I want to migrate a Tarantool table to a different format. Currently (Tarantool 2.8), this must be done manually, by creating a new table, copying the data over, dropping the old table and renaming the new table to the old name. That also means dropping all foreign keys referencing the old table and creating new ones. But an unrelated limitation is that I can't create foreign keys on tables unless those tables are also empty.
Is there any way to solve this other than just not using foreign keys at all?
EDIT: I suppose I could emulate FKs with triggers. Are there any limitations with triggers that would make such emulation impossible?
Related
I wanted to understand that if we drop a PRIMARY KEY using online then what will be the situation for ongoing transactions. Will the maintain data integrity? ideally not cause we have dropped PK. But what could be the real scenario from dropping constraints using online or how we can prevent data conflicts while doing so.
In Oracle, you can keep index but drop Primary Key. In this way you can prevent data conflicts.
ALTER TABLE TBL DROP PRIMARY KEY KEEP INDEX
after that you can add new primary key or unique constraint (whatever it is) and drop former unique constraint
I'm beginer and I'm working on oracle 12c database so, In my database project I want to apply cascade on delete and on update simultaneously as i did in mysql but when i apply tha same technique in oracle it show me the error so how can i do that?
There is no ON UPDATE CASCADE on Oracle. While you can probably argue updating a table's primary key is valid in SQL, you probably should not, hence the decision of Oracle not to implement it.
More info here:
https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:5773459616034
EDIT: As discussed in comments below, think of that constraint as a way Oracle prevents people from doing something wrong (updating primary keys).
The correct way to handle the case of a primary key that might be updated is to create a separate field that will act as the surrogate primary key. The surrogate key, of course, is immutable.
The danger of using a natural key as primary key is discussed there.
I have created database tables manually and i have also created foreign key constraints in tables manually without using migration. now when i am going to delete a record it giving me following error-
Integrity constraint violation: Cannot delete or update a parent row:
a foreign key constraint fails
Note: i can't use migration because database was already created.
Use ON DELETE CASCADE and ON UPDATE CASCADE on foreign keys, like:
...(create/alter Children table query)...
CONSTRAINT FK_ParentChild
FOREIGN KEY (parent_id) REFERENCES Parents(id)
ON UPDATE CASCADE ON DELETE CASCADE;
So you don't have to manually delete children before deleting the parent element. All children records will automatically delete along with the deletion of parent record.
I would suggest investigating this further by doing the follow:
On your development environment, write migrations to create the
tables from scratch 2)
Look at the generated schema and compare it
to the schema of your existing production tables
Note the
differences between the two schema and write migrations to correct
the production tables.
Note:
If you need to correct the data in the meantime and you are confident that the data integrity of your database is not at risk. You can use the following statements to drop the foreign key checks whilst you correct the data.
SET FOREIGN_KEY_CHECKS = 0;
SET FOREIGN_KEY_CHECKS = 1;
I can't stress enough, remember to turn on your foreign key checks once you are finished and do not do this on a production database. Take a copy of the database and try things locally until you are confident that your corrections are 100% safe ;)
The diagram has over 40 tables, most of them have a primary key defined.
For some reason there is this one table, which has a primary key defined, but that's being ignored when I export the model to a DDL script.
This is the "offending" key (even though it's checked it is nowhere to be found on the generated DDL script):
Has anybody had the same problem? Any ideas on how to solve it?
[EDIT] This is where the key is defined:
And this is the DDL preview (yes, the primary key shows up there):
This is what happens if I try to generate the DDL for just that table (primary key still not generated):
I was finally able to identify and reproduce the problem.
It was a simple conflict of constraints.
Table MIEMBROS had a mandatory 1 to n relationship (foreign key) from another table on its primary key column and vice-versa (there was a foreign key on MIEMBROS against the other table's primary key).
This kind of relationship between two tables makes it impossible to add a record to any of them: The insert operation will return an error complaining about the foreign key restriction pointing the other table.
Anyway I realized that one of the relationships was 0 to n so I simply unchecked the "mandatory" checkbox on the foreign key definition and everything went fine.
So, in a nutshell: The Data Modeler "fails" silently if you are defining a mutual relationship (two foreign keys, one on each table against the other table) on non nullable unique columns, by not generating the primary key of one of the tables.
Such an odd behavior, if you ask me!
"This kind of relationship between two tables makes it impossible to add a record to any of them: The insert operation will return an error complaining about the foreign key restriction pointing the other table."
Actually, if you have deferred constraints, this is not impossible. The constraints can be enforced, for example, at commit time rather than immediately at insert time.
From the Data Modeler menu under File, I used Export -> DDL File. The keys appeared in the DDL, then when I went back to the diagram and did DDL Preview, it showed all the missing stuff.
I'm currenly developing on Oracle. I have several tables for which I defined FOREIGN KEY constraints. I have already read this SQL Server-oriented and this MySQL-oriented questions but I could find none about Oracle.
So the question is always the same: in order to optimize query performance, for those columns for which I create a FOREIGN KEY constraint, do I also have to create an explicit secondary index? Doesn't Oracle automatically create an index on FOREIGN KEYed columns to boost performances during JOINs?
I usually perform queries in which the WHERE clause compare against those columns.
No, Oracle doesn't automatically create indexes on foreign key columns, even though in 99% of cases you probably should. Apart from helping with queries, the index also improves the performance of delete statements on the parent table.