I have two tables, TABLE_1 and TABLE_2. Then we have a synonym called TABLE that points to either TABLE_1 or TABLE_2. When TABLE_1 is active, an ETL populates TABLE_2 and when the run is complete, it switches the TABLE synonym to TABLE_2 to make it the active table. Then I have a materialized view that does something like this as the SQL: select * from TABLE. What I am seeing happen is that after the materialize view runs the first time, it caches the actual table the synonym is pointing too. So when the ETL runs and flips the synonym to point at TABLE_2, when a complete refresh is done on the materialized view, it still thinks the synonym is pointed at TABLE_1. Why when I do a complete refresh does the materialized view not pick up the new synonym pointer to TABLE_2?
Here is a workaround to Oracle's bug:
alter materialized view MV_NAME nocache;
BEGIN DBMS_SNAPSHOT.REFRESH( MV_NAME,'C'); end;
alter materialized view MV_NAME cache;
I cannot find anything useful in Oracle documentation but just enable the logic:
I firstly create materialized view
In reality to support this materialized view Oracle makes the TABLE with some specific structure (dependent on your query structure) and the rule to refresh the table (as far as I remember it is called SUMMARY, but the name does not play any role here)
I change synonym target to the table which has another structure
The new target table is not "fitting" to the previous structure (e.g. it has totally different amount/data types of columns), the previous structure is invalid then and not working anymore. The underground table must be recreated then.
That's why I would say it is the only way to prevent the errors: reference the real target instead of synonym for creating materialized view.
So, the answer is: because the materialized view is a static table-like object dependent on the data set it is selecting; that's why to prevent the inconsistencies materialized view references the real object.
Sometimes I really wonder how many details Oracle hides inside.
Related
Good day,
Is it technically possible to create a materialized view against a another view in the master database (as opposed to a solid table)?
My DBA advises that oracle does not allow creation of materialized view logs against a view. IMO a view is pretty much the same as a table, so it ought to be possible.
Has anyone ever done this successfully (Oracle 11g).
The documentation is pretty clear about this:
Restrictions on Master Tables of Materialized View Logs
The following restrictions apply to master tables of materialized view logs:
You cannot create a materialized view log for a temporary table or for a view.
You cannot create a materialized view log for a master table with a virtual column.
A view is not pretty much the same as a table. It's a stored query, so it has no storage and DML is only possible through instead-of triggers. It doesn't have the basis for noticing an underlying data change.
Just thinking about it, in order to support a view log - even for a simple single-table view - an update to the view's base table would have to check if there were any views; check if any of those had materialized view logs; and then work out if the change needed to be logged - which would mean executing each view's query and looking at the entire result set. Imagine doing that for every change to the underlying table. And then imagine a more complicated view query, with aggregates or multiple tables, etc.
You'd effectively be materialising the view query to decide whether was a change that needed to be logged, to update an actual materialized view. This is partly what actual materialized views do - stop you having to repeatedly re-execute an expensive view query by tracking changes on the master table(s).
You have to create the materialized view logs on the (normal) view's base tables.
I have a materialized view that I need to redefine the SQL for. We have an external system that hits the view over a db link, and the monster view takes 5 minutes to refresh the data in the view. The only way I know how to redefine the SQL for a view is to drop it and recreate it, but it would be very bad if the external system couldn't find the table, or it didn't have a complete data set. I need to have as little downtime as possible.
Is there any way to do this natively or more elegantly than:
Create public synonym for materialized view and make everything that uses the view use the synonym instead.
Create new materialized view with new SQL
Change the synonym to point to the new view
Drop the old view.
I've got code to do this dynamically but it is getting really ugly. It seems like there should be a better way to handle this.
Oracle has a build in solution for that. Keep in mind that the mview declaration is separate from that of the table.
The original mview
create materialized view mv1 as select dept , count(*) as cnt from scott.emp;
we want to change the declaration so that only dept over 5 will be calculated
drop materialized view mv1 preserve table;
notice the PRESERVE TABLE clause - the table mv1 is not droped - only the mview layer.
desc mv1
now we create the mview with a different query on top of the existing table
create materialized view mv1 on prebuilt table as
select dept , count(*) as cnt from scott.emp where dept > 5;
notice the on prebuilt table clause. the mview is using the existing object.
exec dbms_mview.refresh_mview('mv1');
So I'm pretty sure Oracle supports this, so I have no idea what I'm doing wrong. This code works:
CREATE MATERIALIZED VIEW MV_Test
NOLOGGING
CACHE
BUILD IMMEDIATE
REFRESH FAST ON COMMIT
AS
SELECT V.* FROM TPM_PROJECTVERSION V;
If I add in a JOIN, it breaks:
CREATE MATERIALIZED VIEW MV_Test
NOLOGGING
CACHE
BUILD IMMEDIATE
REFRESH FAST ON COMMIT
AS
SELECT V.*, P.* FROM TPM_PROJECTVERSION V
INNER JOIN TPM_PROJECT P ON P.PROJECTID = V.PROJECTID
Now I get the error:
ORA-12054: cannot set the ON COMMIT refresh attribute for the materialized view
I've created materialized view logs on both TPM_PROJECT and TPM_PROJECTVERSION. TPM_PROJECT has a primary key of PROJECTID and TPM_PROJECTVERSION has a compound primary key of (PROJECTID,VERSIONID). What's the trick to this? I've been digging through Oracle manuals to no avail. Thanks!
To start with, from the Oracle Database Data Warehousing Guide:
Restrictions on Fast Refresh on Materialized Views with Joins Only
...
Rowids of all the tables in the FROM list must appear in the SELECT
list of the query.
This means that your statement will need to look something like this:
CREATE MATERIALIZED VIEW MV_Test
NOLOGGING
CACHE
BUILD IMMEDIATE
REFRESH FAST ON COMMIT
AS
SELECT V.*, P.*, V.ROWID as V_ROWID, P.ROWID as P_ROWID
FROM TPM_PROJECTVERSION V,
TPM_PROJECT P
WHERE P.PROJECTID = V.PROJECTID
Another key aspect to note is that your materialized view logs must be created as with rowid.
Below is a functional test scenario:
CREATE TABLE foo(foo NUMBER, CONSTRAINT foo_pk PRIMARY KEY(foo));
CREATE MATERIALIZED VIEW LOG ON foo WITH ROWID;
CREATE TABLE bar(foo NUMBER, bar NUMBER, CONSTRAINT bar_pk PRIMARY KEY(foo, bar));
CREATE MATERIALIZED VIEW LOG ON bar WITH ROWID;
CREATE MATERIALIZED VIEW foo_bar
NOLOGGING
CACHE
BUILD IMMEDIATE
REFRESH FAST ON COMMIT AS SELECT foo.foo,
bar.bar,
foo.ROWID AS foo_rowid,
bar.ROWID AS bar_rowid
FROM foo, bar
WHERE foo.foo = bar.foo;
Have you tried it without the ANSI join ?
CREATE MATERIALIZED VIEW MV_Test
NOLOGGING
CACHE
BUILD IMMEDIATE
REFRESH FAST ON COMMIT
AS
SELECT V.*, P.* FROM TPM_PROJECTVERSION V,TPM_PROJECT P
WHERE P.PROJECTID = V.PROJECTID
You will get the error on REFRESH_FAST, if you do not create materialized view logs for the master table(s) the query is referring to. If anyone is not familiar with materialized views or using it for the first time, the better way is to use oracle sqldeveloper and graphically put in the options, and the errors also provide much better sense.
The key checks for FAST REFRESH includes the following:
1) An Oracle materialized view log must be present for each base table.
2) The RowIDs of all the base tables must appear in the SELECT list of the MVIEW query definition.
3) If there are outer joins, unique constraints must be placed on the join columns of the inner table.
No 3 is easy to miss and worth highlighting here
USE THIS CODE
CREATE MATERIALIZED VIEW MV_ptbl_Category2
BUILD IMMEDIATE
REFRESH FORCE
ON COMMIT
AS
SELECT *
FROM ptbl_Category2;
Note- MV_ptbl_Category2 is the Materialized view name
Ptbl is the table name.
Say that you have two Oracle databases, DB_A and DB_B. There is a table named TAB1 in DB_A with a materialized view log, and a materialized view named SNAP_TAB1 in DB_B created with
CREATE SNAPSHOT SNAP_TAB1
REFRESH FAST
AS SELECT * FROM TAB1#DB_A;
Is there a way to query in DB_B the changes made to SNAP_TAB1 after each call to fast-refresh the materialized view ?
DBMS_SNAPSHOT.REFRESH( 'SNAP_TAB1', 'F' );
In DB_A, prior to the refresh, you can query the materialized view log table, MLOG$_TAB1, to see which rows have been changed in TAB1. I'm looking for a way to query in DB_B, after each refresh, which rows have been refreshed in SNAP_TAB1.
Thanks!
I think the lines below work with prebuilt table:
You can add a column in the table SNAP_TAB1.
For inserts You can put it on default sysdate => for every insert you'll have the timestamp of the insert.
For updates you can use a trigger. Because the column is not involved in the Materialized View, updating the column with the trigger won't be a problem.
Probaly better, with the trigger you can use an unique id to store in that column, incremented before every new refresh.(Obtaining the unique id may have different aproaches.)
Obviously, you can't track deletes with this idea.
I have a table xml_documents with two columns: a document_id column (primary key) and an xml column with some XML data, which is a schema-less XMLType. I can create a materialized view with just the document_id with:
create materialized view mv refresh fast on commit as
select document_id
from xml_documents
This works fine, but isn't very useful. As you might expect, I'd like the materialized view to extract data from the XML, and for this I use extractValue(). I am trying the following:
create materialized view mv refresh fast on commit as
select document_id, extractValue(xml, '/my/gaga') gaga
from xml_documents
This fails with:
ORA-12054: cannot set the ON COMMIT refresh attribute for the materialized view
How should I go about to create a fast refresh on commit materialized view that extract values from XML?
Your XMLType is (probably) stored as a CLOB. Find the hidden column with a query like this:
select * from user_tab_cols where table_name = 'XML_DOCUMENTS';
Then create a function to convert a CLOB into an XMLType, and extract the value. Note that the "deterministic" keyword is necessary, although
I'm not sure why. Passing data back and forth between SQL and PL/SQL will be slow, but if you're using a materialized view things are probably
already slow.
create or replace function extract_from_clob(p_xml in clob) return varchar2 deterministic
is
begin
return XMLType(p_xml).extract('/my/gaga/text()').getStringVal();
end;
/
Then drop and create your materialized view with the system column passed into the function:
create materialized view mv refresh fast on commit as
select document_id, extract_from_clob(SYS_NC00003$) gaga
from xml_documents;
I'm unsure about using a system-generated hidden column. It works, but doesn't seem like a really good idea. At the very least it will make it
difficult to create the object on different systems - you'll need to find the new column name each time.
It seems weird that XMLTypes don't work when LOBs work fine. I can't find any documentation about this; I'm not sure if it's a bug, an unimplemented feature, or if there's some magic setting that will make it work. If no one else can provide a better answer, it might be worth checking with Oracle support before you use the above approach.