My professor whom is teaching a database course asked the following question - currently I have no idea where to start as this seems like an unusual question to ask.
I understand what foreign keys are and how they work, however I am not sure how to answer the below question:
ho and hi are public synonyms for two tables owned by the BLURP schema. Execute one query (even if nested, it can be considered as "one" query) on DD view all_constraints, and discuss whether or not table hi currently satisfies an FK constraint on column hi.olord.
Each column has identical value type CHAR(6) & NOT NULL constraint.
Any help would be appreciated.
First, read the documentation about the all_constraints view.
My guess . . . You professor wants you to query the all_constraints view, and
determine whether there's a referential integrity constraint
from hi.olord
to some column in ho
And, in addition, he expects you to comment on the values you find in other relevant columns. I'd expect you to comment on STATUS and VALIDATED, among others.
If you worked for me, and I were testing you on this, I'd expect you to be able to justify why you included some columns in your query, and why you left others out. You might omit SEARCH_CONDITION, for example, because it applies to CHECK constraints, not to referential integrity constraints. You might include STATUS, because the issue of whether a particular constraint is enabled or disabled is relevant to determining whether "table hi currently satisfies an FK constraint on column hi.olord".
Related
I have one big table A who has PK (C1, C2, C3) and many other columns, to make the select faster, a smaller table B was created with PK (C1, C2). So we can do a select by joining the two tables to find a row in A.
But the problem now is that it can happen that if data is corrupted in B which results in a joint select returns nothing but we still have a row in A.
Am I doing something wrong with this design and how can I ensure the data in those two tables are consistent?
Thanks a lot.
Standard way - if those tables are in a master-detail relationship - is to create a foreign key constraint which will prevent deleting master if details exist.
If you can fix it now, do it - then create the constraint.
If you can't, then create foreign key constraint using INITIALLY DEFERRED DEFERRABLE option so that current values aren't checked, but future DML will be.
Finally, to fetch data although certain rows don't exist any more, use outer join.
"Am I doing something wrong with this design"
Well it's hard to be sure without more details about your scenario but probably you just needed a non-unique index on A(C1, C2).
Although I would like to see some benchmarking which proves an index-range scan on your primary key index was not up to the job. Especially as it seems likely the join on table B is using that access path.
Performance tuning an Oracle database is a matter of understanding and juggling many variables. It's not just a case of "bung on another index". We need to understand what the database is actually doing and why the optimiser made that choice. So, please read this post on asking Oracle tuning questions which will give you some insight into how to approach query optimisation.
I fail to understand the logic of the unique constraint when it's based on 2 fields.
I have the following table named DESCRIPTIONS including 3 columns: ID_DESCRIPTION, NAME, ID_DESCRIPTION_TYPE
Now ID_DESCRIPTION is the primary key, and there is a unique constraint UK_DESCRIPTION on couple (ID_DESCRIPTION, NAME).
If I try to run the following query:
UPDATE DESCRIPTIONS SET NAME = 'USA' WHERE ID_DESCRIPTION = 9255813
I'm getting an ORA-00001 exception, saying that unique constraint UK_DESCRIPTION is violated.
Now this would mean that the couple (9255813,'USA') already exists right ?
However, I don't see how this is possible since the ID_DESCRIPTION is a primary key and therefore unique AND the results of the query
SELECT * FROM DESCRIPTIONS WHERE ID_DESCRIPTION = 9255813
only return 1 result, the one I want to update.
What am I failing to understand here ?
I am going to guess that uk_description is in fact a unique key based on the single column of NAME.
"It is unfortunately not."
Okay, the other explanation is that it is a multi-column key based on a different set of columns from what you think. (NAME, ID_DESCRIPTION_TYPE) would also fit the described behaviour.
To be fair, a unique key on(NAME, ID_DESCRIPTION_TYPE) makes more sense. For example, this is the key you'd want when the table is a single reference data look-up (which is a horrible model but common enough). Whereas a compound key of ID_DESCRIPTION, NAME) would do nothing but undermine the primary key.
I have searched for solution to my problem and this question describes it perfectly.
Let´s say I have one table called ProjectTimeSpan (which I haven´t, just as example!) containing the columns StartDate and EndDate.
And that I have another table called SubProjectTimeSpan, also containing columns called StartDate and EndDate, where I would like to set a Check constraint that makes it impossible to set StartDate and EndDate to values "outside" the ProjectTimeSpan.StartDate to ProjectTimeSpan.EndDate
Kind of a Check constraint that knows about another tables values...
Is this possible?
But I have a hard time to implement the solution to oracle. I've got even more puzzled when other articles stated that check constraint can not relate to other tables.
No it can't.
A FOREIGN KEY constraint can (and must) relate to another table, but it can only perform equiality checks.
I.e. you can test that a column (or a set of columns) are equal to those in the other table but not more complex conditions (like inside a span or whatever).
You'll have to implement a trigger for that.
I have a situation like this:
create table a(
a_id number(38) not null,
constraint pk_a primary key (id)
);
create table b(
a_id number(38) not null
);
create index b_a_id_index on b(a_id);
Now b.a_id is in fact meant to be a foreign key referencing a.a_id, but it isn't formally declared as such. Obviously, it should be for integrity reasons. But does a foreign key constraint also improve join performance in general or in specific cases? If yes, for what types of query transformations?
Is there any relevant documentation about this topic?
I'm using Oracle 11g (11.2.0.2.0)
Yes, having foreign key constraints in place can improve query performance. There are various transforms that are open to the optimizer when appropriate foreign key constraints exist that are not generally available. For example, if you were to join A and B but only select data from B, the optimizer could eliminate A from the query plan entirely if there was a foreign key constraint in place (this sort of thing comes in very handy when you've got useful views that join in more tables than your current query strictly needs because you don't have to trade the performance costs of the extra joins against the code reuse from using an existing view). They also come in handy when you're doing things like using things like query rewrite to rewrite a query to use a materialized view in a data warehouse/ DSS type system.
Tom Kyte has a presentation Metadata Matters that talks about how various types of constraints, along with other pieces of metadata, can influence the optimizer.
As Justin already pointed out, JOIN elimination is an essential non-cost based SQL transformation, which can be applied based on the presence of meta data only. I have blogged about this more recently:
JOIN Elimination: An Essential Optimiser Feature for Advanced SQL Usage
10 Cool SQL Optimisations That do not Depend on the Cost Model
As I originally assumed, there are a lot of SQL transformations that depend on meta data, so adding foreign key constraints (and other constraints) definitely can impact performance in a positive way.
I did not create the database I'm working with, but I would like to see the details of a check constraint.
I know a check constraint on a column is enforcing a set of specific values, and I'd like to know what those values are. For example, if a check constraint is enforcing the character 'Y' and 'N', I want to be able to query the database and see that the accepted values are 'Y' and 'N.'
Is this possible to do through a query?
select constraint_name,search_condition
from all_constraints
where table_name='NAME_OF_YOUR_TABLE'
and constraint_type='C';
Will list the check and the constraint name of all check constraints on a specific table.
Don't forget that the columns in the all_constraints table are case-sensitive. If your select statement returns nothing, that may be why.
(If I had enough rep to comment, on DBA's answer, this would go there.)