DB2中如何查看执行计划
说明：
本文讲述了DB2中查看执行计划的方法，包括动态SQL的执行计划，存储过程、包的执行计划，以及如何查看package cache中的执行计划测试环境：
DB2 10.5fp8/Linux前提条件:
查看执行计划需要先创建explain表：$ db2 -tvf $HOME/sqllib/misc/EXPLAIN.DDL
详解：
1.查看普通SQL语句的执行计划
例如下面的例子是查看"SELECT NAME FROM T2 WHERE ID > 15" 的执行计划inst105@db2a:~$ db2 explain plan for "SELECT NAME FROM T2 WHERE ID > 15"
DB20000I The SQL command completed successfully.
inst105@db2a:~$ db2exfmt -d sample -o db2exfmt.out
DB2 Universal Database Version 10.5, 5622-044 (c) Copyright IBM Corp. 1991, 2012
Licensed Material - Program Property of IBM
IBM DATABASE 2 Explain Table Format Tool
Connecting to the Database.
Connect to Database Successful.
Enter up to 26 character Explain timestamp (Default -1) ==>
Enter up to 128 character source name (SOURCE_NAME, Default %%) ==>
Enter source schema (SOURCE_SCHEMA, Default %%) ==>
Enter section number (0 for all, Default 0) ==>
Output is in db2exfmt.out.
Executing Connect Reset -- Connect Reset was Successful.
inst105@db2a:~$ more db2exfmt.out
DB2 Universal Database Version 10.5, 5622-044 (c) Copyright IBM Corp. 1991, 2012
Licensed Material - Program Property of IBM
IBM DATABASE 2 Explain Table Format Tool
******************** EXPLAIN INSTANCE ********************
DB2_VERSION: 10.05.8
FORMATTED ON DB: SAMPLE
SOURCE_NAME: SQLC2K26
SOURCE_SCHEMA: NULLID
SOURCE_VERSION:
EXPLAIN_TIME: 2017-06-30-09.09.31.533874
EXPLAIN_REQUESTER: INST105
Database Context:
----------------
Parallelism: None
CPU Speed: 1.298947e-07
Comm Speed: 100
Buffer Pool size: 1000
Sort Heap size: 256
Database Heap size: 1200
Lock List size: 4096
Maximum Lock List: 10
Average Applications: 1
Locks Available: 13107
Package Context:
---------------
SQL Type: Dynamic
Optimization Level: 5
Blocking: Block All Cursors
Isolation Level: Cursor Stability
---------------- STATEMENT 1 SECTION 203 ----------------
QUERYNO: 1
QUERYTAG:
Statement Type: Select
Updatable: No
Deletable: No
Query Degree: 1
Original Statement:
------------------
SELECT
NAME
FROM
T2
WHERE
ID > 15
Optimized Statement:
-------------------
SELECT
Q1.NAME AS "NAME"
FROM
INST105.T2 AS Q1
WHERE
(15 < Q1.ID)
Access Plan:
-----------
Total Cost: 6.81246
Query Degree: 1
Rows
RETURN
( 1)
Cost
I/O
|
1
FETCH
( 2)
6.81246
1
/----+----\
1 3
IXSCAN TABLE: INST105
( 3) T2
0.00553156 Q1
0
|
3
INDEX: INST105
IDX2
Q12. 存储过程/package执行计划
存储过程执行计划是放在包里面的，因此，要首先找到对应的包名，然后使用db2expln查看包的执行计划：
存储过程定义如下：
找到存储过程对应的包：
存储过程定义如下：
inst105@db2a:~$ cat createproce.txt
CREATE OR REPLACE PROCEDURE PROCEDURE01 ()
DYNAMIC RESULT SETS 2
P1: BEGIN
-- Declare cursor
DECLARE cursor1 CURSOR WITH RETURN for
SELECT ID FROM T1;
DECLARE cursor2 CURSOR WITH RETURN for
SELECT NAME FROM T2 WHERE ID > 15;
-- Cursor left open for client application
OPEN cursor1;
OPEN cursor2;
END P1
@inst105@db2a:~$ db2 "select trim (substr (r.routineschema, 1, 10)) as routineschema,trim (substr (r.routinename, 1, 30)) as routinename, r.valid, trim (substr (p.pkgschema, 1, 15)) as pkgschema, trim (substr (p.pkgname, 1, 15)) as pkgname, p.valid from syscat.routines r, syscat.packages p, syscat.procedures a,syscat.routinedep b where b.specificname=r.specificname and r.specificname=a.specificname and r.routinetype = 'P' and b.bname=p.pkgname and a.procname='PROCEDURE01' order by p.create_time desc fetch first 5 rows only"
ROUTINESCHEMA ROUTINENAME VALID PKGSCHEMA PKGNAME VALID
------------- ------------------------------ ----- --------------- --------------- -----
INST105 PROCEDURE01 Y INST105 P1884571328 Y
1 record(s) selected.查看包的执行计划，其中INST105是模式名，因为存储过程中有两条SQL，所以结果里有两个section：
inst105@db2a:~$ db2expln -database SAMPLE -schema INST105 -package P1884571328 -graph -terminal
DB2 Universal Database Version 10.5, 5622-044 (c) Copyright IBM Corp. 1991, 2012
Licensed Material - Program Property of IBM
IBM DB2 Universal Database SQL and XQUERY Explain Tool
DB2 Universal Database Version 10.5, 5622-044 (c) Copyright IBM Corp. 1991, 2012
Licensed Material - Program Property of IBM
IBM DB2 Universal Database SQL and XQUERY Explain Tool
******************** PACKAGE ***************************************
Package Name = "INST105"."P1884571328" Version = ""
Prep Date = 2017/06/30
Prep Time = 08:46:52
Bind Timestamp = 2017-06-30-08.46.52.002217
Isolation Level = Cursor Stability
Blocking = Block Unambiguous Cursors
Query Optimization Class = 5
Partition Parallel = No
Intra-Partition Parallel = No
Reoptimization = No
SQL Path = "SYSIBM", "SYSFUN", "SYSPROC", "SYSIBMADM",
"INST105"
-------------------- SECTION ---------------------------------------
Section = 1
Statement:
DECLARE CURSOR1 CURSOR
WITH RETURN
for
SELECT ID
FROM T1
Section Code Page = 1208
Estimated Cost = 6.812466
Estimated Cardinality = 3.000000
Access Table Name = INST105.T1 ID = 3,5
| #Columns = 1
| Skip Inserted Rows
| May participate in Scan Sharing structures
| Scan may start anywhere and wrap, for completion
| Fast scan, for purposes of scan sharing management
| Scan can be throttled in scan sharing management
| Relation Scan
| | Prefetch: Eligible
| Lock Intents
| | Table: Intent Share
| | Row : Next Key Share
| Sargable Predicate(s)
| | Return Data to Application
| | | #Columns = 1
Return Data Completion
End of section
Optimizer Plan:
Rows
Operator
(ID)
Cost
3
RETURN
( 1)
6.81247
|
3
TBSCAN
( 2)
6.81247
|
3
Table:
INST105
T1
-------------------- SECTION ---------------------------------------
Section = 2
Statement:
DECLARE CURSOR2 CURSOR
WITH RETURN
for
SELECT NAME
FROM T2
WHERE ID > 15
Section Code Page = 1208
Estimated Cost = 6.812640
Estimated Cardinality = 1.000000
Access Table Name = INST105.T2 ID = 3,6
| #Columns = 1
| Skip Inserted Rows
| Evaluate Block/Data Predicates Before Locking Committed Row
| May participate in Scan Sharing structures
| Scan may start anywhere and wrap, for completion
| Fast scan, for purposes of scan sharing management
| Scan can be throttled in scan sharing management
| Relation Scan
| | Prefetch: Eligible
| Lock Intents
| | Table: Intent Share
| | Row : Next Key Share
| Sargable Predicate(s)
| | #Predicates = 1
| | Return Data to Application
| | | #Columns = 1
Return Data Completion
End of section
Optimizer Plan:
Rows
Operator
(ID)
Cost
1
RETURN
( 1)
6.81264
|
1
TBSCAN
( 2)
6.81264
|
3
Table:
INST105
T2 可以看到，package当中，SQL语句 "SELECT NAME FROM T2 WHERE ID > 15"的执行计划并未用到索引扫描，原因是索引是在存储过程创建之后才创建的。
3.查看package cache中的执行计划
如果应用程序的SQL中，使用了邦定变量的方式，必须得从package cache中抓取执行计划a. 刷新 package cache：
db2 flush package cache dynamic
b. 执行SQL语句，并且不要在其他地方执行这条SQL
c. 使用下面的查询找到SQL对应的executable_id
SELECT executable_id,STMT_EXEC_TIME,Total_cpu_time,varchar(stmt_text,400) as stmt_text FROM TABLE(MON_GET_PKG_CACHE_STMT ('D', NULL,NULL,-1)) AS T
d. 根据上一步中的executable_id，运行下面的存储过程:
CALL EXPLAIN_FROM_SECTION(
e. 使用db2exfmt生成执行计划：
db2exfmt -d
示例：
inst105@db2a:~$ db2 "flush package cache dynamic"
DB20000I The SQL command completed successfully.'
inst105@db2a:~$ db2 "SELECT NAME FROM T2 WHERE ID > 15"
NAME
--------------------
qing
song
2 record(s) selected.
inst105@db2a:~$ db2 "SELECT executable_id,STMT_EXEC_TIME,Total_cpu_time,varchar(stmt_text,50) as stmt_text FROM TABLE(MON_GET_PKG_CACHE_STMT ('D', NULL,NULL,-1)) AS T "
EXECUTABLE_ID STMT_EXEC_TIME TOTAL_CPU_TIME STMT_TEXT
------------------------------------------------------------------- -------------------- -------------------- --------------------------------------------------
x'0100000000000000110100000000000000000000020020170630092845270960' 0 0 SELECT executable_id,STMT_EXEC_TIME,Total_cpu_time
SQL0445W Value "SELECT executable_id,STMT_EXEC_TIME,Total_cpu_time,varchar(s"
has been truncated. SQLSTATE=01004
x'0100000000000000100100000000000000000000020020170630092841223134' 0 73 SELECT NAME FROM T2 WHERE ID > 15
2 record(s) selected with 1 warning messages printed.
inst105@db2a:~$ db2 "CALL EXPLAIN_FROM_SECTION(x'0100000000000000100100000000000000000000020020170630092841223134' , 'M', NULL, 0, 'INST105', ?, ?, ?, ?, ? )"
Value of output parameters
--------------------------
Parameter Name : EXPLAIN_SCHEMA
Parameter Value : INST105
Parameter Name : EXPLAIN_REQUESTER
Parameter Value : INST105
Parameter Name : EXPLAIN_TIME
Parameter Value : 2017-06-30-09.29.03.900808
Parameter Name : SOURCE_NAME
Parameter Value : SQLC2K26
Parameter Name : SOURCE_SCHEMA
Parameter Value : NULLID
Parameter Name : SOURCE_VERSION
Parameter Value :
Return Status = 0
inst105@db2a:~$ db2exfmt -d sample -o packagecache.out1
DB2 Universal Database Version 10.5, 5622-044 (c) Copyright IBM Corp. 1991, 2012
Licensed Material - Program Property of IBM
IBM DATABASE 2 Explain Table Format Tool
Connecting to the Database.
Connect to Database Successful.
Enter up to 26 character Explain timestamp (Default -1) ==>
Enter up to 128 character source name (SOURCE_NAME, Default %%) ==>
Enter source schema (SOURCE_SCHEMA, Default %%) ==>
Enter section number (0 for all, Default 0) ==>
Output is in packagecache.out1.
Executing Connect Reset -- Connect Reset was Successful.
inst105@db2a:~$ more packagecache.out1
DB2 Universal Database Version 10.5, 5622-044 (c) Copyright IBM Corp. 1991, 2012
Licensed Material - Program Property of IBM
IBM DATABASE 2 Explain Table Format Tool
******************** EXPLAIN INSTANCE ********************
DB2_VERSION: 10.05.8
FORMATTED ON DB: SAMPLE
SOURCE_NAME: SQLC2K26
SOURCE_SCHEMA: NULLID
SOURCE_VERSION:
EXPLAIN_TIME: 2017-06-30-09.29.03.900808
EXPLAIN_REQUESTER: INST105
Database Context:
----------------
Parallelism: None
CPU Speed: 1.298947e-07
Comm Speed: 100
Buffer Pool size: 1000
Sort Heap size: 256
Database Heap size: 1200
Lock List size: 4096
Maximum Lock List: 10
Average Applications: 1
Locks Available: 13107
Package Context:
---------------
SQL Type: Dynamic
Optimization Level: 5
Blocking: Block All Cursors
Isolation Level: Cursor Stability
---------------- STATEMENT 1 SECTION 201 ----------------
QUERYNO: 0
QUERYTAG: CLP
Statement Type: Select
Updatable: No
Deletable: No
Query Degree: 1
Original Statement:
------------------
SELECT
NAME
FROM
T2
WHERE
ID > 15
Optimized Statement:
-------------------
SELECT
Q1.NAME AS "NAME"
FROM
INST105.T2 AS Q1
WHERE
(15 < Q1.ID)
Explain level: Explain from section
Access Plan:
-----------
Total Cost: 6.81246
Query Degree: 1
Rows
RETURN
( 1)
Cost
I/O
|
1
FETCH
( 2)
6.81246
NA
/----+----\
1 3
IXSCAN TABLE: INST105
( 3) T2
0.00553156 Q1
NA
|
3
INDEX: INST105
IDX2
Q1