": "scan", "rows": 3, "access_type": "ref", "index": "f1",
"cost": 2.6051, "chosen": true "rows": 2, "cost": 3.4698, "chosen": true
} ] /* considered_access_paths */ }, {
} /* best_access_path */, "access_type": "scan", "using_join_cache": true,
"cost_for_plan": 2.6051, "rows": 2, "cost": 3.8087,
"rows_for_plan": 3, "chosen": true
"rest_of_plan": [ { } ] } /* best_access_path */,
… next iteration … => "cost_for_plan": 6.4138,
} ] /* rest_of_plan */ "rows_for_plan": 6,
"chosen": true
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12. Optimizer trace: example
join_optimization / plan refinement
{ {
"attaching_conditions_to_tables": { "refine_plan": [
"original_condition": {
"((`t1`.`f1` = 1) and (`t2`.`f2` = 1))", "table": "`t1`"
"attached_conditions_summary": [ { },
"table": "`t1`", "attached": null {
}, { "table": "`t2`",
"table": "`t2`", "attached": "(`t2`.`f2` = 1)" "access_type": "table_scan"
} }
] /* attached_conditions_summary */ ] /* refine_plan */
} /* attaching_conditions_to_tables */ }
},
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13. Optimizer trace: controls
 optimizer-trace
 optimizer-trace-features
– greedy_search
– range_optimizer
– dynamic_range
– repeated_subselect
 optimizer-trace-limit
 optimizer-trace-offset
 optimizer-trace-max-mem-size
 end-markers-in-json
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14. Optimizer trace: exploring
 mysql> pager less
 SELECT TRACE INTO DUMPFILE <filename> FROM
INFORMATION_SCHEMA.OPTIMIZER_TRACE;
 JSONView for Firefox
 Pretty JSON for Chrome
 A registry patch for IE
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15. EXPLAIN in JSON format
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16. Traditional EXPLAIN
id select type table type possible key key len ref rows extra
keys
1 PRIMARY cust index cust_id, cust_name 40 const 10 Using where; Start
cust_name materialize; Scan
1 PRIMARY orders ref order_id order_id 8 ordid 32 Using where; End
materialize; Using join
buffer (Block Nested
Loop)
1 PRIMARY <derived2> ref <auto_key0> <auto_key0> 23 vc1 100 Using index
2 DERIVED flights ALL NULL NULL NULL NULL 10000 Using where; Start
materialize; Scan
2 DERIVED storage eq_ref PRIMARY PRIMARY 8 fl_store 1 Using where; End
materialize; Using join
buffer (Block Nested
Loop)
4 SUBQUERY buzz range buzzez buzzez 11 NULL 42 Using index; Using
where
6 SUBQUERY shortage index shrt_idx shrt_idx 100 NULL 243 Using index; Using
where
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17. Traditional EXPLAIN: Cons
 Hard to read in complex cases
 Unclear/non-obvious query structure in complex cases
– Could take quite some time to see what’s going on
 Limited information about query plan
– How conditions are split? When a subquery is evaluated?
 Hard to impossible to extend
– Indication of a feature being used usually is limited to “Using <something>”
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18. EXPLAIN in JSON: Basics
mysql> EXPLAIN SELECT * FROM t1;
id select type table type possible keys key key len ref rows extra
1 SIMPLE t1 index NULL i 5 NULL 3 NULL
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19. EXPLAIN in JSON: Basics
EXPLAIN
"query_block": {
"select_id": 1,
"table": {
"table_name": "t1",
mysql> EXPLAIN FORMAT=JSON "access_type": "index",
SELECT * FROM t1; "key": "i",
"key_length": "5",
"rows": 3,
"filtered": 100,
"using_index": true
} /* table */
} /* query_block */
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20. EXPLAIN in JSON: Basics
mysql> set end_markers_in_json = on;
end_markers_in_json = off end_markers_in_json = on
} } /* table */
} } /* query_block */
} }
] ] /* query_specifications */
} } /* union_result */
} } /* query_block */
} } /* materialized_from_subquery */
} } /* table */
} } /* grouping_operation */
} } /* ordering_operation */
} } /* query_block */
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21. EXPLAIN in JSON: Basics
mysql> EXPLAIN SELECT * FROM t1 WHERE str IS NULL AND id > 0;
id select type table type possible keys key key len ref rows extra
1 SIMPLE t1 ref str str 11 const 1 Using index
condition;
Using where
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22. EXPLAIN in JSON: Basics
EXPLAIN
"query_block": {
"select_id": 1,
"table": {
"table_name": "t1",
mysql> EXPLAIN FORMAT=JSON "access_type": "ref",
SELECT * FROM t1 WHERE str IS "possible_keys": [ "str“ ],
"key": "str", "key_length": "11",
NULL AND id > 0;
"ref": [ "const"],
"rows": 1, "filtered": 100,
"index_condition": "isnull(`test`.`t1`.`str`)“
"attached_condition": "(`test`.`t1`.`id` > 0)"
} /* table */
} /* query_block */
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23. EXPLAIN in JSON: Basics
mysql> EXPLAIN PARTITIONS SELECT * FROM t1 WHERE a
BETWEEN '0000-00-00' AND '1001-01-01';
possible key
id select type partitions table type key ref rows extra
keys len
1 SIMPLE pNULL, t1 index a a 4 NULL 6 Using where;
p0001-01-01, Using index
p1001-01-01
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24. EXPLAIN in JSON: Basics
EXPLAIN
"query_block": {
"select_id": 1,
"table": {
"table_name": "t1",
mysql> EXPLAIN FORMAT=JSON "partitions": [ "pNULL", "p0001-01-01",
SELECT * FROM t1 WHERE a "p1001-01-01"],
"access_type": "index",
BETWEEN '0000-00-00' AND '1001-
"possible_keys": [ "a" ],
01-01'; "key": "a", "key_length": "4",
"rows": 6, "filtered": 33.333,
"using_index": true,
"attached_condition": "(`test`.`t1`.`a` between
'0000-00-00' and '1001-01-01')"
}}
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25. EXPLAIN in JSON: Basics
mysql> EXPLAIN SELECT * FROM t1 JOIN t2 USING(i);
possible key
id select type table type key ref rows extra
keys len
1 SIMPLE t2 ALL NULL NULL NULL NULL 3 NULL
1 SIMPLE t1 index i i 5 NULL 3 Using where; Using
index; Using join
buffer (Block Nested
Loop)
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26. EXPLAIN in JSON: Basics
EXPLAIN
{ {
"query_block": { "table": {
"select_id": 1, "table_name": "t1",
"nested_loop": [ "access_type": "index",
{ "possible_keys": [ "i" ],
"table": { "key": "i", "key_length": "5",
"table_name": "t2", "rows": 3, "filtered": 100,
"access_type": "ALL", "using_index": true,
"rows": 3, "using_join_buffer": "Block Nested Loop",
"filtered": 100 "attached_condition":
} /* table */ "(`test`.`t1`.`i` = `test`.`t2`.`i`)"
}, } /* table */
… next table … => }
] } /* nested_loop */ } /* query_block */
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27. EXPLAIN in JSON: Basics
MySQL Workbench 5.2
SELECT * FROM t1 JOIN t2 USING(i);
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28. EXPLAIN in JSON: more complex queries
mysql> EXPLAIN FORMAT=JSON SELECT t1.a, MIN(t2.b)
FROM t1 LEFT JOIN t2 ON t1.a=t2.a GROUP BY t1.a ORDER BY t1.b;
id select type table type possible keys key key len ref rows extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using temporary;
Using filesort
1 SIMPLE t2 ref PRIMARY PRIMARY 4 test.t1.a 1 Using index
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29. EXPLAIN in JSON: more complex queries
EXPLAIN
"query_block": {
mysql> EXPLAIN FORMAT=JSON "select_id": 1,
SELECT t1.a, MIN(t2.b) "ordering_operation": {
"using_temporary_table": true,
FROM t1 LEFT JOIN t2 "using_filesort": true,
"grouping_operation": {
ON t1.a=t2.a
"using_filesort": true,
GROUP BY t1.a ORDER BY t1.b; "nested_loop": [
{ “table” : {…} },
{ “table” : {…} } ] /* nested_loop */
} /* grouping_operation */
} /* ordering_operation */
} /* query_block */
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30. EXPLAIN in JSON: more complex queries
MySQL Workbench 5.2
SELECT t1.a, MIN(t2.b)
FROM t1 LEFT JOIN t2
ON t1.a=t2.a
GROUP BY t1.a ORDER BY t1.b;
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31. EXPLAIN in JSON: more complex queries
mysql> EXPLAIN SELECT a, b FROM
(SELECT 1 AS a, 2 AS b UNION ALL SELECT 1 AS a, 2 AS b) t1
GROUP BY a ORDER BY b DESC;
possible key
id select type table type key ref rows extra
keys len
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 2 Using temporary;
Using filesort
2 DERIVED NULL NULL NULL NULL NULL NULL NULL No tables used
3 UNION NULL NULL NULL NULL NULL NULL NULL No tables used
NULL UNION <union2,3> ALL NULL NULL NULL NULL NULL Using temporary
RESULT
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32. EXPLAIN in JSON: more complex queries
EXPLAIN
"query_block": { "union_result": {
"ordering_operation": { "using_temporary_table": true,
"using_filesort": true, "table_name": "<union2,3>",
"grouping_operation": { "query_specifications": [ {
"using_temporary_table": true, "query_block": {
"using_filesort": false, "table": {
"table": { "message": "No tables used"
"table_name": "t1", } } }, {
"materialized_from_subquery": { "query_block": {
"using_temporary_table": true, "table": {
"query_block": { "message": "No tables used"
… here goes union_result… => } } } ] } /* union_result */
}}}}}}
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33. EXPLAIN in JSON: more complex queries
MySQL Workbench 5.2
SELECT a, b FROM
(SELECT 1 AS a, 2 AS b UNION ALL
SELECT 1 AS a, 2 AS b) t1
GROUP BY a ORDER BY b DESC;
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34. EXPLAIN in JSON: subqueries
mysql> EXPLAIN FORMAT=JSON SELECT derived.vc
FROM (SELECT * FROM t1) AS derived WHERE
derived.vc IN (SELECT t2.vc1 FROM t2 JOIN t3 ON t2.vc2=t3.vc);
select possible key
id table type key ref rows extra
type keys len
1 PRIMARY t2 ALL NULL NULL NULL NULL 2 Using where;
Start materialize;
Scan
1 PRIMARY t3 ALL NULL NULL NULL NULL 3 Using where; End
materialize; Using
join buffer (Block
Nested Loop)
1 PRIMARY <derived2> ref <auto_key0> <auto_key0> 23 t2.vc1 2 NULL
2 DERIVED t1 ALL NULL NULL NULL NULL 2 NULL
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35. EXPLAIN in JSON: subqueries
EXPLAIN
"query_block": { "table": {
"select_id": 1, "table_name": "derived",
"nested_loop": [ "materialized_from_subquery": {
{ "table": { "using_temporary_table": true,
"table_name": "<subquery3>", "query_block": {
"materialized_from_subquery": { "select_id": 2,
"query_block": { "table": {
"nested_loop": [ "table_name": "t1“,
{ "table": { "table_name": "t2“ } }, …
{ "table": { "table_name": "t3“ } } }
] } /* query_block */ } } /* table */ } /* query_block */
}, { … next table goes here … => } } /* materialized_from_subquery */
] /* nested_loop */ } /* query_block */ } /* table */
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36. EXPLAIN in JSON: subqueries
MySQL Workbench 5.2
SELECT derived.vc
FROM (SELECT * FROM t1) AS
derived WHERE
derived.vc IN (SELECT t2.vc1 FROM
t2 JOIN t3 ON t2.vc2=t3.vc);
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37. EXPLAIN in JSON: subqueries
mysql> EXPLAIN SELECT * FROM t1, t2 WHERE t2.a IN (SELECT a
FROM t1 WHERE t1.b <> 30) AND t1.b=t2.a GROUP BY t1.a;
select possible
id table type key key len ref rows extra
type keys
1 SIMPLE t2 index a a 5 NULL 4 Using where; Using index;
Using temporary; Using filesort
1 SIMPLE t1 ref a a 5 test.t2.a 101 Using where; Using index; Start
temporary; End temporary
1 SIMPLE t1 index NULL a 10 NULL 10004 Using where; Using index;
Using join buffer (Block Nested
Loop)
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38. EXPLAIN in JSON: subqueries
EXPLAIN
"query_block": { {
"select_id": 1, "duplicates_removal": {
"grouping_operation": { "using_temporary_table": true,
"using_temporary_table": true, "table": {
"nested_loop": [ "table_name": "t1",
{ "attached_condition": "(`test`.`t1`.`b` <> 30)"
"table": { } /* table */
"table_name": "t2", } /* duplicates_removal */
"attached_condition": "(`test`.`t2`.`a` is not null)" }, {
} /* table */ "table": {
}, "table_name": "t1",
… next 2 tables => … "attached_condition": "(`test`.`t1`.`b` = `test`.`t2`.`a`)"
] /* nested_loop */ } /* grouping_operation */ } /* table */
} /* query_block */ }
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39. EXPLAIN in JSON: subqueries
MySQL Workbench 5.2
SELECT * FROM t1, t2 WHERE t2.a
IN (SELECT a FROM t1 WHERE t1.b
<> 30) AND t1.b=t2.a GROUP BY t1.a;
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40. EXPLAIN in JSON: subqueries
EXPLAIN
"query_block": { "query_block": {
"select_id": 1, "select_id": 1,
"nested_loop": [ "nested_loop": [
{ {
"table": { "table": {
"table_name": "t2", "table_name": "t1",
} /* table */ "access_type": "range", "loosescan": true,
}, { } /* table */
"table": { }, {
"table_name": "t1", "table": {
"first_match": "t2", "table_name": "t2",
"attached_condition": "(`test`.`t1`.`b` <> 30)" } /* table */
} /* table */ }
} ] /* nested_loop */ } /* query_block */ ] /* nested_loop */ } /* query_block */
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41. EXPLAIN
for non-select queries
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42. EXPLAINing non-select queries
 INSERT/REPLACE and INSERT/REPLACE SELECT
 Single- and multi-table UPDATE
 Single- and multi-table DELETE
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43. EXPLAINing non-select queries
mysql> EXPLAIN UPDATE t1 SET a = 10 WHERE a < 10;
id select type table type possible keys key key len ref rows extra
1 SIMPLE t1 range a,a_2 a 5 NULL 2 Using where;
Using
temporary
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44. EXPLAINing non-select queries
EXPLAIN
"query_block": {
"select_id": 1,
"table": {
"update": true,
mysql> EXPLAIN FORMAT=JSON "table_name": "t1",
"access_type": "range",
UPDATE t1 SET a = 10 WHERE
"possible_keys": [ "a", "a_2" ],
a < 10; "key": "a", "key_length": "5",
"rows": 2, "filtered": 100,
"using_temporary_table": "for update",
"attached_condition": "(`test`.`t1`.`a` < 10)"
} /* table */
} /* query_block */
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45. EXPLAINing non-select queries
EXPLAIN
"query_block": {
"select_id": 1,
"table": {
"delete": true,
mysql> EXPLAIN FORMAT=JSON "table_name": "t1",
"access_type": "range",
DELETE FROM t1 WHERE a < 10;
"possible_keys": [ "a", "a_2“ ],
"key": "a", "key_length": "5",
"rows": 2, "filtered": 100,
"attached_condition": "(`test`.`t1`.`a` < 10)"
} /* table */
} /* query_block */
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46. EXPLAINing non-select queries
mysql> EXPLAIN UPDATE t1 t11, (SELECT * FROM t2) t12 SET t11.a =10
WHERE t11.a = 1;
select possible
id table type key key len ref rows extra
type keys
1 PRIMARY t11 ref a,a_2 a 5 const 1 NULL
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 3 Using join buffer (Block
Nested Loop)
2 DERIVED t2 ALL NULL NULL NULL NULL 3 NULL
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47. EXPLAINing non-select queries
EXPLAIN
"query_block": { {
"select_id": 1, "table": {
"nested_loop": [ { "table_name": "t12",
"table": { "access_type": "ALL",
"update": true, "rows": 3, "filtered": 100,
"table_name": "t11", "using_join_buffer": "Block Nested Loop",
"access_type": "ref", "materialized_from_subquery": {
"possible_keys": [ "a", "a_2“ ], "using_temporary_table": true,
"key": "a", "key_length": "5", "query_block": {
"ref": ["const"] /* ref */, "select_id": 2,
"rows": 1, "filtered": 100 "table": { … } /* table */
} /* table */ }, } /* query_block */
… here goes 2nd table…=> } /* materialized_from_subquery */
] /* nested_loop */ } /* query_block */ } /* table */ }
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48. EXPLAINing non-select queries
mysql> EXPLAIN UPDATE t1 SET a = 10 WHERE a IN
(SELECT a FROM t2);
id select type table type possible keys key key len ref rows extra
1 PRIMARY t1 ALL a,a_2 NULL NULL NULL 3 NULL
1 DEPENDENT t2 ALL NULL NULL NULL NULL 3 Using where
SUBQUERY
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49. EXPLAINing non-select queries
EXPLAIN
"query_block": { [{
"select_id": 1, "dependent": true, "cacheable": false,
"table": { "query_block": {
"update": true, "select_id": 2,
"table_name": "t1", "table": {
"possible_keys": [ "a", "a_2“ ], "table_name": "t2",
"attached_condition": "access_type": "ALL",
"<in_optimizer>(`test`.`t1`.`a`, "rows": 3, "filtered": 100,
<exists>(select 1 from `test`.`t2` where "attached_condition":
(<cache>(`test`.`t1`.`a`) = "(<cache>(`test`.`t1`.`a`) = `test`.`t2`.`b`)"
`test`.`t1`.`a`)))", } /* table */
"attached_subqueries": [ … => …] } /* query_block */
} /* table */ }]
} /* query_block */
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