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Questions tagged [databases]

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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 ...
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1answer
39 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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23 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 ...
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27 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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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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22 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 ...
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0answers
13 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 ...
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0answers
23 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 ...
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1answer
54 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]...
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21 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 ...
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1answer
36 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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view serialisable and conflict serialisable

I recently was solving some problems on these topic and realised that in cases where conflict serialisable schedule existed; the number of view serialisable schedule was actually equal to the number ...
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31 views

When BCNF decomposition preserves Functional Dependencies?

It is known that BCNF decomposition does not preserve functional dependencies (FDs). For example, for FDs on R(A,B,C), ...
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3answers
58 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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71 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 $\...
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67 views

Any Recoverable schedule which is not a Cascadeless schedule?

When I read about Cascadeless schedule, it says "Every Cascadeless schedule is also recoverable schedule." and the examples are also almost same. I want to know an example of Recoverable schedule ...
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Is a schedule with autocommit and no aborts automatically recoverable?

As in the title, since we won't have problems with cascading aborts and each transaction is committed automatically, doesn't this mean that the schedule will always be recoverable?
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1answer
29 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 ...
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1answer
21 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 $2500$ ...
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1answer
39 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 ...
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1answer
51 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 ...
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1answer
35 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 ...
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2answers
180 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 ...
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1answer
53 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
52 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
24 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 ...
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0answers
22 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 ...
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1answer
35 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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89 views
0
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1answer
79 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 ...
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2answers
219 views

How many candidate keys are there in the table shown below?

One candidate key is (report_no, author_id). What are the other candidate keys?
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1answer
48 views

Checking claim in paper “A Read-Only Transaction Anomaly Under Snapshot Isolation” by Alan Fekete et al

In the 2004 paper "A Read-Only Transaction Anomaly Under Snapshot Isolation" by Alan Fekete et al. (which can be found here and here) the authors state: We also note that any execution of T1 and T2 ...
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1answer
50 views

What are the essential differences and similarities among “data”, “Information” and “knowledge” in the context of Computer Science?

The terms "data", "information" and "knowledge" are quite interchangeably used in Computer Science. Strictly speaking, they do carry a very different meaning. I want to know their precise meanings and ...
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1answer
20 views

Is relation following given characteristic already in 2NF, 3NF and BCNF?

The definitions of 2NF, 3NF and BCNF goes like this: 2 Normal Form (NF) definition A relation is in second normal form if and only if it is in first normal form and all the non-key attributes ...
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1answer
112 views

Use of existential quantifier in tuple relational calculus

It is somewhat confusing to me when to use quantifiers in tuple relational calculus (TRC). I stumbled upon following problem and I am still scratching my head thinking why not TRC query can be ...
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1answer
103 views

Query Super Type and Subtype in Mysql

Assuming I have a schema depicted in the image , showing relationships(specialization) between Staffs, Academic Staffs and Non Academic Staffs. Is it possible to have a query that retrieves the ...
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87 views

Data structure for a single elimination tournament and its implementation

I want to implement a tournament managing software , something like challonge(https://challonge.com/) and am currently stuck at the data structure to be used. I read up on tournament trees and a ...
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65 views

Databases - How to generalize an sql query from a specific input? find project number, number of employees

Schema: http://comet.lehman.cuny.edu/jung/cmp420758/DBMidexamFa14-SelectedSolution.pdf Write SQL code to find the following details: For each project: find the project number and the number of ...
2
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1answer
70 views

Fastest way to search a word in a word list?

I have a word list like "English Open Word List" used to create word games. The list is in txt file with space between words, like; aa aah aahed .........and so on. I want to search it while typing ...
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33 views

Any elegant approach to search where users have permissions on subset of records?

I work in Search and have seen following problem appear in different forms. There are many users in an enterprise; they have ability to create records. Each user has variable level of access to some ...
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1answer
124 views

What is the equivalent form of full outer join in relational algebra?

Considering only basic relational algebra operations, how can we implement the full outer-join ?
2
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1answer
82 views

How to do compare-and-increment different locations with in memory MVCC?

I'm reading the paper An Empirical Evaluation of In-Memory Multi-Version Concurrency Control which is about implementations of MVCC for in-memory databases. Section 3.3 on Two-phase Locking (MV2PL) ...
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0answers
110 views

Question regarding ER Diagram design

I'm working through a Database Design textbook, and one of the problems involving ER Diagram design is giving me some trouble. The diagram is for a UNIVERSITY database with the following rules: 1) ...
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1answer
271 views

Transitive dependency in 3NF

Came across this question : State whether True or False A prime attribute can be transitively dependent on a key in 3NF Answer: True How could this be true? Because as far as I know, A relation ...
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16 views

concurrency control in real time database

Can Lamport's Bakery algorithm be used for controlling concurrency in soft real time database system (case study of stock trading)?
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1answer
1k views

Why must a Primary Index be sparse?

Reading Fundamentals of Database Systems 7th Edition, on Page 603, it says, Indexes can also be characterized as dense or sparse. A dense index has an index entry for every search key value (and ...
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2answers
214 views

Does 2NF require 1NF?

From Database System Concepts, by Silberschatz, Korth and Sudarshan : A domain is atomic if elements of the domain are considered to be indivisible units. We say that a relation schema ...
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1answer
172 views

Are all relational schemas with partial dependencies not in 3NF?

From Database Management Systems, by Raghu Ramakrishnan, ‎Johannes Gehrk Let R be a relation schema, F be the set of ...
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4k views

Order of a leaf node in B+ Tree

Going with the definition, that order of a B+ tree is the maximum number of children a node can have. What is exactly meant by the order of a leaf node? As per my understanding order of a leaf node ...
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114 views

Domain relational calculus to relational algebra

I have an expression in DRC $\{\langle a,b,c \rangle \mid \langle a,b \rangle \in r \land \langle a,b \rangle \in s\}$, where $r = (A,B)$ and $s = (A,C)$. I want to convert it onto relational algebra ...