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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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