In DB2, a materialized query table (MQT) is a table whose definition is based on the result of a query. As such, the MQT typically contains precomputed results based on the data existing in the table or tables that its definition is based on. If the query compiler determines that a query will run more efficiently against an MQT than the base table or tables, the query executes against the MQT and you obtain the result faster than you otherwise would. This concept is identical to the materialized view concept in Oracle.
The DB2 materialized query table in relation to the Oracle materialized view
In DB2, the DDL syntax to create an MQT is similar to that of creating an Oracle materialized view. There are syntactical differences though. Let's look at a real-world Oracle materialized view DDL converted to DB2 and compare the sections of the body (see Table 1):
Table 1. A DB2 materialized query table converted from an Oracle materialized view
|DB2 materialized query table (MQT)||Oracle materialized view|
CREATE (1) TABLE (2) PROD.DAILYSUM (3) AS( SELECT A.ORGUNT_SID AS ORGUNT_SID, C.DPTCHRT_SID AS DPTCHRT_SID, C.ANCDPT_SID AS DPT_SID, B.TMFRAM_SID AS TMFRAM_SID, B.STRTDT AS STRTDT, SUM(A.ONSALEFLG) AS ONSALEFLG, COUNT(A.ONSALEFLG) AS ONSALEFLGCNT, SUM(A.SALES) AS SALES, COUNT(A.SALES) AS SALESCNT, SUM(A.SHRINK) AS SHRINK, COUNT(A.SHRINK) AS SHRNKCNT, SUM(A.SHRINKQTY) AS SHRINKQTY, COUNT(A.SHRINKQTY) AS SHRINKQTYCNT, COUNT(*) AS RECORDCNT, FROM PROD.DAYTOT A, PROD.CALDTL B, PROD.DPTCHR C WHERE B.CAL_SID = 100 AND B.TMFRAM_SID <> 10 AND A.DT BETWEEN B.STRTDT AND B.ENDDT AND A.DPT_SID = C.RPTDPT_SID GROUP BY A.ORGUNT_SID, C.DPTCHRT_SID, C.ANCDPT_SID, B.TMFRAM_SID, B.STRTDT) (4) [INTIALLY DEFERRED] (5) REFRESH DEFERRED (6) [MAINTAINED BY SYSTEM] (7) [ENABLE QUERY OPTIMIZATION] (8) IN TABLESPACE1 INDEXES IN TABLESPACE2 ;
CREATE (1) MATERIALIZED VIEW (2) PROD.DAILYSUM (8) TABLESPACE TABLESPACE1 (4) BUILD IMMEDIATE (5) REFRESH FAST ON DEMAND (3) AS SELECT A.ORGUNT_SID ORGUNT_SID, C.DPTCHRT_SID DPTCHRT_SID, C.ANCDPT_SID DPT_SID, B.TMFRAM_SID TMFRAM_SID, B.STRTDT STRTDT, SUM(A.ONSALEFLG) ONSALEFLG, COUNT(A.ONSALEFLG) ONSALEFLGCNT, SUM(A.SALES) SALES, COUNT(A.SALES) SALESCNT, SUM(A.SHRINK) SHRINK, COUNT(A.SHRINK) SHRNKCNT, SUM(A.SHRINKQTY) SHRINKQTY, COUNT(A.SHRINKQTY) SHRINKQTYCNT, COUNT(*) RECORDCNT FROM PROD.DAYTOT A, PROD.CALDTL B, PROD.DPTCHR C WHERE B.CAL_SID = 100 AND B.TMFRAM_SID != 10 AND A.DT BETWEEN B.STRTDT AND B.ENDDT AND A.DPT_SID = C.RPTDPT_SID GROUP BY A.ORGUNT_SID, C.DPTCHRT_SID, C.ANCDPT_SID, B.TMFRAM_SID, B.STRTDT ;
CREATE TABLEsyntax rather than
CREATE MATERIALIZED VIEWsyntax that Oracle uses. DB2 calls MQTs "tables" rather than "views," thus it uses the
CREATE TABLEsyntax when creating these. DB2 knows this is a materialized query table rather than a regular table because, first, MQTs are always created from another table or set of tables and, second, the MQT creation DDL did not say at the end of it "
FOR DEFINITION ONLY". If you want to create a regular (non-MQT) table, as defined from another table, then use the
FOR DEFINITION ONLYkeywords.
DB2 then allows you to define the MQT using whatever schema qualifier and name you desire. As in Oracle, the
ASprecedes the entire
SELECTclause that creates the MQT.
DB2 uses the
SELECTclause to define the body of the MQT. Selecting subsets, full selects, and even joins of tables is permissible. This is identical in concept to Oracle. Note: There are many exceptions and caveats as to what can and cannot go into a DB2 MQT (just as in Oracle), although this article does not go into detail of these things. You can get clear rules of MQT creation in the DB2 documentation.
DATA INTIALLY DEFERRED
In DB2, the default is to immediately place data into the new MQT upon its creation. If you do not want this to occur, you should use
DATA INITIALLY DEFERREDsubcommand. In Oracle, the
BUILD IMMEDIATEis also the default, but you can say this specifically and is often used by Oracle DBAs. For the example in Table 1, data is to be built immediately upon MQT creation, so you can just leave the
DATA INTIALLY DEFERREDoption out of the MQT DDL. (The option is shown in brackets to indicate it is optional.)
REFRESH [DEFERRED / IMMEDIATE]
DB2 controls the way the MQT is refreshed with the
REFRESH [DEFERRED / IMMEDIATE]options. The example use the
REFRESH DEFERREDoption because the source Oracle materialized view was defined with the
ON DEMANDsubcommand, which means the same thing.
REFRESH DEFERREDin DB2 just means that you have to use a
REFRESH TABLEstatement in order to get the MQT have the latest changes to the data applied to it. (See the "
REFRESH TABLEcommand" section for more information.)
REFRESH IMMEDIATEmeans that DB2 changes the MQT every time the base table or tables change and does not require the use of the
Note: In Oracle, there is a
FASTkeyword, which is shown in the example. This particular feature uses special logs to accomplish a "delta" refresh, which can speed this process up. In order to get this "fast" refresh in DB2, you can use a staging table. (See the "Staging tables" section for more information.)
MAINTAINED BY [SYSTEM / USER / FEDERATED TOOL]
DB2 MQTs can be "maintained" three different ways:
MAINTAINED BY SYSTEMis the default, so it did not really have to be put in this output DDL example. It is shown in brackets to indicate that it is optional in this situation. This just means that only DB2 controls what is in the MQT by doing the
REFRESHoptions (which are either
MAINTAINED BY USERallows the DBA to perform direct
DELETESto the MQT
MAINTAINED BY FEDERATED TOOLindicates that the DB2 replication tool maintains the MQT
ENABLE QUERY OPTIMIZATION
DB2 can now use the MQT in its query optimization steps by using
ENABLE QUERY OPTIMIZATION. This is the default behavior and is shown in brackets to indicate that it was optional in this situation. There are some circumstances where you would
DISABLE QUERY OPTIMIZATION, but this article does not cover that.
IN [TABLE SPACE NAME] INDEXES IN [TABLE SPACE NAME]
Finally, DB2 places the MQT data and the MQT indexes in their designated table spaces. Oracle DBAs should note that in DB2, whether it is a base table or an MQT, you always define which table spaces the table data, indexes, and long objects (like lobs or XML) are assigned during table creation time.
REFRESH TABLE command
For refresh deferred MQTs, you have to tell DB2 when and how you want them
to be refreshed because DB2 does not do this automatically. You can
specify this with the
REFRESH TABLE command.
The details of this command are shown in Listing 1, below:
REFRESH TABLE command
.-,-------------------------------------------------------------. V | >>-REFRESH TABLE----table-name--| online-options |--| query-optimization-options |-+--> >--+-----------------+----------------------------------------->< +-INCREMENTAL-----+ '-NOT INCREMENTAL-' online-options: .-ALLOW NO ACCESS----. |--+--------------------+---------------------------------------| +-ALLOW READ ACCESS--+ '-ALLOW WRITE ACCESS-' query-optimization-options: |--+---------------------------------------------------------------------------------+--| | .-ALLOW QUERY OPTIMIZATION-. .-WITH REFRESH AGE ANY-. | '-+--------------------------+--USING REFRESH DEFERRED TABLES--+----------------------+-'
INCREMENTALspecifies an incremental refresh for the table by considering only the delta portion (if any) of its underlying tables or the content of an associated staging table (if one exists and its contents are consistent). If such a request cannot be satisfied (that is, the system detects that the materialized query table definition needs to be fully recomputed), an error (SQLSTATE 55019) is returned
NON INCREMENTALdoes a full refresh on the MQT
ALLOW [NO ACCESS /
READ ACCESS /
These keywords indicate the degree to which others can use the MQT during its refresh:
ALLOW NO ACCESSis the default and provides the fastest refresh of the MQT. Its description is self explanatory
ALLOW READ ACCESSis the next fastest refresh of the MQT. You can read the MQT during the refresh itself, but you cannot do a user-maintained DML operation against the MQT
ALLOW WRITE ACCESSis the slowest refresh. It allows complete DML against an MQT during its refresh
How to do DBMS_MVIEW.REFRESH in DB2
REFRESH TABLE statement is similar to
using the Oracle package
REFRESH as illustrated below.
A full refresh example:
REFRESH TABLE PROD.DAILYSUM NON INCREMENTAL;
An incremental refresh example:
REFRESH TABLE PROD.DAILYSUM INCREMENTAL;
AGE register for DEFERRED MQTs
CURRENT REFRESH AGE register specifies a
timestamp duration value with a data type of DECIMAL(20,6). It is the
maximum duration since a particular timestamped event occurred to a cached
data object (for example, a
statement processed on a system-maintained refresh deferred materialized
query table), such that the cached data object can be used to optimize the
processing of a query. If
CURRENT REFRESH AGE
has a value of 99 999 999 999 999, and the query optimization class is 5
or more, the types of tables specified in
CURRENT MAINTAINED TABLE TYPES FOR OPTIMIZATION
are considered when optimizing the processing of a dynamic SQL query.
The value of
CURRENT REFRESH AGE must be 0 or 99
999 999 999 999. The initial value is 0. You can invoke the
SET CURRENT REFRESH AGE statement to change the
When you define the MQT, you can define a second table that will be the
staging table for that MQT. A staging table allows incremental maintenance
support for the deferred MQT. The staging table collects changes that need
to be applied to the MQT to synchronize it with the contents of underlying
tables. Using staging tables eliminates the high lock contention caused by
immediate maintenance content when an immediate refresh of the MQT is
requested. Also, the MQTs no longer need to be entirely regenerated
REFRESH TABLE is performed.
How MQTs are used by the DB2 optimizer
The Oracle DBA already knows how materialized views work, so let's see how an MQT works in DB2. From a DB2 "beginners" perspective with little experience in using DB2 and its tools, let's look at the Control Center, the Command Editor, as well as the text and visual explain utilities.
Setting up the MQT example
- First, launch the DB2 Control Center. Select Start >
All Programs > IBM DB2 General Administration
Tools > Control Center.
You can also launch the DB2 Control Center using a Windows command prompt. Enter
- Right-click on the SAMPLE database > Query. This opens the Command Editor and connects to the SAMPLE database.
- Copy and paste in the Command Editor’s command window the SQL from
Listing 2. Creating MQT example base tables
--#SET TERMINATOR ! DROP TABLE SHIPPING.BILL ! DROP TABLE SHIPPING.CHARGE_BILL ! CREATE TABLE "SHIPPING"."BILL" ("BILL_GK" INTEGER NOT NULL , "TD_IND" DOUBLE , "CREATION_TS" TIMESTAMP NOT NULL ) ! CREATE UNIQUE INDEX "SHIPPING"."IX_BILL" ON "SHIPPING"."BILL" ("BILL_GK" ASC) ALLOW REVERSE SCANS ! CREATE TABLE "SHIPPING"."CHARGE_BILL" ( "BILL_GK" INTEGER NOT NULL , "BILL_CHARGE_IND" INTEGER NOT NULL , "RATE_PRICE" DOUBLE ) ! ALTER TABLE "SHIPPING"."CHARGE_BILL" ADD CONSTRAINT "PK_CHARGEBILL" PRIMARY KEY ("BILL_GK", "BILL_CHARGE_IND") !
This creates two tables (with appropriate indexing) for you to work out the MQT example. Note that the command
--#SET TERMINATORcan change what the SQL delimiter is in the SQL scripting, in this case an exclamation point (!). This is necessary to do when you are creating stored procedures or UDFs in DB2. You can also set this in the Command Editor at the bottom where it says “Statement termination character”, as shown in Figure 1:
Figure 1. Statement terminator character
- Make sure your cursor is at the very beginning of the SQL in the
Command Editor’s command window and click on the green arrow to run
it. It should say "The SQL command completed successfully" for each
- Copy and paste the SQL from Listing 3, which
inserts data into the tables and collects statistics on them.
- To clear the previous command, right-click anywhere in the Command Editor, then select Select all Use your delete key to clear everything selected.
- To clear the results pane, right-click anywhere in the Command Editor and select Clear results
Run this SQL.
Listing 3. Inserting data into the MQT example base tables
--#SET TERMINATOR ! INSERT INTO SHIPPING.CHARGE_BILL (BILL_GK, BILL_CHARGE_IND, RATE_PRICE) WITH TEMP1 (R1,R2,R3) AS ( VALUES (0, RAND(2), RAND()+(RAND()/1E5) ) UNION ALL SELECT R1+1, RAND(2), RAND()+(RAND()/1E5) FROM TEMP1 WHERE R1 < 25000 ) SELECT R1, R2*23+1, DECIMAL(R3*99999,7,2) FROM TEMP1 ! INSERT INTO SHIPPING.BILL (BILL_GK, TD_IND, CREATION_TS) WITH TEMP1 (R1,R2,R3) AS ( VALUES (0,RAND(), CURRENT TIMESTAMP ) UNION ALL SELECT R1 +1,RAND(),CURRENT TIMESTAMP FROM TEMP1 WHERE R1 < 25000 ) SELECT R1, R2*23+1, CURRENT TIMESTAMP FROM TEMP1 ! RUNSTATS ON TABLE SHIPPING.BILL ON ALL COLUMNS WITH DISTRIBUTION ON ALL COLUMNS AND DETAILED INDEXES ALL ! RUNSTATS ON TABLE SHIPPING.CHARGE_BILL ON ALL COLUMNS WITH DISTRIBUTION ON ALL COLUMNS AND DETAILED INDEXES ALL !
- From the Control Center, you can check out what these tables and the
data looks like. Click on the Tables directory, then on the
column schema to sort by schema. You should be able to find the
two new tables, BILL and CHARGE_BILL, under schema SHIPPING.
Figure 3. How tables look
- Double-click on either table to see the data in it.
Figure 4. Open table "Bill"
- Next, you need to create the explain tables in your schema in order to
work with the DB2 explain facilities. Find the file in this path:
Figure 5. Create explain tables
- Copy the contents of the file EXPLAIN.DDL into the Command Editor and run it. The explain tables are now in your user's schema and ready for use.
- Copy and paste the SQL from Listing 4, and run
Listing 4. Create a stored procedure to test your tables
--#SET TERMINATOR ! CALL SYSPROC.SET_ROUTINE_OPTS('EXPLAIN YES EXPLSNAP YES') ! DROP PROCEDURE SHIPPING.GET_CHARGE_COUNT ! CREATE PROCEDURE SHIPPING.GET_CHARGE_COUNT(OUT v_count INTEGER) LANGUAGE SQL BEGIN SET v_COUNT = (SELECT COUNT(*) FROM ( SELECT DISTINCT B.RATE_PRICE FROM SHIPPING.CHARGE_BILL B, SHIPPING.BILL C WHERE B.BILL_GK = C.BILL_GK ) AS TEMP ); END !
- Notice that this stored procedure is counting all distinct values for
RATE_PRICEthat are in common between the two tables. Test the stored procedure with the following SQL command:
- So, the stored procedure SHIPPING.GET_CHARGE_COUNT is now using your
tables and has an access plan stored in DB2 in what is called a DB2
"package". There are a couple of ways to find out how this access plan
looks in DB2 using the various explain facilities. The first explain
facility is a line command explain tool called
db2exfmt, and the second explain is a visual explain.
Line command explain:
- Enter the following DB2 utility command in a Windows command prompt
anywhere on your C: drive:Note: This is not a Command Editor command, but rather is a DB2 utility executed from the OS.
db2exfmt -D SAMPLE -O MQT_EXFMT_BEFORE.TXT -1
- This DB2 explain utility command produces a Windows text file called
beforemqt_exfmt.txt. You can review the explain output from this file
to see how DB2 is accessing the data in the query.
Figure 6 includes an excerpt from this file's
output, illustrating that the stored procedure is using the two
individual tables from the query in that stored procedure code.
Take note of the very top number in the stored procedure, which is the total timerons, or DB2 cost, for that query. You will reduce it later using an MQT.
Figure 6. Text explain plan output
- Now let's review a visual explain method of seeing the same information as you saw in the text explain output. From the DB2 Control Center, find and expand the directory tree Application Objects. Then find and expand Packages.
- You now see all the packages in DB2 that have bound access plans to
the SAMPLE database. You want to find the one for your stored
procedure you just created. In this case, since it is the only one
that is created by the owner "SHIPPING," you can easily find it. So go
to the only package that is in the "SHIPPING" creator's schema, as
shown in Figure 7, below.
Note: The package name is generated by DB2 when the stored procedure was created, so it will most likely not match the one you have in your database.
- Right-click on this package, and select Show Explainable
Figure 7. Show explain plan
- There is only one explainable statement in this package. Right-click
on it, and select Show Explained Statement History:
Figure 8. Show explain statements history
- There is only one version of this statement in the package.
Right-click on it, and select Show Access Plan:
Figure 9. Show access plan
- This gives you a visual explain output of the SQL statement in the
package for this stored procedure. Notice it tells you the same as the
text output tells you, which is that the base tables are being used
for the stored procedure's SQL statement.
Figure 10. Explain plan graph
How MQTs work: Bringing the example home
So, what does all of this have to do with MQTs? Let's take a look at that now.
- Copy and paste the SQL from Listing 5 into
your Command Editor window, and run it. This creates an MQT for your
two tables, previously shown in the example above.
FROMclause in the MQT DDL. Earlier in this article, you learned what each part of this DDL means. So now you will see it in action.
Listing 5. Creating the MQT
--#SET TERMINATOR ! DROP TABLE SHIPPING.BILL_MQT ! CREATE TABLE SHIPPING.BILL_MQT AS ( SELECT B.RATE_PRICE, COUNT(*) CNT FROM SHIPPING.CHARGE_BILL B, SHIPPING.BILL C WHERE B.BILL_GK = C.BILL_GK GROUP BY B.RATE_PRICE ) DATA INITIALLY DEFERRED REFRESH IMMEDIATE ENABLE QUERY OPTIMIZATION MAINTAINED BY SYSTEM ! REFRESH TABLE SHIPPING.BILL_MQT ! RUNSTATS ON TABLE SHIPPING.BILL_MQT ON ALL COLUMNS WITH DISTRIBUTION ON ALL COLUMNS AND DETAILED INDEXES ALL !
- You now have three tables: two base tables and one MQT for a summary
join of both of them. Figure 11 illustrates how these three tables
look in the Control Center:
Figure 11. MQT
Note that the MQT has a DB2 type of "S," which means it is a summary table. You can double-click on the MQT table to view the data if you like.
- The DB2 optimizer will now use this MQT any time it knows that it can,
rather than use the base tables automatically. Your query does not
have to change to use the MQT.
To prove this, let's simply rebind the package for the stored procedure that you already created. You won't change the SQL in that package, just tell DB2 to give the access path another look now that you have an MQT based on your two tables.
Execute the following command from the Command Editor to first clear out the explain tables:Note: This is an optional step. If you do not do it, DB2 keeps both versions of your access path in your package, but will uses the latest one from the last rebind.
--#SET TERMINATOR ! DELETE FROM EXPLAIN_INSTANCE ! COMMIT !
- Now, let's rebind the package for the stored procedure. Issue the
following command from the Command Editor:
- If you got a 0 return code from this last command, the rebind is successful. DB2 has now changed your SQL in the package to utilize the MQT.
- So, how do you check this all out? You can run the following text
explain utility from a Windows command prompt (not the Command
Editor) and produce another text output file with your new access
db2exfmt -D SAMPLE -O MQT_EXFMT_AFTER.TXT -1
- Figure 12 depicts an excerpt from the output. Notice the MQT is now
being used by the package used by the stored procedure:
Figure 12. MQT used by optimizer
- You can alternatively use the visual explain method shown previously.
(Remember, it is a package in the Application Objects tree in the
Control Center.) Figure 13 shows how the output now looks:
Figure 13. Show explain plan
Note the timeron count again. It should be greatly reduced because DB2 is now utilizing the MQT and not the individual tables. This is, of course, just a small example of what savings you can get in a real world.
The DB2 materialized query table is exactly the same in concept as the Oracle materialized view. In this article, you have learned what the practical differences are between the two and how MQTs work in DB2 so that you can build your new DB2 database with confidence in using DB2 MQTs. To learn more about materialized query tables, or for more detailed information about any of the topics covered in this article, see the DB2 Information Center.
|Sample DB2 scripts to create MQT||DB2MQT.zip||10KB|
- "Use materialized query tables to speed up queries in DB2" (developerWorks, August 2002): Learn how to use MQTs to speed up queries.
- "An introduction to materialized query tables" (developerWorks, September 2005): Gain an understanding of MQTs, summary tables, and staging tables. By way of working examples, see how to get up and running with materialized query tables
- "Maximize the performance of WebSphere Information Integrator with materialized query tables" (developerWorks, May 2006): Learn about how the use of MQTs can help improve performance of federated system.
- "Improve DB2 query performance in a business intelligence environment" (developerWorks, May 2006): Improve performance of queries in a business intelligence environment. Walk through various methods, step-by-step, then experiment on your own system.
- DB2 Information Center: Find the information that you need to use the DB2 family of products and features, as well as related WebSphere Information Integration products and features.
- db2ude: Vikram Khatri's blog: Stay tuned to learn about tips and techniques with DB2 9.
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