Questions tagged [databases]

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Example for Database architecture

PHPMyadmin is stated as a 3 tier database architecture.How is this possible,Isn't it a 2 tier architecture.What are the real world examples of 1-tier and 2-tier architecture.
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What does the three schema architecture of DBMS refer to?

This architecture contains three layers or levels of the database management system: External level Conceptual level Internal level Can some one explain to me with a practical real example so that ...
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Why is Read(Q) timestamp is not checked when a transaction issues a command reading Q?

Consider a concurrent schedule of two transactions $T_1,T_2$: $R_x(Q): T_x$ Reads $Q$ $W_x(Q): T_x$ Writes $Q$ $S:R_2(A),R_1(A),W_1(A),W_2(A)$, this schedule is not conflict serializable.However, if ...
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how two phase commit ensure serializability

Two phase commit is used in distributed transaction. For example, a client sends a transaction to two databases with a coordinator. step1: client get a global transaction id from coordinator step2: ...
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Bits needed to send parameters of perfect hash functions

Suppose that there is a server that has $n$ files. The server is used to construct a perfect hash function for those files and then the computed parameters will be sent to a user. These parameters ...
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Efficiently storing and modifying a reorderable data structure in a database

I'm trying to create a list curation web app. One thing that's important to me is being able to drag-and-drop reorder items in the list easily. At first I thought I could just store the order of each ...
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Databases and B-Trees: What are Keys and how are they related

I confused about the description & definition of "key" occuring as terminology for databases and b-trees. In first case dealing with theory of databases a key is defined as a choice for ...
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44 views

What is the maximum number of indices one can create on a table with N columns?

Say, I have a database table with $N$ columns. What is the (theoretical) maximum number of indices I can create on that table? For $N = 1,2,3$ it's easy enough to calculate the answer $(1, 4, 15)$, ...
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Confused between 2 phase locking and 2 phase commit

I understand that both algorithms are very different, but what I don't understand is whether they achieve the same thing in the end. 2PC is for atomic commits and 2PL is for serializable isolation. ...
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Parsing text, then searching it: one entry per position, vs. 1 JSON column per text

I have a Rails application using Postgresql. Texts get added to the application (ranging in size -- as short as a few words, to as long as, say, 5,000 words?). The text gets parsed, both ...
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Relation Closure for F+

$A \rightarrow B C$ $C D \rightarrow E$ $B \rightarrow D$ $E \rightarrow A$ I have these Relations for R={A,B,C,D,E} Now I have to calculate the closure $F^+$ and give the candidate keys I came ...
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How to create a database of meta-model transformations with real data?

I am currently working on meta-model transformation and the coevolution of models. I have already done some kind of a state of the art, and the next step in my research would be to collect some (...
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Incremental View Maintenance for projection

I was reading https://www2.cs.duke.edu/courses/spring02/cps296.1/lectures/08-view.pdf Or in the paper "Incremental Maintenance of Views with Duplicates," by Griffin and Libkin, SIGMOD, 1995. figure 2....
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Evolving graph rewritings between trees (representing JSON) and graphs (representing relational database schemas)

I have in input different, say 100, types of trees with labeled nodes (representing JSON files). I need to transform the information contained in the trees into graphs with labeled nodes (representing ...
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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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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?
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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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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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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: $$Workshop(\...
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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 ...
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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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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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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 ...
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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 ...
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381 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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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 ...
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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 ...
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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,...
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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 ...
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1answer
583 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 ...
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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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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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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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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 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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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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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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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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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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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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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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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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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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545 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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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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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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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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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 ...