Questions tagged [databases]
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171
questions
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19 views
Why am I apparently the only one who would VASTLY prefer polishing the existing product instead of adding more features? [closed]
Take PostgreSQL as just one example out of countless. It's more than "mature" now. Yet the upgrade process is a painful ordeal which makes me upgrade once every few years instead of months. It really ...
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1answer
21 views
Need help understanding the tuple relational calculus
We say that a query in a TRC is a statement of the form $\{T: P(T)\}$, where $T$ is a tuple variable and $P(T)$ is a formula.
Now my question is, what are the possible tuples $T$ to be tested i.e., ...
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9 views
Why only one sparse Index possible for any data file and two for dense index?
Why only one sparse Index possible for any data file and two for dense index?
Can some one please explain in intuitive way?
0
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0answers
26 views
How does database indexing improve performance for lookup?
I read that indexing a database will improve its performance by assigning a number to each row to loop through. Even after indexing, to get the data at say row 1024, shouldn't the memory reader go ...
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0answers
16 views
Linking Subtype table to another table in EERD
I made this Enhanced Entity relationship diagram for my database project, but there is a problem that I have been mulling over. Over here, it can be seen that MANAGER is under the supertype of ...
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0answers
48 views
Using relational algebra to express all with a condition
I encountered this question while revising for my finals exam on database theory.
The following database contains information about car repair workshops. The following tables are used:
$$...
1
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1answer
47 views
Database Theory - Does the dependency preservation and lossless join properties hold for every decomposition of a relation into 3NF?
I was reading Database Systems, 6th ed by Ramez Elmasri and Shamkant Navathe. In the textbook they make a claim (in Chapter 15, section 15.2 Properties of Relational Decompositions) which I quote ...
0
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1answer
18 views
DBMS: MINIMUM NUMBER OF TABLES REQUIRED TO REALIZE THIS ER MODEL
I was following a discussion on a forum where they have posted a question like this-->
Some people are saying that we can merge A,B,C in a single table and m:n relation of A and C in another table ...
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0answers
19 views
Consistency CAP vs ACID
I understand that the consistency property specifies what values can be read, depending on what has been written before. For instance, if we consider a simple abstract read/write service. It enjoys ...
0
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0answers
13 views
Does graph based protocol in DBMS suffer from starvation issues?
I know that graph based protocols require prior knowledge of access of data items. Besides when a transaction locks some data item in graph based protocol it also locks some unnecessary data items ...
1
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1answer
26 views
Does basic 2 PL always ensure conflict serializability?
While going through the DBMS book by Korth, it was mentioned :
" For a set of transactions, there may be conflict-serializable schedules that
cannot be obtained through the two-phase locking ...
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0answers
77 views
What is difference between r, and r(R) in DBMS?
Sorry, for asking such a basic question.
I am really confused between small 'r' and 'r(R)' being interchangeably denoted as 'relation' or 'relational instance'. I know, that R is a relational schema. ...
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0answers
19 views
What is the difference between Instance of a relation and Instance of a relation schema?
Following question was asked in the exam. The correct answer to this question is 2. But, I feel that it is a wrong answer. Option 2 is an example of instance of a relation and not an example of ...
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0answers
29 views
How to decide if two conjunctive queries cannot have any result in common
Consider the following queries:
Q1: age > 18 & age < 24 & gender = 'male'
Q2: age > 25 & gender = 'male' & major = 'CS'
Q3: age = 20 & major = 'CS'
Clearly, given any ...
0
votes
1answer
32 views
Prime attribute dependant on another prime attribute allowed under BCNF?
I have been studying DBMS ans I have a question that I got conflicting answers to:
Is prime attribute that is dependant on another prime attribute allowed under BCNF?
Meaning P->P.
I know that NP->P ...
0
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1answer
130 views
Join with Bloom-filters
can you tell me how i can use a bloom-filter by a joining operation with two tables?
let's assume that:
Table A ={A_id,B_id,age}, Table B = {B_id, color}.
now i want to use a bloom-filter so i can ...
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0answers
13 views
Which Window models in stream data processing increasing monotonically?
Good morning,
at the moment I try to understand which of the types of windows are increasing monotonically (not a strict increase)
In Scholl we hat the following windows
Tuple-based Sliding ...
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0answers
45 views
How do i solve this task about the the Reservoir Sampling algorithm?
Hello I have the following task (no homework).
You are using the Reservoir Sampling algorithm. You process the 2000th element in the data stream and are maintaining a
sample of size 500. What is ...
1
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1answer
37 views
In a DBMS what are the implementation details that make set operations faster than cursors?
What are the low level implementation details that make set based operations exponentially faster than iterative processes, like cursors?
At a memory level how does a set operation "touch" the data ...
1
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1answer
20 views
Adding an ordered into a relationship without violating the functional dependencies
Assuming that we have the relationship $R(A,B,C,D)$, the tuple $(W,X,Y,NULL)$ and the following functional dependencies:
$$A \to C$$
$$A \to B,D$$
is it possible to insert a tuple with $NULL$ values,...
1
vote
1answer
47 views
Find candidate keys given functional dependencies
In my database class we practiced how to find the candidate keys, given the functional dependencies. However they were easy examples, based on a single attribute being functionally dependent on a ...
1
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1answer
340 views
How to find highest normal form of a relation, given its functional dependencies
Let's say that I have a relational model, defined as:
$$X(A,B,C,D,E,F,G)$$ with the functional dependencies:
$$f_1: \{A,B,C\} \to \{D,E,F,G\}$$
$$f_2: \{A,B\} \to \{C\}$$
Using this information I ...
0
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0answers
18 views
Need an optimized way to handle combination of entities to improve performance
So, I am working on a feature in a web application. The problem is like this-
I have four different entities. Let's say those are - Item1, ...
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0answers
26 views
Doubt on Lossless or Lossy Decomposition
Consider a relation $R\left ( A,B,C,D,E \right )$ and functional dependencies are
$F=\left \{ AC\rightarrow B,C\rightarrow D,A\rightarrow E,C\rightarrow B \right \}$
Relation $R$ is decomposed into ...
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0answers
14 views
Doubt on the definition of Partial funtional dependency
I have some doubt regarding the definition of partial functional dependency.
According to the book Fundamentals of Database Systems by Elmasri and Navathe
the definition of partial functional ...
2
votes
1answer
242 views
Why do OS and DBMS have their own synchronisation mechanisms, when OS can alone do it and it sits at the bottom of DB?
I know that synchronization is important in the OS, but why do we need a separate topic synchronization and concurrency in the DBMS? The OS concentrates on program synchronization while the DBMS ...
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0answers
101 views
Relational Database to Aggregate Data Model
Can any application currently using a relational database be rewritten to use an Aggregate Data Model, by Focusing on data modelling aspects only (ignoring other features like salability or ...
1
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0answers
50 views
How do I implement fixed block storage file system in database from typical file-level file system?
I've a typical parent ID based file system in database and files are physically stored on disks. What I want is to store files in blocks (4096K) of fixed size.
Chunking can be used I found, but how ...
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0answers
29 views
Understanding non repeatable read anomaly in databases
This paper discusses in detail ANSI isolation levels discussed in ANSI SQL standard on page 84.
Paper introduces following shorthand for describing histories which contain reads, writes, commits and ...
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0answers
82 views
When cascadeless and / or recoverable schedules can contain blind writes?
Some quick definitions / theory:
If instructions $I_i$ and $I_j$ of two different transactions operate on same data item and if any of them is a write operation then they are conflicting ...
1
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0answers
31 views
Understanding connection between different database concurrency control protocol
I have came across following diagram related to timestamp based concurrency control protocols in database systems:
I don't know whether its correct or not.
As per my interpretation, it means all ...
1
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0answers
32 views
Understanding validation based database concurrency control protocol
Database book by Korth explains validation protocol as follows :
To perform the validation test, we need to know when the various phases of transactions took place. We shall, therefore, associate ...
0
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1answer
201 views
Why “ every non-prime attribute of R is non-transitively dependent on every key of R” in the original def of 3NF?
I have read the definition in of Third_normal_form here
, which shows
The third normal form (3NF) is a normal form used in database normalization. 3NF was originally defined by E.F. Codd in 1971.[2]...
1
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0answers
25 views
Why don't RDBMS use a quorum based solution to do failover
For RDBMS viz. MySql, PostgreSql a common solution for scalability and high availability is to run a primary instance as master that is a read-write instance and one or more read replicas. If the ...
0
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1answer
94 views
Doubt regarding foreign key and referential integrity
I am studying database from Database System Concepts.
The author was explaining foreign keys and referential integrity with the help of following schemas:
$$ instructor(\underline{ID},\space name,\...
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3answers
317 views
Functional Dependencies
Let relational schema R(A,B,C,D,E,F,G,H,I).
The functional dependencies are: A -> BC, AD -> BE, C ->F , A->F , D-> E
Find the number of non-trivial functional dependencies ? I know that R has only ...
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0answers
99 views
How are these two definitions of the Third Normal Form (3NF) equivalent?
According to the textbook by Silberschatz,
A relation schema R is in third normal form with respect to a set $F$ of functional dependencies if, for all functional dependencies in $F^+$ of the form $\...
0
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1answer
34 views
Given a relation and asked to enumerate all FD's and mark the trivial FD's
Given a relation R(A,B,C) and a FD set (A->B, B->C), find the FD Closure. Enumerate all the FD's and organize them accordingly to the LHS of the FD's. Mark all the trivial FD's.
I am a little ...
0
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1answer
22 views
Maximum join of relations
I have a question as follows
Consider the relations $r1(P, Q, R)$ and $r2(R, S, T)$ with primary keys $P$ and $R$ respectively. The relation $r1$ contains $2000$ tuples and $r2$ contains $2500$ ...
0
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1answer
424 views
How to decompose into BCNF while preserving all functional dependencies
I have a relation R = {A,B,C} and F={AB -> C , C -> B}.
In order to check if R is in BCNF.
I checked if AB and C are both superkeys and since C is not a superkey I conclude R is not in BCNF.
After ...
1
vote
1answer
154 views
Is my decomposition of R into 3NF is lossless and preserving all dependencies?
I have a schema R = {A,B,C,D,E} and the set F of FDs:
F = {A --> BCDE, CD --> E, EC --> B}.
Candidate key = A
Non - Prime Attributes = BCDE
Prime Attributes = A
According to me, there is no partial ...
0
votes
1answer
44 views
Did Date and Darwen's “Third Manifesto” have a lasting impact?
In 1995, two DB theory researchers, C.J. Date and H. Darwen, published a text with the rather impressive-sounding name "The Third Manifesto" with their ideas regarding future DBMSes, which would ...
-1
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2answers
531 views
Is it possible to do a NULL comparison in Relational Algebra?
Please consider the following question given in a booklet:
I have read about databases from Database System Concepts - by Henry F. Korth, but never saw an instance where we could do a NULL comparison ...
1
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1answer
115 views
Raft algorithm: What's the data format of `term`?
Time is divided into terms, and each term begins with an election. After a successful election, a single leader manages the cluster until the end of the term. Some elections fail, in which case the ...
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0answers
123 views
How to determine conflict serializability without read and writes
I'm learning about conflict serializability in an introduction to databases class. I know how to check if a schema of transactions is conflict serializable when there's reads and writes. Draw the ...
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0answers
35 views
normalization: how does nf work?
I once did normalization for my Computer Science project, and I had to turn 0NF to 1NF, 2NF and 3NF.
What must we actually do to convert 0NF to 1NF, 2NF and 3NF? Must we rearrange the information and ...
1
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0answers
37 views
Merge unstructured with structured data
We have a database with well-formated and structured data. From time to time we get Excel - files from our clients. This Excel Files have to be imported to the database. It it very important that we ...
-2
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1answer
39 views
How to find information quickly in a table
I have a table and asked me to add my information to it. And then asked me to find my information in the easiest way (after adding your details to the table, assume that the table is arranged ...
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0answers
100 views
How is Eventual Consistency (EC) better than Strong Consistency (SC) when N = 3, R=W=2 when SC gives better consistency gurantee in same config?
In dynamo paper, it says that in production most of the users set N = 3, R = 2 and W = 2.
...
0
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1answer
83 views
Where do you exactly use data structures?
I know what data structures is, How it works & importance of it. I just have some doubts on where do we actually use it.
When database can do filtering, fetching, sorting efficiently, Why we ...
