The last post about Db2 was about getting it up and running. As the issue we had to solve was about performance, getting an explain plan for the problematic statement(s) was the obvious step to do. In PostgreSQL you can just use EXPLAIN for that, and you’re done. In Db2 the procedure is a bit different, but once you know the tools, it is quite easy as well.
Let’s start by creating a new database:
db2@sles15db2:~> . sqllib/db2profile
db2@sles15db2:~> db2
(c) Copyright IBM Corporation 1993,2007
Command Line Processor for DB2 Client 11.5.6.0
You can issue database manager commands and SQL statements from the command
prompt. For example:
db2 => connect to sample
db2 => bind sample.bnd
For general help, type: ?.
For command help, type: ? command, where command can be
the first few keywords of a database manager command. For example:
? CATALOG DATABASE for help on the CATALOG DATABASE command
? CATALOG for help on all of the CATALOG commands.
To exit db2 interactive mode, type QUIT at the command prompt. Outside
interactive mode, all commands must be prefixed with 'db2'.
To list the current command option settings, type LIST COMMAND OPTIONS.
For more detailed help, refer to the Online Reference Manual.
db2 => create database db1
DB20000I The CREATE DATABASE command completed successfully.
As we need something to explain, we’ll copy the “tables” catalog table without any data:
db2 => connect to db1
Database Connection Information
Database server = DB2/LINUXX8664 11.5.6.0
SQL authorization ID = DB2
Local database alias = DB1
db2 => create table t like syscat.tables
DB20000I The SQL command completed successfully.
db2 => select count(*) from t;
1
-----------
0
1 record(s) selected.
Generate some data:
db2 => insert into t select * from syscat.tables
DB20000I The SQL command completed successfully.
db2 => insert into t select * from syscat.tables
DB20000I The SQL command completed successfully.
db2 => insert into t select * from t
DB20000I The SQL command completed successfully.
db2 => insert into t select * from t
DB20000I The SQL command completed successfully.
db2 => insert into t select * from t
DB20000I The SQL command completed successfully.
db2 => insert into t select * from t
DB20000I The SQL command completed successfully.
db2 => insert into t select * from t
DB20000I The SQL command completed successfully.
db2 => select count(*) from t
1
-----------
14016
1 record(s) selected.
The structure of the table is like this:
db2 => describe table t
Data type Column
Column name schema Data type name Length Scale Nulls
------------------------------- --------- ------------------- ---------- ----- ------
TABSCHEMA SYSIBM VARCHAR 128 0 No
TABNAME SYSIBM VARCHAR 128 0 No
OWNER SYSIBM VARCHAR 128 0 No
OWNERTYPE SYSIBM CHARACTER 1 0 No
TYPE SYSIBM CHARACTER 1 0 No
STATUS SYSIBM CHARACTER 1 0 No
BASE_TABSCHEMA SYSIBM VARCHAR 128 0 Yes
BASE_TABNAME SYSIBM VARCHAR 128 0 Yes
ROWTYPESCHEMA SYSIBM VARCHAR 128 0 Yes
ROWTYPENAME SYSIBM VARCHAR 128 0 Yes
CREATE_TIME SYSIBM TIMESTAMP 10 6 No
ALTER_TIME SYSIBM TIMESTAMP 10 6 No
INVALIDATE_TIME SYSIBM TIMESTAMP 10 6 No
STATS_TIME SYSIBM TIMESTAMP 10 6 Yes
COLCOUNT SYSIBM SMALLINT 2 0 No
TABLEID SYSIBM SMALLINT 2 0 No
TBSPACEID SYSIBM SMALLINT 2 0 No
CARD SYSIBM BIGINT 8 0 No
NPAGES SYSIBM BIGINT 8 0 No
MPAGES SYSIBM BIGINT 8 0 No
FPAGES SYSIBM BIGINT 8 0 No
NPARTITIONS SYSIBM BIGINT 8 0 No
NFILES SYSIBM BIGINT 8 0 No
TABLESIZE SYSIBM BIGINT 8 0 No
OVERFLOW SYSIBM BIGINT 8 0 No
TBSPACE SYSIBM VARCHAR 128 0 Yes
INDEX_TBSPACE SYSIBM VARCHAR 128 0 Yes
LONG_TBSPACE SYSIBM VARCHAR 128 0 Yes
PARENTS SYSIBM SMALLINT 2 0 Yes
CHILDREN SYSIBM SMALLINT 2 0 Yes
SELFREFS SYSIBM SMALLINT 2 0 Yes
KEYCOLUMNS SYSIBM SMALLINT 2 0 Yes
KEYINDEXID SYSIBM SMALLINT 2 0 Yes
KEYUNIQUE SYSIBM SMALLINT 2 0 No
CHECKCOUNT SYSIBM SMALLINT 2 0 No
DATACAPTURE SYSIBM CHARACTER 1 0 No
CONST_CHECKED SYSIBM CHARACTER 32 0 No
PMAP_ID SYSIBM SMALLINT 2 0 Yes
PARTITION_MODE SYSIBM CHARACTER 1 0 No
LOG_ATTRIBUTE SYSIBM CHARACTER 1 0 No
PCTFREE SYSIBM SMALLINT 2 0 No
APPEND_MODE SYSIBM CHARACTER 1 0 No
REFRESH SYSIBM CHARACTER 1 0 No
REFRESH_TIME SYSIBM TIMESTAMP 10 6 Yes
LOCKSIZE SYSIBM CHARACTER 1 0 No
VOLATILE SYSIBM CHARACTER 1 0 No
ROW_FORMAT SYSIBM CHARACTER 1 0 No
PROPERTY SYSIBM VARCHAR 32 0 No
STATISTICS_PROFILE SYSIBM CLOB 10485760 0 Yes
COMPRESSION SYSIBM CHARACTER 1 0 No
ROWCOMPMODE SYSIBM CHARACTER 1 0 No
ACCESS_MODE SYSIBM CHARACTER 1 0 No
CLUSTERED SYSIBM CHARACTER 1 0 Yes
ACTIVE_BLOCKS SYSIBM BIGINT 8 0 No
DROPRULE SYSIBM CHARACTER 1 0 No
MAXFREESPACESEARCH SYSIBM SMALLINT 2 0 No
AVGCOMPRESSEDROWSIZE SYSIBM SMALLINT 2 0 No
AVGROWCOMPRESSIONRATIO SYSIBM REAL 4 0 No
AVGROWSIZE SYSIBM SMALLINT 2 0 No
PCTROWSCOMPRESSED SYSIBM REAL 4 0 No
LOGINDEXBUILD SYSIBM VARCHAR 3 0 Yes
CODEPAGE SYSIBM SMALLINT 2 0 No
COLLATIONSCHEMA SYSIBM VARCHAR 128 0 No
COLLATIONNAME SYSIBM VARCHAR 128 0 Yes
COLLATIONSCHEMA_ORDERBY SYSIBM VARCHAR 128 0 No
COLLATIONNAME_ORDERBY SYSIBM VARCHAR 128 0 Yes
ENCODING_SCHEME SYSIBM CHARACTER 1 0 No
PCTPAGESSAVED SYSIBM SMALLINT 2 0 No
LAST_REGEN_TIME SYSIBM TIMESTAMP 10 6 Yes
SECPOLICYID SYSIBM INTEGER 4 0 No
PROTECTIONGRANULARITY SYSIBM CHARACTER 1 0 No
AUDITPOLICYID SYSIBM INTEGER 4 0 Yes
AUDITPOLICYNAME SYSIBM VARCHAR 128 0 Yes
AUDITEXCEPTIONENABLED SYSIBM CHARACTER 1 0 No
DEFINER SYSIBM VARCHAR 128 0 No
ONCOMMIT SYSIBM CHARACTER 1 0 No
LOGGED SYSIBM CHARACTER 1 0 No
ONROLLBACK SYSIBM CHARACTER 1 0 No
LASTUSED SYSIBM DATE 4 0 No
CONTROL SYSIBM CHARACTER 1 0 No
TEMPORALTYPE SYSIBM CHARACTER 1 0 No
TABLEORG SYSIBM CHARACTER 1 0 No
EXTENDED_ROW_SIZE SYSIBM CHARACTER 1 0 No
PCTEXTENDEDROWS SYSIBM REAL 4 0 No
REMARKS SYSIBM VARCHAR 254 0 Yes
85 record(s) selected.
If we create an index on the “tabname” column and later filter on that column we should get an index access:
db2 => create index i on t(tabname) DB20000I The SQL command completed successfully. db2 => quit DB20000I The QUIT command completed successfully.
You have several options to create explain plans in Db2 and one of the options is to use the CURRENT EXPLAIN MODE special register, so let’s try that:
db2@sles15db2:~> echo "select count(*) from t where tabname='t';" > 1.sql db2@sles15db2:~> db2 set current explain mode explain DB20000I The SQL command completed successfully. db2@sles15db2:~> db2 -tvf 1.sql select count(*) from t where tabname='t' SQL0219N The required Explain table "DB2.EXPLAIN_INSTANCE" does not exist. SQLSTATE=42704
The error message is pretty clear, somehow we need to create the explain tables. There are two options for this and we’ll use the EXPLAIN.DDL script which comes with the Db2 installation:
db2@sles15db2:~> ls -l ./sqllib/misc/EXPLAIN.DDL -r--r--r-- 1 db2 db2 48371 Jun 11 2021 ./sqllib/misc/EXPLAIN.DDL db2@sles15db2:~> grep -i "create table" ./sqllib/misc/EXPLAIN.DDL CREATE TABLE EXPLAIN_INSTANCE ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL, CREATE TABLE EXPLAIN_STATEMENT ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL, CREATE TABLE EXPLAIN_ARGUMENT ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL, CREATE TABLE EXPLAIN_OBJECT ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL, CREATE TABLE EXPLAIN_OPERATOR ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL, CREATE TABLE EXPLAIN_PREDICATE ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL, CREATE TABLE EXPLAIN_STREAM ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL, CREATE TABLE EXPLAIN_DIAGNOSTIC ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL, CREATE TABLE EXPLAIN_DIAGNOSTIC_DATA ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL, CREATE TABLE OBJECT_METRICS ( EXECUTABLE_ID VARCHAR(32 OCTETS) FOR BIT DATA NOT NULL, CREATE TABLE ADVISE_INSTANCE ( CREATE TABLE ADVISE_INDEX( CREATE TABLE ADVISE_WORKLOAD ( CREATE TABLE ADVISE_MQT ( CREATE TABLE ADVISE_PARTITION ( CREATE TABLE ADVISE_TABLE ( CREATE TABLE EXPLAIN_ACTUALS ( EXPLAIN_REQUESTER VARCHAR(128 OCTETS) NOT NULL,
Quite a few tables get created by this script and we’ll not go into the details of which table contains what (this is out of the scope of this post), but just execute it:
db2@sles15db2:~> db2 -tvf ./sqllib/misc/EXPLAIN.DDL ******* IMPORTANT ********** USAGE: db2 -tf EXPLAIN.DDL ******* IMPORTANT ********** UPDATE COMMAND OPTIONS USING C OFF DB20000I The UPDATE COMMAND OPTIONS command completed successfully. ... COMMIT WORK DB20000I The SQL command completed successfully.
All done, lets try again:
db2@sles15db2:~> db2 set current explain mode explain DB20000I The SQL command completed successfully. db2@sles15db2:~> db2 -tvf 1.sql select count(*) from t where tabname='t' SQL0217W The statement was not executed as only Explain information requests are being processed. SQLSTATE=01604
Looks better, but how do we get the explain plan? One option is to use db2exfmt:
db2@sles15db2:~> db2exfmt -d DB1 -# 0 -w -1 -g TIC -n % -s % -o explain.txt DB2 Universal Database Version 11.5, 5622-044 (c) Copyright IBM Corp. 1991, 2017 Licensed Material - Program Property of IBM IBM DATABASE 2 Explain Table Format Tool Connecting to the Database. Connect to Database Successful. Binding package - Bind was Successful Output is in explain.txt. Executing Connect Reset -- Connect Reset was Successful. db2@sles15db2:~> db2 set current explain mode no DB20000I The SQL command completed successfully.
This generates the “explain.txt” file with lots of information. What we are looking for is the explain plan and there is a graphical representation in the file, which is exactly what we need:
Access Plan:
-----------
Total Cost: 6.77696
Query Degree: 1
Rows
RETURN
( 1)
Cost
I/O
|
1
GRPBY
( 2)
6.77689
1
|
35.8378
IXSCAN
( 3)
6.77614
1
|
14016
INDEX: DB2
I
Q1
Another option is to use db2expln:
db2@sles15db2:~> db2expln -database db1 -statement "select count(*) from t where tabname = 't'" -terminal
DB2 Universal Database Version 11.5, 5622-044 (c) Copyright IBM Corp. 1991, 2017
Licensed Material - Program Property of IBM
IBM DB2 Universal Database SQL and XQUERY Explain Tool
DB2 Universal Database Version 11.5, 5622-044 (c) Copyright IBM Corp. 1991, 2017
Licensed Material - Program Property of IBM
IBM DB2 Universal Database SQL and XQUERY Explain Tool
******************** DYNAMIC ***************************************
==================== STATEMENT ==========================================
Isolation Level = Cursor Stability
Blocking = Block Unambiguous Cursors
Query Optimization Class = 5
Partition Parallel = No
Intra-Partition Parallel = No
SQL Path = "SYSIBM", "SYSFUN", "SYSPROC", "SYSIBMADM",
"DB2"
Statement:
select count(*)
from t
where tabname ='t'
Section Code Page = 1208
Estimated Cost = 6.775021
Estimated Cardinality = 1.000000
Access Table Name = DB2.T ID = 2,4
| Index Scan: Name = DB2.I ID = 1
| | Regular Index (Not Clustered)
| | Index Columns:
| | | 1: TABNAME (Ascending)
| #Columns = 0
| Skip Inserted Rows
| Avoid Locking Committed Data
| Currently Committed for Cursor Stability
| #Key Columns = 1
| | Start Key: Inclusive Value
| | | 1: 't'
| | Stop Key: Inclusive Value
| | | 1: 't'
| Index-Only Access
| Index Prefetch: Sequential(1), Readahead
| Lock Intents
| | Table: Intent Share
| | Row : Next Key Share
| Sargable Index Predicate(s)
| | Predicate Aggregation
| | | Column Function(s)
Aggregation Completion
| Column Function(s)
Return Data to Application
| #Columns = 1
End of section
Compared to db2exfmt this gives a much more compact output.
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