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A materialized query table (MQT) is a table whose definition is based upon the result of a query. The data that is contained in an MQT is derived from one or more tables on which the materialized query table definition is based. Summary tables (or automatic summary tables, ASTs), which are familiar to IBM® DB2® Universal Database™ (UDB) for Linux, UNIX®, and Windows® (DB2 UDB) users, are considered to be a specialized type of MQT. The fullselect that is part of the definition of a summary table contains a GROUP BY clause summarizing data from the tables that are referenced in the fullselect.
You can think of an MQT as a kind of materialized view. Both views and MQTs are defined on the basis of a query. The query on which a view is based is run whenever the view is referenced; however, an MQT actually stores the query results as data, and you can work with the data that is in the MQT instead of the data that is in the underlying tables.
Materialized query tables can significantly improve the performance of queries, especially complex queries. If the optimizer determines that a query or part of a query could be resolved using an MQT, the query might be rewritten to take advantage of the MQT.
An MQT can be defined at table creation time as maintained by the system or maintained by the user. The following sections introduce you to these two types of MQTs, as well as summary tables and staging tables. The examples that follow require a connection to the SAMPLE database; if you don’t have the SAMPLE database created on your system, you can create it by entering the db2sampl command from any command prompt.
The data in this type of materialized query table is maintained by the system. When you create this type of MQT, you can specify whether the table data will be a REFRESH IMMEDIATE or REFRESH DEFERRED. The REFRESH keyword lets you specify how the data is to be maintained. DEFERRED means that the data in the table can be refreshed at any time using the REFRESH TABLE statement. Neither REFRESH DEFERRED nor REFRESH IMMEDIATE system-maintained MQTs allow insert, update, or delete operations to be executed against them. However, REFRESH IMMEDIATE system-maintained MQTs are updated with changes made to the underlying tables as a result of insert, update, or delete operations.
Listing 1 shows an example of creating a REFRESH IMMEDIATE system-maintained MQT. The table, which is named EMP, is based on the underlying tables EMPLOYEE and DEPARTMENT in the SAMPLE database. Because REFRESH IMMEDIATE MQTs require that at least one unique key from each table referenced in the query appear in the select list, we first define a unique constraint on the EMPNO column in the EMPLOYEE table and on the DEPTNO column in the DEPARTMENT table. The DATA INITIALLY DEFERRED clause simply means that data will not be inserted into the table as part of the CREATE TABLE statement. After being created, the MQT is in check pending state (see Demystifying table and table space states), and cannot be queried until the SET INTEGRITY statement has been executed against it. The IMMEDIATE CHECKED clause specifies that the data is to be checked against the MQT's defining query and refreshed; the NOT INCREMENTAL clause specifies that integrity checking is to be done on the whole table. A query executed against the EMP materialized query table shows that it is now fully populated with data.
Listing 1. Creating an MQT that is to be maintained by the system
connect to sample ... alter table employee add unique (empno) alter table department add unique (deptno) create table emp as (select e.empno, e.firstnme, e.lastname, e.phoneno, d.deptno, substr(d.deptname, 1, 12) as department, d.mgrno from employee e, department d where e.workdept = d.deptno) data initially deferred refresh immediate set integrity for emp immediate checked not incremental select * from emp EMPNO FIRSTNME LASTNAME PHONENO DEPTNO DEPARTMENT MGRNO ------ ------------ --------------- ------- ------ ------------ ------ 000010 CHRISTINE HAAS 3978 A00 SPIFFY COMPU 000010 000020 MICHAEL THOMPSON 3476 B01 PLANNING 000020 000030 SALLY KWAN 4738 C01 INFORMATION 000030 000050 JOHN GEYER 6789 E01 SUPPORT SERV 000050 000060 IRVING STERN 6423 D11 MANUFACTURIN 000060 000070 EVA PULASKI 7831 D21 ADMINISTRATI 000070 000090 EILEEN HENDERSON 5498 E11 OPERATIONS 000090 000100 THEODORE SPENSER 0972 E21 SOFTWARE SUP 000100 000110 VINCENZO LUCCHESSI 3490 A00 SPIFFY COMPU 000010 000120 SEAN O'CONNELL 2167 A00 SPIFFY COMPU 000010 000130 DOLORES QUINTANA 4578 C01 INFORMATION 000030 ... 000340 JASON GOUNOT 5698 E21 SOFTWARE SUP 000100 32 record(s) selected. connect reset |
The data in this type of materialized query table is maintained by the user. Only a REFRESH DEFERRED materialized query table can be defined as MAINTAINED BY USER. The REFRESH TABLE statement (used for system-maintained MQTs) cannot be issued against user-maintained MQTs. User-maintained MQTs do allow insert, update, or delete operations to be executed against them.
Listing 2 shows an example of creating a REFRESH DEFERRED user-maintained MQT. The table, which is named ONTARIO_1995_SALES_TEAM, is based on the underlying tables EMPLOYEE and SALES in the SAMPLE database. Again, the DATA INITIALLY DEFERRED clause means that data will not be inserted into the table as part of the CREATE TABLE statement. After being created, the MQT is in check pending state (see Demystifying table and table space states), and cannot be queried until the SET INTEGRITY statement has been executed against it. The MATERIALIZED QUERY IMMEDIATE UNCHECKED clause specifies that the table is to have integrity checking turned on, but is to be taken out of check pending state without being checked for integrity violations.
Next, to populate the MQT with some data, we will import data that had been exported from the EMPLOYEE and SALES tables. The exporting query matches the defining query for the MQT. Then we will insert another record into the ONTARIO_1995_SALES_TEAM table.
A query executed against the ONTARIO_1995_SALES_TEAM materialized query table shows that it is now fully populated with the imported and inserted data, demonstrating that user-maintained MQTs can indeed be modified directly.
Listing 2. Creating an MQT that is to be maintained by the user
connect to sample
...
create table ontario_1995_sales_team as (select distinct e.empno, e.firstnme,
e.lastname, e.workdept, e.phoneno, 'Ontario' as region,
year(s.sales_date) as year from employee e, sales s
where e.lastname = s.sales_person and year(s.sales_date) = 1995
and left(s.region, 3) = 'Ont')
data initially deferred refresh deferred maintained by user
set integrity for ontario_1995_sales_team materialized query immediate
unchecked
export to ontario_1995_sales_team.del of del
select distinct e.empno, e.firstnme, e.lastname, e.workdept, e.phoneno,
'Ontario' as region, year(s.sales_date) as year from employee e,
sales s
where e.lastname = s.sales_person and year(s.sales_date) = 1995
and left(s.region, 3) = 'Ont'
...
Number of rows exported: 2
import from ontario_1995_sales_team.del of del insert into
ontario_1995_sales_team
...
Number of rows committed = 2
insert into ontario_1995_sales_team
values ('006900', 'RUSS', 'DYERS', 'D44', '1234', 'Ontario', 1995)
select * from ontario_1995_sales_team
EMPNO FIRSTNME LASTNAME WORKDEPT PHONENO REGION YEAR
------ ------------ --------------- -------- ------- ------- -----------
000110 VINCENZO LUCCHESSI A00 3490 Ontario 1995
000330 WING LEE E21 2103 Ontario 1995
006900 RUSS DYERS D44 1234 Ontario 1995
3 record(s) selected.
connect reset
|
You will recall that a summary table is a specialized type of MQT whose fullselect contains a GROUP BY clause summarizing data from the tables that are referenced in the fullselect. Listing 3 shows a simple example of creating a summary table. The table, which is named SALES_SUMMARY, is based on the underlying table SALES in the SAMPLE database. Once again, the DATA INITIALLY DEFERRED clause means that data will not be inserted into the table as part of the CREATE TABLE statement. The REFRESH DEFERRED clause means that the data in the table can be refreshed at any time using the REFRESH TABLE statement. A query against this MQT right after it was created, but before the REFRESH TABLE statement was issued, returns an error. After the REFRESH TABLE statement executes, the query runs successfully.
A subsequent insert operation into the SALES table, followed by a summary table refresh and a query against the summary table, shows that the change to the underlying table is reflected in the summary table: salesperson Lee's total sales in the Ontario-South region have increased by 100. Similar behavior can be observed in response to update or delete operations against the underlying SALES table.
Listing 3. Creating a summary table
connect to sample
...
create table sales_summary as (select sales_person, region, sum(sales)
as total_sales
from sales group by sales_person, region)
data initially deferred refresh deferred
select * from sales_summary
SALES_PERSON REGION TOTAL_SALES
--------------- --------------- -----------
SQL0668N Operation not allowed for reason code "1" on table
"MELNYK.SALES_SUMMARY". SQLSTATE=57016
refresh table sales_summary
select * from sales_summary
SALES_PERSON REGION TOTAL_SALES
--------------- --------------- -----------
GOUNOT Manitoba 15
GOUNOT Ontario-North 1
GOUNOT Ontario-South 10
GOUNOT Quebec 24
LEE Manitoba 23
LEE Ontario-North 8
LEE Ontario-South 34
LEE Quebec 26
LUCCHESSI Manitoba 3
LUCCHESSI Ontario-South 8
LUCCHESSI Quebec 3
11 record(s) selected.
insert into sales values ('06/28/2005', 'LEE', 'Ontario-South', 100)
refresh table sales_summary
select * from sales_summary
SALES_PERSON REGION TOTAL_SALES
--------------- --------------- -----------
...
LEE Ontario-North 8
LEE Ontario-South 134
LEE Quebec 26
...
11 record(s) selected.
update sales set sales = 50 where sales_date = '06/28/2005' and
sales_person = 'LEE'
and region = 'Ontario-South'
refresh table sales_summary
select * from sales_summary
SALES_PERSON REGION TOTAL_SALES
--------------- --------------- -----------
...
LEE Ontario-North 8
LEE Ontario-South 84
LEE Quebec 26
...
11 record(s) selected.
delete from sales where sales_date = '06/28/2005' and sales_person = 'LEE'
and region = 'Ontario-South'
refresh table sales_summary
select * from sales_summary
SALES_PERSON REGION TOTAL_SALES
--------------- --------------- -----------
...
LEE Ontario-North 8
LEE Ontario-South 34
LEE Quebec 26
...
11 record(s) selected.
connect reset
|
You can incrementally refresh a REFRESH DEFERRED MQT if it has a staging table associated with it. The staging table collects changes that need to be applied to synchronize the MQT with its underlying tables. You can create a staging table using the CREATE TABLE statement; then, when the underlying tables of the MQT are modified, the changes are propagated and immediately appended to the staging table. The idea is to use the staging table to incrementally refresh the MQT, rather than regenerate the MQT from scratch. Incremental maintenance provides significant performance improvement. The staging table is pruned when the refresh operation is complete.
After it is created, a staging table is in a pending (inconsistent) state; it must be brought out of this state before it can start collecting changes to its underlying tables. You can accomplish this by using the SET INTEGRITY statement.
Listing 4 shows an example of using a staging table with a summary table. The summary table, which is named EMP_SUMMARY, is based on the underlying table EMPLOYEE in the SAMPLE database. You'll recall that the DATA INITIALLY DEFERRED clause means that data will not be inserted into the table as part of the CREATE TABLE statement. The REFRESH DEFERRED clause means that the data in the table can be refreshed at any time using the REFRESH TABLE statement. The staging table, which is named EMP_SUMMARY_S, is associated with the summary table EMP_SUMMARY. The PROPAGATE IMMEDIATE clause specifies that any changes made to the underlying table as part of an insert, update, or delete operation are cascaded to the staging table. SET INTEGRITY statements are issued against both tables to take them out of their pending states.
Not unexpectedly, a query against the summary table at this point returns no data. The REFRESH TABLE statement returns a warning, a reminder that the "integrity of non-incremental data remains unverified." This, too, is not unexpected. Another query against the summary table returns no data as well. However, after we insert a new row of data into the underlying EMPLOYEE table, a query against the staging table EMP_SUMMARY_S returns one row, corresponding to the data that was just inserted. The staging table has the same three columns that its underlying summary table has, plus two additional columns that are used by the system: GLOBALTRANSID (the global transaction ID for each propagated row) and GLOBALTRANSTIME (the timestamp of the transaction). Another query against the summary table returns no data, but after the REFRESH TABLE statement executes this time, the query runs successfully.
Listing 4. Using a staging table with a summary table
connect to sample
...
create table emp_summary as (select workdept, job, count(*) as count
from employee group by workdept, job)
data initially deferred refresh deferred
create table emp_summary_s for emp_summary propagate immediate
set integrity for emp_summary materialized query immediate unchecked
set integrity for emp_summary_s staging immediate unchecked
select * from emp_summary
WORKDEPT JOB COUNT
-------- -------- -----------
0 record(s) selected.
refresh table emp_summary
SQL1594W Integrity of non-incremental data remains unverified by the
database manager. SQLSTATE=01636
select * from emp_summary
WORKDEPT JOB COUNT
-------- -------- -----------
0 record(s) selected.
insert into employee
values ('006900', 'RUSS', 'L', 'DYERS', 'D44', '1234', '1960-05-05',
'FIELDREP', 5, 'M', '1940-04-02', 10000, 100, 1000)
select * from emp_summary_s
WORKDEPT JOB COUNT GLOBALTRANSID GLOBALTRANSTIME
-------- -------- ----------- -------------------... -----------------------------...
D44 FIELDREP 1 x'00000000000000CD' x'20050822201344536158000000'
1 record(s) selected.
select * from emp_summary
WORKDEPT JOB COUNT
-------- -------- -----------
0 record(s) selected.
refresh table emp_summary
SQL1594W Integrity of non-incremental data remains unverified by the database
manager. SQLSTATE=01636
select * from emp_summary
WORKDEPT JOB COUNT
-------- -------- -----------
D44 FIELDREP 1
1 record(s) selected.
connect reset
|
The SYSCAT.TABDEP system catalog view contains a row for every dependency that a materialized query table has on some other object. You can query this view to obtain a dependency summary for the MQTs that we have created (Listing 5). MQTs have a DTYPE value of 'S.' The TABNAME column lists the names of the MQTs, and the BNAME column lists the names of the database objects on which the corresponding MQTs depend. The BTYPE column identifies the object type: 'T' for table, 'I' for index, and 'F' for function instance.
Listing 5. Querying the SYSCAT.TABDEP system catalog view to see MQT dependencies on other database objects
connect to sample ... select substr(tabname,1,24) as tabname, dtype, substr(bname,1,24) as bname, btype from syscat.tabdep where tabschema = 'MELNYK' and dtype = 'S' TABNAME DTYPE BNAME BTYPE ------------------------ ----- ------------------------ ----- EMP S DEPARTMENT T EMP S EMPLOYEE T EMP S SQL050829104058970 I EMP S SQL050829104058800 I EMP_SUMMARY S EMPLOYEE T ONTARIO_1995_SALES_TEAM S LEFT1 F ONTARIO_1995_SALES_TEAM S SALES T ONTARIO_1995_SALES_TEAM S EMPLOYEE T SALES_SUMMARY S SALES T 9 record(s) selected. connect reset |
We have seen that a materialized query table, whose definition is based upon the result of a query, can be thought of as a kind of materialized view. MQTs are important because they can significantly decrease the response time for complex queries. This article has introduced you to the basic concepts around maintained by system MQTs and maintained by user MQTs, as well as summary tables and staging tables, and these concepts were illustrated by working examples that you can run yourself. 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.
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Roman B. Melnyk , Ph.D., is a senior member of the DB2 Information Development team, specializing in database administration, DB2 utilities, and SQL. During more than nine years at IBM, Roman has written numerous DB2 books, articles, and other related materials. Roman coauthored DB2 Version 8: The Official Guide (Prentice Hall Professional Technical Reference, 2003), DB2: The Complete Reference (Osborne/McGraw-Hill, 2001), DB2 Fundamentals Certification for Dummies (Hungry Minds, 2001), and DB2 for Dummies (IDG Books, 2000).
