MySQL EXPLAIN Explained-Norvald H. Ryeng

MySQL EXPLAIN Explained
Norvald H. Ryeng
Software Engineer

Copyright © 2014, Oracle and/or its affiliates. 2 All rights reserved.
議程
 傳統的, 結構化的, 和視覺化的EXPLAIN
 用EXPLAIN來優化查詢
 EXPLAIN和子查詢
 對INSERT/UPDATE/DELETE的EXPLAIN
 Optimizer trace

查詢優化器
優化器做啥?
Optimizer
SELECT a, b
FROM t1, t2, t3
WHERE t1.a = t2.b
AND t2.b = t3.c
AND t2.d > 20
AND t2.d < 30;
Metadata:
- Index information
- Uniqueness
- Nullability
Statistics:
- Table sizes
- Cardinality
JOIN
JOIN
Table
scan
Range
scan
Ref
access
t2 t3
t1
Query
Query Execution
Plan (QEP)

MySQL EXPLAIN
不同的佈局
+----+-------------+------------+-------+---------------+-------------+---------+-------------------------+---------+…
| id | select_type | table | type | possible_keys | key | key_len | ref | rows |…
+----+-------------+------------+-------+---------------+-------------+---------+-------------------------+---------+…
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 1050000 |…
| 2 | DERIVED | customer | index | PRIMARY | PRIMARY | 4 | NULL | 150000 |…
| 2 | DERIVED | orders | ref | i_o_custkey | i_o_custkey | 5 | dbt3.customer.c_custkey | 7 |…
+----+-------------+------------+-------+---------------+-------------+---------+-------------------------+---------+…
{
"query_block": {
"select_id": 1,
"cost_info": {
"query_cost": "262510.00"
},
"ordering_operation": {
"using_filesort": true,
"grouping_operation": {
"using_temporary_table": true,
"using_filesort": false,
"table": {
"table_name": "c_orders",
"access_type": "ALL",
"rows_examined_per_scan": 1050000,
"rows_produced_per_join": 1050000,
"filtered": 100,
"cost_info": {
"read_cost": "52510.00",
"eval_cost": "210000.00",
"prefix_cost": "262510.00",
"data_read_per_join": "24M”
...

傳統的EXPLAIN
film
film_
category
JOIN
category
JOIN
EXPLAIN為在SELECT指令中的每
個表傳回一列訊息
這個表可能是真的表,也可以是洐生的,臨時
表,子查詢,或一個union的結果

了解EXPLAIN
EXPLAIN SELECT l_returnflag, l_linestatus, SUM(l_quantity) AS sum_qty
FROM lineitem WHERE l_shipdate <= DATE_SUB('1998-12-01', INTERVAL '118' DAY)
GROUP BY l_returnflag, l_linestatus ORDER BY l_returnflag, l_linestatusG
id: 1
select_type: SIMPLE
table: lineitem
type: ALL
possible_keys: i_l_shipdate
key: NULL
key_len: NULL
ref: NULL
rows: 6001215
Extra: Using where; Using temporary; Using filesort
Full table scan
Considered indexes
Number of rows to be read
Chosen index (none)

傳統的EXPLAIN的結果可能會很複雜
id select type table type possible
keys
key key len ref rows extra
1 PRIMARY cust index cust_id,
cust_name
cust_name 40 const 10 Using where; Start
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

傳統的EXPLAIN: 缺點
 複雜的查詢很難讀得懂
 複雜的查詢所產生的結果不清楚/不明顯
– 可能要用很長的時間了解發生了什麼事
 能為查詢計畫提供的資訊有限
– 條件是如何劃分? 子查詢是何時評估?
 非常難擴充
– 顯示用到了一個功能僅限於 “Using <something>”

EXPLAIN
{ "query_block": {
"select_id": 1,
"ordering_operation": {
"using_filesort": false,
"grouping_operation": {
"using_temporary_table": true,
"using_filesort": true,
"table": {
"table_name": "lineitem“,
"access_type": "ALL“,
"possible_keys": [
"i_l_shipdate”
],
"rows": 2829575,
"filtered": 50,
"attached_condition":
"(`dbt3`.`lineitem`.`l_shipDATE` <=
<cache>(('1998-12-01' - interval '118' day)))”
} /* table */
} /* grouping_operation */
} /*ordering_operation */
} /*query_block */ }
結構化 EXPLAIN
EXPLAIN FORMAT=JSON
SELECT l_returnflag, l_linestatus,
SUM(l_quantity) AS sum_qty
FROM lineitem
WHERE l_shipdate <=
DATE_SUB('1998-12-01',
INTERVAL '118' DAY)
GROUP BY l_returnflag, l_linestatus
ORDER BY l_returnflag, l_linestatus;
MySQL 5.6的新功能

圖示EXPLAIN
MySQL Workbench
Cost estimate
( MySQL 5.7)

圖示EXPLAIN
滑鼠掃過就能看到詳情

單一個表的查詢
SELECT * FROM customer WHERE c_custkey = 570887;
主鍵查找
id
select
type
table type
possible
keys
1 SIMPLE customer const PRIMARY PRIMARY 4 const 1 NULL

次要鍵查找
SELECT * FROM orders WHERE o_orderdate = '1992-09-12';
id
select
type
table type
possible
keys
key
key
len
ref rows extra
1 SIMPLE orders ref i_o_orderdate i_o_orderdate 4 const 6271 NULL

索引區域掃瞄
SELECT * FROM orders
WHERE o_orderdate BETWEEN '1997-05-01' AND '1997-05-31‘;
id
select
type
table type
possible
keys
key
key
len
ref rows extra
1 SIMPLE orders range i_o_orderdate i_o_orderdate 4 NULL 376352 Using index
condition

範圍優化器: 案例
WHERE YEAR(o_orderdate) = 1997 AND MONTH(o_orderdate) = 5 AND
o_clerk LIKE '%01866';
為何要做表掃?
id
select
type
table type
possible
keys
1 SIMPLE orders ALL NULL NULL NULL NULL 15000000 Using where
mysql> SELECT * FROM orders WHERE year(o_orderdate) = 1997 AND MONTH(…
...
15 rows in set (8.91 sec)
未考量索引

WHERE o_orderdate BETWEEN '1997-05-01' AND '1997-05-31'
AND o_clerk LIKE '%01866';
重寫查詢以避免把加了索引的欄位放入function的參數中
id
select
type
table type
possible
keys
key
key
len
ref rows extra
1 SIMPLE orders range i_o_orderdate i_o_orderdate 4 NULL 376352 Using index
condition;
Using where
mysql> SELECT * FROM orders WHERE o_orderdate BETWEEN '1997-05-01' AND …
...

CREATE INDEX i_o_clerk ON orders(o_clerk);
AND o_clerk LIKE '%01866';
加上另一個索引
id
select
type
table type
possible
keys
key
key
len
ref rows extra
1 SIMPLE orders range i_o_orderdate i_o_orderdate 4 NULL 376352
Using index
condition;
Using where
New index not considered

AND o_clerk LIKE 'Clerk#000001866';
再次重寫查詢
id
select
type
table type possible keys key
key
len
ref rows extra
1 SIMPLE orders range i_o_orderdate,
i_o_clerk
i_o_clerk 16 NULL 1504 Using index
condition;
Using where
...

CREATE INDEX i_o_clerk_date
ON orders(o_clerk, o_orderdate);
建立多欄位索引
id
select
type
key
len
ref rows extra
i_o_clerk,
i_o_clerk_date
i_o_clerk_date 20 NULL 14 Using index
condition
...

多欄位索引
CREATE INDEX i_o_clerk_status_date
ON orders(o_clerk, o_orderstatus, o_orderdate);
key_len對應使用索引欄位
id
select
type
key
len
ref rows extra
i_o_clerk_status
_date
i_o_clerk_status
_date
16 NULL 1504 Using index
condition

Multi-Column index
CREATE INDEX i_o_clerk_date_status
ON orders(o_clerk, o_orderdate, o_orderstatus);
key_len對應使用索引欄位 - 續
id
select
type
key
len
ref rows extra
i_o_clerk_status
_date,i_o_clerk_
date_status
i_o_clerk_date_
status
20 NULL 15 Using index
condition

使用加了索引欄位
結構化的 EXPLAIN
EXPLAIN
{ "query_block": {
"select_id": 1,
"table": {
"table_name": "orders",
"access_type": "range",
"possible_keys": [
"i_o_orderdate",
"i_o_clerk_status_date"
],
"key": "i_o_clerk_status_date",
"used_key_parts": [
"o_clerk"
],
"key_length": "16",
"rows": 1504,
"filtered": 100,
"index_condition": "((`dbt3`.òrders`.ò_orderDATE` between
'1997-05-01' and '1997-05-31') and (`dbt3`.òrders`.ò_clerk` like
'Clerk#000001866'))"
} } }
EXPLAIN
{ "query_block": {
"select_id": 1,
"table": {
"table_name": "orders",
"possible_keys": [
"i_o_orderdate",
"i_o_clerk_status_date",
"i_o_clerk_date_status"
],
"key": "i_o_clerk_date_status",
"used_key_parts": [
"o_clerk",
"o_orderDATE"
],
"key_length": "20",
"rows": 15,
"filtered": 100,
"index_condition": "((`dbt3`.òrders`.ò_orderDATE` between
'1997-05-01' and '1997-05-31') and (`dbt3`.òrders`.ò_clerk` like
'Clerk#000001866'))"
} } }

索引合併
SELECT COUNT(*) FROM lineitem
WHERE l_shipdate = '1997-05-01'
AND l_commitdate = '1997-05-01';
id
select
type
key
len
ref rows extra
1 SIMPLE lineitem index_
merge
i_l_shipdate,
i_l_commitdate
i_l_shipdate,
i_l_commitdate
4,4 NULL 43 Using intersect
(i_l_shipdate,
i_l_commitdate);
Using where;
Using index

Join 查詢
SELECT *
FROM orders JOIN customer
ON c_custkey = o_custkey
WHERE o_orderdate = '1992-09-12';
id
select
type
key
len
ref rows extra
1 SIMPLE orders ref i_o_orderdate,
i_o_custkey
i_o_orderdate 4 const 6271 Using
where
1 SIMPLE customer eq_ref PRIMARY PRIMARY 4 dbt3.orders.
o_custkey
1 NULL

Join查詢 (續)
SELECT *
FROM orders JOIN customer
WHERE c_acctbal < -1000;
id
select
type
table type
possible
keys
key
key
len
ref rows extra
1 SIMPLE customer ALL PRIMARY NULL NULL NULL 1500000 Using
where
1 SIMPLE orders ref i_o_custkey i_o_custkey 5 dbt3.customer
.c_custkey
7 NULL

EXPLAIN
{
"query_block": {
"select_id": 1,
"nested_loop": [
{
"table": {
"table_name": "customer“,
"access_type": "ALL“,
"possible_keys": [
"PRIMARY”
],
"rows": 147973,
"filtered": 100,
"attached_condition":
"(`dbt3`.`customer`.`c_acctbal` < <cache>(-(1000)))”
} /* table */
},
{
"table": {
"table_name": "orders“,
"access_type": "ref“,
"possible_keys": [
"i_o_custkey”
],
"key": "i_o_custkey“,
"used_key_parts": [
"o_custkey”
],
"key_length": "5“,
"ref": [
"dbt3.customer.c_custkey
],
"rows": 7,
"filtered": 100
} /* table */
}
] /* nested_loop */
} /* query_block */
}
結構化EXPLAIN: Join 查詢

EXPLAIN: 子查詢
SELECT SUM(l_extendedprice) / 7.0 AS avg_yearly
FROM lineitem JOIN part ON l_partkey = p_partkey
WHERE p_brand = 'Brand#33‘ AND p_container = 'LG CAN'
AND l_quantity <
(SELECT 0.2 * AVG(l_quantity) FROM lineitem
WHERE l_partkey = p_partkey);
依附的子查詢 (DBT-3 Query 17)
id
select
type
key
len
ref rows extra
1 PRIMARY
part ALL PRIMARY 2000000 Using where
1 PRIMARY lineitem ref i_l_suppkey_partkey i_l_suppkey_partkey 5 dbt3.part.
p_partkey
14 Using where
2 DEPENDENT
SUBQUERY
lineitem ref i_l_suppkey_partkey i_l_suppkey_partkey 5 dbt3.part.
p_partkey
14

圖示 EXPLAIN
DBT-3 Query 17
SELECT SUM(l_extendedprice) / 7.0 AS avg_yearly
FROM lineitem JOIN part ON l_partkey = p_partkey
WHERE p_brand = 'Brand#33‘ AND p_container = 'LG CAN'
AND l_quantity <
(SELECT 0.2 * AVG(l_quantity) FROM lineitem
WHERE l_partkey = p_partkey);

依附的子查詢
DBT-3 Query 11
SELECT ps_partkey, SUM(ps_supplycost * ps_availqty) AS value
FROM partsupp JOIN supplier ON ps_suppkey = s_suppkey
JOIN nation ON s_nationkey = n_nationkey
WHERE n_name = 'KENYA'
GROUP BY ps_partkey HAVING SUM(ps_supplycost * ps_availqty) >
(SELECT SUM(ps_supplycost * ps_availqty) * 0.0001000000
FROM partsupp JOIN supplier ON ps_suppkey = s_suppkey
JOIN nation ON s_nationkey = n_nationkey
WHERE n_name = 'KENYA‘)
ORDER BY value DESC;

EXPLAIN: 獨立的子查詢
DBT-3 Query 11
id
select
type
table type
possible
keys
key
key
len
ref rows extra
1 PRIMARY nation ALL PRIMARY 25 Using where; Using
temporary; Using filesort
1 PRIMARY supplier ref PRIMARY,
i_s_nationkey
i_s_nationkey 5 dbt3.nation.
n_nationkey
2000 Using index
1 PRIMARY partsupp ref PRIMARY,
i_ps_partkey,
i_ps_suppkey
i_ps_suppkey 4 dbt3.supplier.
s_suppkey
40
2 SUBQUERY nation ALL PRIMARY 25 Using where
2 SUBQUERY supplier ref PRIMARY,
i_s_nationkey
n_nationkey
2000 Using index
2 SUBQUERY partsupp ref i_ps_suppkey i_ps_suppkey 4 dbt3.supplier.
s_suppkey
40

Semi-join
SELECT s_name, s_address
FROM supplier JOIN nation ON s_nationkey = n_nationkey
WHERE s_suppkey IN
(SELECT DISTINCT (ps_suppkey)
FROM partsupp JOIN part ON ps_partkey=p_partkey
WHERE p_name like 'grey%’ AND ps_availqty >
(SELECT 0.5 * SUM(l_quantity)
FROM lineitem
WHERE l_partkey = ps_partkey AND l_suppkey = ps_suppkey
AND l_shipdate >= '1995-01-01
AND l_shipdate < date_add( '1995-01-01’, interval '1' year)))
AND n_name = 'SAUDI ARABIA‘
ORDER BY s_name;
DBT-3 Query 20

圖示EXPLAIN: Semi-join
DBT-3 Query 20

EXPLAIN: Semi-join
DBT-3 Query 20
id select type table type possible keys key
key
len
ref rows extra
1 PRIMARY nation ALL PRIMARY 25 Using where;
Using temporary;
Using filesort
1 PRIMARY supplier ref PRIMARY,
i_s_nationkey
n_nationkey
2000
1 PRIMARY partsupp ref PRIMARY,
i_ps_partkey,
i_ps_suppkey
i_ps_suppkey 4 dbt3.supplier.
s_suppkey
40 Using where
1 PRIMARY part eq_ref PRIMARY PRIMARY 4 dbt3.partsupp.
ps_partkey
1 Using where;
FirstMatch
(supplier)
3 DEPENDENT
SUBQUERY
lineitem ref i_l_shipdate,
i_l_partkey_suppkey,
i_l_partkey,
i_l_suppkey
i_l_partkey_suppkey 10 dbt3.partsupp.
ps_partkey,
dbt3.partsupp.
ps_suppkey
3 Using where

洐生性表 (自FROM子句的查詢)
SELECT c_count, COUNT(*) AS custdist
FROM (SELECT c_custkey,
COUNT(o_orderkey) AS c_count
FROM customer LEFT JOIN orders
AND o_comment NOT LIKE
'%express%requests%’
GROUP BY c_custkey
) AS c_orders
GROUP BY c_count
ORDER BY custdist DESC, c_count DESC;
DBT-3 Query 13

EXPLAIN: 衍生的性表
DBT-3 Query 13
id
select
type
table type
possible
keys
key
key
len
ref rows extra
1 PRIMARY <derived2> ALL 10500000 Using temporary;
Using filesort
2 DERIVED customer index PRIMARY,
i_c_nationkey
PRIMARY 4 1500000 Using index
2 DERIVED orders ref i_o_custkey i_o_custkey 5 dbt3.customer.
c_custkey
7 Using where
SELECT c_count, COUNT(*) AS custdist
FROM (SELECT c_custkey, COUNT(o_orderkey) AS c_count
FROM customer LEFT JOIN orders ON c_custkey = o_custkey
AND o_comment NOT LIKE '%express%requests%’
GROUP BY c_custkey
) AS c_orders
GROUP BY c_count
ORDER BY custdist DESC, c_count DESC;

EXPLAINing 非Select查詢
INSERT/REPLACE
INSERT/REPLACE … SELECT FROM
UPDATE (Single- and multi-table)
DELETE (Single- and multi-table)

EXPLAINing非Select查詢
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
EXPLAIN UPDATE t1 SET a = 10 WHERE a < 10;
1 SIMPLE t1 range a,a_2 a 5 NULL 2 Using where
EXPLAIN DELETE FROM t1 WHERE a < 10;
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL No tables used
EXPLAIN INSERT INTO t1 VALUES (13,'m'), (14,'n');

EXPLAIN
"query_block": {
"select_id": 1,
"table": {
"update": true,
"table_name": "t1",
"possible_keys": [ "a", "a_2" ],
"key": "a", "key_length": "5",
"rows": 2, "filtered": 100,
"using_temporary_table": "for update",
"attached_condition": "(`test`.`t1`.`a` < 10)"
} /* table */
} /* query_block */
EXPLAIN FORMAT=JSON
UPDATE t1
SET a = 10
WHERE a < 10;
結構化的EXPLAIN

UPDATE part
SET p_retailprice = p_retailprice*1.10
WHERE p_partkey IN
(SELECT ps_partkey
FROM partsupp JOIN supplier
ON ps_suppkey = s_suppkey
WHERE s_nationkey = 4);
Plans may differ from similar SELECT query
SELECT * FROM part
WHERE p_partkey IN
(SELECT ps_partkey
FROM partsupp JOIN supplier
ON ps_suppkey = s_suppkey
WHERE s_nationkey = 4);

優化器Traces: 查詢計畫偵錯
 EXPLAIN 顯示優化器所選擇的計畫
 TRACE 顯示為何會選擇這個計畫:
– 可選擇的計畫
– 預估的成本
– 所下的決定
 JSON格式

優化器Trace:範例
mysql> SET optimizer_trace= “enabled=on“,
end_markers_in_json=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";
QUERY SELECT * FROM t1,t2 WHERE f1=1 AND f1=f2 AND f2>0;
TRACE “steps”: [ { "join_preparation": { "select#": 1,… } … } …]
MISSING_BYTES_BEYOND_MAX_MEM_SIZE 0
INSUFFICIENT_PRIVILEGES 0

優化器Trace:範例
join_optimization / row_estimation / table : t1, range_analysis
"table_scan": {
"rows": 5,
"cost": 5.1085
} /* table_scan */,
"potential_range_indices": [ {
"index": "f1", "usable": true,
"key_parts": [ "f1“ ] /* key_parts */ }
] /* potential_range_indices */,
"best_covering_index_scan": {
"index": "f1", "cost": 2.093, "chosen": true
} /* best_covering_index_scan */,
"group_index_range": {
"chosen": false, "cause": "not_single_table"
} /* group_index_range */,
"analyzing_range_alternatives": {
"range_scan_alternatives": [ {
"index": "f1",
"ranges": [ "1 <= f1 <= 1“ ] /* ranges */,
"index_dives_for_eq_ranges": true,
"rows": 2, "cost": 3.41,
"chosen": false,
"cause": "cost"
}
] /* range_scan_alternatives */,
"analyzing_roworder_intersect": {
"usable": false, "cause": "too_few_roworder_scans"
} /* analyzing_roworder_intersect */
} /* analyzing_range_alternatives */

優化器Trace: 範例
join_optimization / considered_execution_plan
"plan_prefix": [ ] /* plan_prefix */,
"table": "`t2`",
"best_access_path": {
"considered_access_paths": [ {
"access_type": "scan", "rows": 3,
"cost": 2.6051, "chosen": true
} ] /* considered_access_paths */
} /* best_access_path */,
"cost_for_plan": 2.6051,
"rows_for_plan": 3,
"rest_of_plan": [ {
… next iteration … =>
} ] /* rest_of_plan */
"plan_prefix": [ "`t2`“ ] /* plan_prefix */,
"table": "`t1`",
"best_access_path": {
"considered_access_paths": [ {
"access_type": "ref", "index": "f1",
"rows": 2, "cost": 3.4698, "chosen": true
}, {
"access_type": "scan", "using_join_cache": true,
"rows": 2, "cost": 3.8087,
"chosen": true
} ] } /* best_access_path */,
"cost_for_plan": 6.4138,
"rows_for_plan": 6,
"chosen": true

優化器Trace: 控制
 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

優化器Trace: 探索
 mysql> pager less
 mysql> SELECT TRACE INTO DUMPFILE <filename> FROM
INFORMATION_SCHEMA.OPTIMIZER_TRACE;
 JSONView for Firefox
 Pretty JSON for Chrome
 A registry patch for IE

 MySQL 5.7 Reference Manual
– http://dev.mysql.com/doc/refman/5.7/en/index.html
 Optimizer team blog:
– http://mysqloptimizerteam.blogspot.com/
 MySQL Server Blog:
– http://mysqlserverteam.com
進一步的資訊

The preceding is intended to outline our general product direction. It is
intended for information purposes only, and may not be incorporated
into any contract. It is not a commitment to deliver any material, code,
or functionality, and should not be relied upon in making purchasing
decisions. The development, release, and timing of any features or
functionality described for Oracle’s products remains at the sole
discretion of Oracle.

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MySQL EXPLAIN Explained-Norvald H. Ryeng

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