I installed Oracle on my system, so now orcl is the SID, which is the unique identifier of my database instance.
Now starter db was created as part of the installation. I created 2 users user1 and user2 using the system account.
Using SQL developer I am accessing the users, this shows me 2 different connections with all the database objects like tables, stored procedures views etc.
so
When using these 2 users, am I accessing the same database? I am giving all the ddl commands by logging into the user1 or user 2, does all this data goes into the same .dbf file?
The database instance can be connected to only one database, then does this essentially mean that everytime I create a new database, to make a database instance to point to that, I need to do a configuration change?
In my experience with Oracle, the typical unit of division is a schema. Schemas in Oracle are used more like you would use databases in SQL Server or PostgreSQL. They represent both users and a logical separation of objects. Physical separation would usually be done using tablespaces. Tablespaces are a group of physical files where data is stored. Schemas can share or use different tablespaces. Having one tablespace per schema is uncommon; they usually share a few tablespaces or often even just one.
With that in mind, to answer your questions more directly,
1) Like in any other database, you can specify the schema the object belongs to:
CREATE TABLE MY_SCHEMA.TABLE_X ( X NUMBER )
If the schemas on two CREATE statements are different, then it will create different objects. What's different in Oracle is that the default schema changes for every user. The default schema is always the currently connected schema/user. So if you omit the schema like so:
CREATE TABLE TABLE_X ( X NUMBER )
then the implied schema is the currently connected schema/user. So if I'm logged in as MY_SCHEMA, then the above is equivalent to the first example. When connecting as two different users, then the implied schema will be different and the DDL is not equivalent between the two users. So running the same statement would create two different objects if you do not specify a schema.
The two objects may be stored in the same physical file if they are in the same tablespace. (They are most likely in the USERS tablespace if you did not create one explicitly and did not specify a different default tablespace when creating the schemas.) Regardless, they are still two completely separate objects.
If you specify the schema explicitly like in the first example, then the DDL is equivalent regardless of who executes it (although permissions may prevent some users from executing it). So it would result in creating the object once, and attempting to create it a second time would result in an error unless you're using CREATE OR REPLACE or something similar.
2) I don't know the answer to this question, but as I said, in Oracle, the basic unit of separation is usually the schema, not a database. I believe the question you're asking is a large part of the reason why the schemas are used in the way they are. Having multiple actual databases on the same machine/instance is far more difficult in Oracle than in other databases (if not impossible), so it's much simpler to have a single database with many schemas.
Related
If I have a schema_search_path set and I wish to create a bunch of tables using a common script by setting the schema and not explicating the
schema in the table create (common script could be used in multiple schemas, this also sets the schema_search_path to just the specified schema.
This seems like an undesirable side affect.
Value set by SET SCHEMA_SEARCH_PATH is not affected by any other commands.
But this value is only used when an identified is not qualified with the schema and an object with this name doesn't exist in the current schema (affected by SET SCHEMA command).
For example, tables referenced by non-qualified names are searched in the following order:
Tables of the current schema.
Local temporary tables. (Currently they also include query aliases from the WITH clauses, but this may be changed when somebody will implement a separate scope of identifiers for these views.)
Tables of each schema from SCHEMA_SEARCH_PATH, if any. When multiple schemas are specified, they order has a meaning, they are processed in the same order.
Legacy or compatibility tables, such as DUAL or SYSDUMMY1 in DB2 and Derby compatibility modes.
The first table matched by its name will be used.
This is a complex case, for the most types of database objects only steps (1) and (3) are performed.
If you think that something is not going as described here and you can create a standalone test case (Java / JDBC / SQL only, no third-party libraries), you can create a bug report on GitHub:
https://github.com/h2database/h2database/issues
I'm totally novice in terms of Oracle DB knowledge. Trying to understand IMPDB command and its scope.
Issue: Suppose there are 500 tables in a particular DB, many of them (60% - 70% or more) are coming as zero records when we're importing the data into a fresh Oracle DB (getting the data from one vendor who has the DB). The doubt is, how can most of the tables be zero records in a DB (why were they created at the first place then?). Also, we're assuming maybe the vendor is using a specific user while generating the .DMP files who has no access to those tables and hence the 0 count. When we asked the vendor, they said, that's not how Oracle works, they've provided user export dump and said, "Schema is a collection of database objects owned by a specific user. Those objects include tables, indexes, views, functions, stored procedures, etc."
When asked about the zero records issue, they said they're pulling correctly and have no understanding as to why so many tables are zero. The SO community has great experts in Oracle DB, can anyone shed some light as to:
What might be the issue?
Is our assumption correct (i.e, that user doesn't have access to those tables which got zero records)?
What's the right way forward?
4) Anything else you want to add.
The vendor is correct - the utility used to generate the export, EXPDP (the compliment to IMPDP) can create a full dump of all of the database objects of a specific user. However, the parameters used to generate the export can vary greatly, and it's absolutely possible for an export to not include table data IF the EXPDP command/parameters used to create the export are specified in that way. For example, let's imagine that someone wants to export a specific schema using the following commmand:
expdp [USER]#[DATABASE] schemas=test directory=DATA_PUMP_DIR dumpfile=test.dmp logfile=test.log query=TEST.TABLE:'"WHERE row_date>sysdate"'
While the export is being generated, all of the rows in that specific table will be evaluated based on the where condition. Unless rows have a date that is in the future, none of the rows dated prior and up to the sysdate will be exported. If a where condition like that is applied to the entire export, you'll have tables with 0 rows in the dump file.
That is just an example - it might also be the case that the tables really have 0 rows. This is possible for a lot of reasons - perhaps it is an older schema with tables that have previously been truncated. Perhaps that particular database isn't used often, and the tables within the schema are empty because rows were never added to the tables. Maybe a developer or another DBA created a bunch of unnecessary tables and they simply were never dropped. It could be a plethora of potential reasons/issues for a schema to have empty tables, and that doesn't mean there is something wrong with the database or the export file being generated. Applications and their technical requirements change all the time, and it's possible that the schema simply wasn't updated when those tables were no longer needed.
The first thing I would recommend is:
Ask the vendor to provide record counts of each table in that schema from their end for validation purposes. This will tell you if the tables are empty in the database. If they are empty in the database, they will be empty in your export. This is very simple and can be achieved with a query like select owner, table_name, num_rows, sample_size, last_analyzed from all_tables where owner=[SCHEMA]; provided that their table statistics are up to date.
If this is a big concern for you, you can always ask them to exclude those tables in the export with a command like:
expdp [USER]#[DATABASE] schemas=test exclude=TABLE:"IN ('Table1', 'Table2')" directory=DATA_PUMP_DIR dumpfile=test.dmp logfile=test.log
Or simply exclude them during your import with a command like:
impdp [USER]#[DATABASE] schemas=test exclude=TABLE:"IN ('Table1', 'Table2')" directory=DATA_PUMP_DIR dumpfile=test.dmp logfile=test.log
Either way should work, but be careful and ensure that there will be no issues from a constraint/child record perspective. You can also exclude the constraints. There are many ways to work around it.
IF THERE ARE INCONSISTENCIES BETWEEN THE COUNTS AND THE ROWS IMPORTED, I would recommend asking the vendor for the specific EXPDP command or parameter file that was used to generate the export. This will let you know if the empty rows are being caused by a clause in the export command.
It's impossible to know if your assumption is correct without knowing more about the database the export is coming from or seeing the the commands being used to generate the export. I would ask the vendor to verify record counts before assuming that it's a permission issue. Empty tables are created all the time.
We are writing some interfacing routines in PL/SQL to transfer data between several oracle database by using another oracle database as host. (ie hr -> host -> finance)
the transfers are happening over db_links
essentially
insert into schema.tablname#dblink1 select * from schema.tablename#dblink2;
(its more complicated then that with multiple tables and transformations etc.. but that's the general idea)
the discussion we have been having here is which of the following should do
reference "schema.tablename#dblink" everywhere in out code
create synonyms (public or private) "create synonym tablename for schema.tablename#dblink"
create views on the object " create view tablename as select * from schema.tablename#dblink"
are there any other options?
are any inherently better then the others?
NB:the dblink names are standardised throughout each level dev/test/prod
so that dblink 'server1' goes to the dev server on the dev host and the test server on test host etc..
none of the table names should ever exist on multiple servers
Location transparency is easiest setup by creating synonyms for your remote objects. That is easier to maintain than having the remote addresses in every SQL. How would you make a quick test for something in an other remote database? Just re creating the involved database links is enough to accomplish that.
An other option could be to create snapshots are materialized views from the remote tables in the local database but that also requires a database link. It would have a good performance at the cost of extra space.
In oracle what's the differences between a database and a table space?
A little terminology:
Oracle defines a database as the set of files that you find on your Oracle system. This means all data in your Oracle system is in these database files, commonly known as "data files". There are other files in the database such as parameter files and redo logs.
On the other hand, an instance consists of the processes and memory areas that Oracle database uses. Together, a database and an instance make up a database system. (For more information, see the Oracle Concept guide)
Logically, you will want to define different spaces within that database. This is done via tablespaces (see Oracle Concept guide). A tablespace usually consists of one or more data files. When you define a table with CREATE TABLE, you can specify in which tablespace the table should be created. This allows you to seperate different applications on the same database system, for example.
The Oracle Concepts guide is an excellent source of information for questions like these. See this picture on how data files and tablespaces are composed.
Let's consider an example of an Ocean consist of lots of water. Now you want that water. For this what you do is collect water in barrel for better usage and better storage purpose. Same here Ocean is database having lots of Data files here data file means water and for better usage and handling you put that into barrel you can relate barrel as Tablespace
An Oracle database consists of one or more logical storage units
called tablespaces, which collectively store all of the database's
data.
Databases, tablespaces, and datafiles are closely related, but they have important differences:
Each tablespace in an Oracle database consists of one or more files
called datafiles, which are physical structures that conform to the
operating system in which Oracle is running.
A database's data is collectively stored in the datafiles that
constitute each tablespace of the database. For example, the simplest
Oracle database would have one tablespace and one datafile. Another
database can have three tablespaces, each consisting of two datafiles
(for a total of six datafiles).
reference link
DATABASES's data are stored in logical storage units called TABLESPACES. A database may contain "one or more" tablespaces. A tablespace may contain one or more datafiles.
A database's data is collectively stored in the datafiles that constitute each tablespace of the database.
Example: the simplest database may have one tablespace and one datafile. On the other hande another database can have 5 tablespaces which may contain two datafiles each (On a total of 10 files)
This question is quite dated. IBM Db2 also has Table spaces. I think there is some commonality between Db2 table spaces and Oracle database table spaces. In that respect this link offers more insight into why table spaces, as storage structures, are needed and how are they distinct from databases. Two benefits are easy recoverability of the database and better storage management.
I trying to take oracle DB backup using expdp. I have a specific case where an application table resides in the SYSTEM tablespace.
The backup export of this schema is successfully created with options SCHEMAS=SYSTEM and INCLUDE=TABLE:"like 'USER%'" which corresponds to my application tables.
I have created another schema with the user impexp which has a different tablespace allocated to it.
when I try to import the .dmp file into impexp, the import is unsuccessful stating "SYSTEM"."USER_SYS_MAST" exists.
Is there a way to import this table in the newly created schema. I also tried using the option REMAP_SCHEMA=SYSTEM:IMPEXP, but it seems to error out saying ORA-39013: Remapping the SYSTEM schema is not supported.
Summarizing : I want to import my application tables in the SYSTEM tablespace into a new TABLESPACE 'IMPEXP'.
Please let me know If I am going wrong somewhere and trying to do something that isn't supported.
any help will be greatly appreciated.
This is one of the reasons why putting application tables in the SYS or SYSTEM schemas is considered bad practice. These schemas are vital to the running of our databases and should not be meddled with.
You have compounded this bloomer by naming your tables with a prefix of USER, which is the same convention the data dictionary uses.
What you need to do is create a new schema to hold these tables. Grant it whatever privileges it needs that made you think it had to be owned by SYSTEM. Then move those tables out of the SYSTEM schema.
To do a proper job you should change your application to use this new schema, but as temporary fix you could give SYSTEM rights on the tables and build synonyms for them. If you have the time, change the application. It will cause you less grief in the long run.
Either way, you will be able to export the data out of the old database and into the target database using this new schema.
Agree with APC.
In your specific case, I would look at DBMS_METADATA.GET_DDL to extract the DDL so I can recreate all the objects in the new schema. There are options to exclude the TABLESPACE component so they would get created in the new schema's default tablespace.
Then I would simply do INSERT /*+APPEND */ INTO newschema.table AS SELECT * FROM SYSTEM.table
If space is an issue, you may need to TRUNCATE or DROP individual tables immediately after they are successfully copied.