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New Postgresql and C Programming Guides From LinuxHint

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  • Postgresql group by

    Postgresql group by clause is a feature that is used to unite/combine those rows in the table that have the same data. This clause is mainly used to remove duplicate data and to maintain concurrency. Whenever we want to calculate sum, or any other aggregate like AVG, etc., this group by clause is always used as there are many clauses used in PostgreSQL. But there exists a hierarchy between each clause.

  • Postgresql stored procedure example

    Postgresql database is more reliable than others because it can create and store the data in the form of relations, procedures, etc. Postgresql is a customizable database, as we can modify the storage containers according to our requirements. The data in Postgresql is managed by the schemas and catalogs. Postgresql supports many languages, which means that we can execute queries in any programming language either in the psql (shell) or on the pgAdmin side.

    Just like the temporary tables, we also use some other features to involve the storage capacity. These are called the “STORED PROCEDURES”. These are not shown like the tables. But silently works with the tables.

    In Postgresql or any other database management system, we use functions to perform operations on the data. These functions are user-created or user-defined. One major drawback of these functions is that we are unable to execute transactions inside the functions. We cannot commit or rollback. That’s why we use the stored procedures. By using these procedures, application performance is increased. Moreover, we can use more than one SQL statement inside a single procedure. There are three types of parameters.

  • PostgreSQL Copy from Stdin

    PostgreSQL like other database management systems supports the standard streams. These streams are responsible for manipulating data for storage in PostgreSQL. These are the input and output channels of communication between the application and the environment that is created at the time of execution.

    Whenever we execute a command in PostgreSQL, the streams make the connection with the text terminal where the psql (shell) is running. However, in the case of inheritance, each child’s process inherits the streams from the parent process. Not every program needs these streams to be introduced in the code, some functions such as getchar() and putchar() use the input and output streams automatically. Streams lie in the category of 3.

  • How do I count unique values in PostgreSQL?

    Postgresql database, like other databases, can store data that remain intact for a long time. This can be done by removing redundancy from the data in the schema by maintaining concurrency and accuracy of data present. While adding data in relations, you might encounter such a situation in which you don’t notice the replication of the inserted data at the time of inserting. To remove the garbage values or similar data, we need some special functions to keep the rest of the values unique and distinct. Count () is a built-in function of PostgreSQL. By using this function, we can get information regarding the data present in the table. Postgresql deals with the count () very effectively by displaying data to the provided conditions. To get the distinct data or the values in terms of discrimination with other data, we need some different commands and the count (). This article will highlight the examples applied on different relations to elaborate the concept of counting unique values in PostgreSQL.

    Firstly, you need to create a database in the installed PostgreSQL. Otherwise, Postgres is the database that is created by default when you start the database. We will use psql to start implementation. You may use pgAdmin.

  • How do I describe a table in PostgreSQL?

    To describe the tables of a database, we don’t need any permissions or privileges of the user. Anyone can describe the information regarding the table. “postgresql describe table” refers to checking the table structure. We can use various examples with variety in their usage leads to gain the description of the data. This will be understood by you while reading the article.

  • PostgreSQL Median Function

    The calculating median is obliging to get the middle value from the data. Like many other functions, Postgresql provides a wide range of versatility in dealing with the median function. It is concerned with finding the median value from the column of the table on which median is used. In contrast with Postgresql aggregate functions COUNT, SUM, there is not a specific function used to calculate the aggregate of the median. Median is a user-defined function.

    In simple arithmetic functions, there are different means and formulas to find the median in any provided data either in plain text or in the tabular form. Whereas in the case of database systems, we use some other built-in functions combined to get the value of the median. This article will elaborate on some of the most efficient techniques and also a manual function created by us to fetch the median from the data in Postgresql.

  • PostgreSQL Create Database if not Exists

    Whenever we talk about PostgreSQL, we always think about databases because it is a database management system. You can create as many databases of your choice on your PostgreSQL server as you want. After creating a database of your choice, you can create multiple tables within it to store your data, and then you can perform different calculations on that data. However, at times, we want to run a query on an existing database, but for that, the existence of such a database is mandatory.

    What we mean to say is that we need a mechanism to check if a database exists on our PostgreSQL server or not. Moreover, there are some situations in which we want a database to be created once we run a query if it does not already exist on our database server. In that case, the “Create Database if not Exists” notation comes into play. This guide aims to introduce you to the usage of this notation with a brief discussion on whether PostgreSQL supports this notation or not. After that, we will share with you a PostgreSQL-supported alternative for this notation.

  • What is the Usage of Extern in C?

    The term “extern” has been used to define the global variables within the C language. The word “extern” indicates that it is abbreviated from the word “external” which means outside. Hence, these variables are globally accessible throughout the C code. These variables are always initialized and defined outside of the main method. This guide is for the users who want to explore the working of the “extern” variable using the C language. So, let’s go ahead.

  • Bind System Call in C

    The bind() system call binds an address or name with any socket in the C language. This system call has mostly been used in socket programming. This address will be bonded to the socket descriptor. The bind() system call contains three parameters in actual. The first one is the descriptor of a socket. The second parameter is the pointer used for the address of a socket. The address must be some local path. The third argument can be the size of a socket address. Today’s guide will see how the “bind()” system call works in Ubuntu 20.04 using the C language. So, log in from the system first.

  • ptrace system call in C

    Ptrace() system call is generally used for debugging breakpoints and tracing system calls. The ptrace() “process trace” system call is frequently used for debugging purposes. It is the main way that native debuggers keep track. Tracees can be paused, registers and memory can be inspected and set, system calls can be monitored, and even system calls can be intercepted using Ptrace system call. The Tracee must first be connected to the tracer. In a multithreaded process, each thread can be separately attached to a possibly distinct tracer or left unattached and therefore un-debugged. As a result, “Tracee” always refers to “a potentially multithreaded process, never or maybe multithreaded process.

    All signals provided to the traced process, except for one, cause it to stop, regardless of its registered signal processing, and deliver an event towards the tracing process, which may be identified using the wait () system function. The SIGKILL signal is an exception, as it is delivered instantly and accomplishes the expected behavior. There has never been a standard for Ptrace system call. Its interface is comparable across operating systems, notably in terms of essential functionality, but it differs slightly from one system to the next.

    System calls can be traced using the Linux edition of ptrace. The PTRACE SYSCALL request restarts the child process in the same way that PTRACE CONT does, but it arranges for it to stop at the next system call entry or exit. This brings up a lot of new opportunities. For PTRACE PEEK requests, ptrace() will return the desired data; it will return zero for all the other requests. All requests that fail return -1, with errno set to the optimum value. In the case of PTRACE PEEK requests, -1 may be a legitimate return value; the program is responsible for determining whether this is an error situation or a valid return value. This guide will explain to you the functionality of the ptrace() system call in C language with one example.

  • Futex system call in C

    In general, thread synchronization primitives necessitate the use of system calls by userspace programs. The system call is inherently inevitable for placing a thread to sleep and waiting for another thread or waking up a thread from sleep. Obtaining an uncontended lock, for example, does not necessitate a system call, at least not naturally. The futex is the remedy to this challenge. A futex is simply an instantaneous integer address. The address is used to identify a queue of threads that are waiting to be processed. The value of the integer at that location is utilized to execute the fast path with atomic operations if available, as well as to handle corner case race situations in the event of a conflict. The futex() system call allows a program to wait for a value at a particular address to change, as well as wake up anyone waiting on that address. It is most commonly used to implement the debatable case of a shared memory lock, as mentioned in futex (7). When a futex(7) action in userspace fails to complete without error, a call to the kernel is required to resolve the issue. Arbitration can be used to either put the calling process to sleep or to wake up a waiting process. The semantics defined in the futex is expected to be followed by callers of this function (7).

    Because these semantics require authoring non-portable assembly instructions, most users will most likely be library authors rather than ordinary application developers. Futex is a single system call that performs many operations. That may appear strange, even perplexing, if not downright. That is, nevertheless, standard procedure for a one-of-a-kind system call: the “ioctl” system call contains considerably more operations than futex. For another instance, programmers are unlikely to discover because Glibc hides it, although the single socket call system implements all socket-related functions. As a result, if threads access it in a single process, it can be specified as a global variable or stored in a shared memory segment if threads from various processes utilize it. A state updated in userspace using atomic operations is stored in the shared variable. There is no need for a system call when the state says that there is no conflict. If the condition indicates a contention, on the other side, a futex system call is made to put the calling task to sleep.

  • Getline function in C

    Getline is the newest and most popular function for reading a string of text (). The getline() function is part of the C library. This function accepts a string from the input stream as an input, so getline() is a better option. The concept of pointers is used by getline(). For reading text, the getline method is the ideal way. The getline method reads a full line from a stream, such as a newline character. To finish the input, use the getline function to generate a stop character. The command will be completed, and this character will be removed from the input. We all have the cin object to take user input; however, the cin object doesn’t allow us to receive user input in many lines; therefore, we can use the getline() function to take input from the input stream in several lines or a string till a delimiter character is discovered.

    The getline function uses the realloc function to automatically increase the memory block as required, ensuring that there is never a space shortage. This is one of the explanations why getline is safe. The value returned inside the second parameter will also inform us of the new block size. It returns -1 if an error appears, such as reaching the end of a file without receiving any bytes. Getline functions cease reading input from the stream when they meet a newline character or the end of a file.Syntax

  • Brk system call in c

    The program discontinuity, which marks the termination of the process’s data segment, i.e., the program breakdown is the first position following the completion of the uninitialized data segment, can be changed using brk(). Increasing the program break allows memory to the process; reducing the break allows memory to the process. When that amount is sensible, the system would have enough memory, and the process does not surpass its maximum data size, brk() sets the ending of the data segment to the value supplied by the addr. Brk() returns zero if it succeeds. If there comes an error, -1 is returned. The system functions brk(), and sbrk() is utilized to regulate the amount of memory allotted towards the process’s data segment. Usually, these functions are invoked from a larger memory management library function like malloc. The program break, which identifies the end of the process’s heap section, is moved about with brk() and sbrk(). brk() assigns the value of addr to the ending of the heap segment. sbrk() increases the heap space of the program by increment bytes. It takes you back to the earlier program break. The present location of the program break can be found by calling sbrk() with just a raise of 0.

  • How to Check for Malloc Error in C

    The malloc is a C language function used to allocate memory to some variable. It also returns a pointer. We can also use the Malloc function to check for errors about memory allocation. When a malloc method finds itself unable to allocate memory, it usually returns NULL. You can also through an error message if the allocation got failed upon managing the pointers. In this guide, we will see the implementation of the malloc function to allocate memory and check the error for memory allocation. So, start the implementation by simply log in from the Linux system.

Anorher later one

  • Foreign key Postgresql

    A foreign key is one or more columns having values that depend on the primary key. This constraint is used for joining two tables. In postgresql, a foreign key helps generate a relationship between the tables, making one table a parent table and the other a child.

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