Materialized view and table with the same name

问题

I kind of understand materialized views and have worked with them before. Recently a question came up as to why a particular report didn't show latest data, I looked into the issue. Apparently, they had a temp table loaded with crontab earlier and switched to Materialized view later.

When I looked into the database with the below query (name of the table changed):

SELECT * FROM all_objects WHERE object_name = 'TEMP_DATA';

This actually showed 2 objects in the same schema: one table and another materialized view

OWNER   OBJECT_NAME  OBJECT_TYPE        DATA_OBJECT_ID  LAST_DDL_TIME     TIMESTAMP
SCHEMA  TEMP_DATA    TABLE                      110623  08/06/2013 15:38  2013-08-06:14:53:01
SCHEMA  TEMP_DATA    MATERIALIZED VIEW                  10/30/2015 00:00  2013-08-06:14:56:33

And, when I try to alter the table to rename it, it said materialized view cannot be renamed.

My question is, whether a materialized view actually creates a table with the same name and if so, when I do SELECT where does the data come from (table or MView)?

Or is it just the leftover table from earlier times? If so, does Oracle allow different type of objects with the same name? (I was really stumped by this, as I thought every object had to have a unique name!).

And just curious, if they are 2 objects, which one is used in the below SQL:

SELECT * FROM TEMP_DATA;

Any insight into it, much appreciated.

UPDATE based on @Alex and @Husqvik's responses: In the Mview definition, I see below:

BUILD IMMEDIATE  
REFRESH COMPLETE  
START WITH TO_DATE('06-Nov-2015','dd-mon-yyyy')  
NEXT trunc(sysdate) + 1  
WITH PRIMARY KEY

Does that mean it should update everyday (here tomorrow)? Will the START WITH change to 07-Nov after tomorrow's refresh?

And, does it automatically refresh and if so, when does it do the refresh?

Is there anything that would prevent it from refreshing, because the user complained they don't see the latest data in a report using this MView and that's why I got this to look at this in the first place?

Here is the full DDL for the MView:

DROP MATERIALIZED VIEW SCHEMA.TEMP_DATA;
CREATE MATERIALIZED VIEW SCHEMA.TEMP_DATA 
TABLESPACE ITS_DATASPACE
PCTUSED    0
PCTFREE    10
INITRANS   2
MAXTRANS   255
STORAGE    (
            INITIAL          64K
            NEXT             1M
            MINEXTENTS       1
            MAXEXTENTS       UNLIMITED
            PCTINCREASE      0
            BUFFER_POOL      DEFAULT
           )
NOCACHE
LOGGING
NOCOMPRESS
NOPARALLEL
BUILD IMMEDIATE
REFRESH COMPLETE
START WITH TO_DATE('06-Nov-2015','dd-mon-yyyy')
NEXT trunc(sysdate) + 1
WITH PRIMARY KEY
AS 
/* Formatted on 2015/11/05 09:35 (Formatter Plus v4.8.8) */
SELECT *
  FROM SCHEMA.h_case_data
 WHERE status LIKE 'M%';

COMMENT ON MATERIALIZED VIEW SCHEMA.TEMP_DATA IS 'snapshot table for snapshot SCHEMA.TEMP_DATA';

CREATE INDEX SCHEMA.CASE_ID_IDX ON SCHEMA.TEMP_DATA
(CASE_ID)
LOGGING
TABLESPACE ITS_DATASPACE
PCTFREE    10
INITRANS   2
MAXTRANS   255
STORAGE    (
            INITIAL          64K
            NEXT             1M
            MINEXTENTS       1
            MAXEXTENTS       UNLIMITED
            PCTINCREASE      0
            BUFFER_POOL      DEFAULT
           )
NOPARALLEL;

CREATE INDEX SCHEMA.STATUS_IDX ON SCHEMA.TEMP_DATA
(STATUS)
LOGGING
TABLESPACE ITS_DATASPACE
PCTFREE    10
INITRANS   2
MAXTRANS   255
STORAGE    (
            INITIAL          64K
            NEXT             1M
            MINEXTENTS       1
            MAXEXTENTS       UNLIMITED
            PCTINCREASE      0
            BUFFER_POOL      DEFAULT
           )
NOPARALLEL;

回答1:

From the documentation:

When you create a materialized view, Oracle Database creates one internal table and at least one index, and may create one view, all in the schema of the materialized view. Oracle Database uses these objects to maintain the materialized view data.

So having the table and materialized view with the same name is normal. The MV needs to store the data somewhere, so having a table makes sense; the MV itself then defines how the table data is maintained.

You can use the ON PREBUILT TABLE clause to create a view over an existing table, which I assume is what "they had a temp table earlier ... and switched to Materialized view later" refers to.

You can also go the other way, with the DROP MATERIALIZED VIEW ... PRESERVE TABLE option, which leaves the underlying table behind.

When you SELECT * FROM TEMP_DATA; you're querying the underlying table, but the distinction isn't really important as they refer to the same combined object.

Based on the definition to added to the question later, it will refresh every day at midnight.

回答2:

There are two schema objects. Physically the materialized view is a table. The materialized view objects contains the metadata about the refresh mode, type, method and other properties. If you check the ALL_OBJECTS you see that MATERIALIZED VIEW object doesn't have its segment. Data dictionary secures that you cannot treat the underlying table as normal table otherwise it can break the consistency between these coupled objects.

You also can create a materialized view on top of a prebuilt table.

UPDATE:

START WITH will update with every refresh. NEXT START is evaluated when view is created or refreshed.

Refresh can fail on any error that can normally occur in the database, e. g, not enough space, lock timeout, changes of the query underlying objects.

来源：https://stackoverflow.com/questions/33552237/materialized-view-and-table-with-the-same-name

标签

Oracle

materialized-views