PostgreSQL Articles - Page 3 of 2

Quite often, you need the current timestamp in PostgreSQL. You do that as follows −SELECT current_timestampIt will output the current time. The output will look like the following −2021-01-30 15:52:14.738867+00Now, what if you want the relative time instead of the current time? For example, if you want the time corresponding to 5 hours prior to the current time, you can get it using intervals.SELECT current_timestamp - interval '5 hours'The output will be different every time. At the time of writing this, the output was −2021-01-30 10:57:13.28955+00 You can also do these operations on date instead of timestampsSELECT current_dateOutput2021-01-30SELECT current_date + ... Read More

Suppose you have two tables: marks and student_info. Examples are given below for the two respectivelynameroll_noperc_marksAniket1224Siddhi4565Yash2642Isha5687nameroll_noagegenderAniket1226MIsha5625FSiddhi4523FYash2625MNow, suppose your manager at work looks at the two tables and tells you, “Why do we have two tables? Simplify things, shift everything to one table!”So you decide to add the perc_marks column to the student_info table.ALTER TABLE student_info ADD COLUMN perc_marks integerNow, how will you populate this column? Will you manually add the marks for each column? That will leave the room open for a lot of errors and will also be very time-consuming. Instead, this is what you could do −UPDATE student_info ... Read More

There are 6 types of constraints that can be generally used with a PostgreSQL table. They are listed and explained below −NOT NULL CONSTRAINTThis is a very common constraint. If there is a particular column that cannot have null values, you add this constraint at the time of table creation. For example, if we create a marks table, which can’t have NULL values for the name, then the table creation command will look like −CREATE TABLE marks(    name VARCHAR NOT NULL,    roll_no INTEGER,    marks_obtained INTEGER );Now, if we try to insert a row into this table, without ... Read More

The ability to define JSON columns in PostgreSQL makes it very powerful and helps PostgreSQL users experience the best of both worlds: SQL and NoSQL.Creating JSON columns is quite straightforward. You just have to create/ define it like any other column and use the data type as JSON.Let us create a new table in PostgreSQL, called json_test −CREATE TABLE json_test(    serial_no SERIAL PRIMARY KEY,    name VARCHAR,    metadata JSON );Now, let us populate it with some data −INSERT INTO json_test(name, metadata) VALUES ('Yash', '{"marks_scored":{"science":50, "maths":65}}'), ('Isha', '{"marks_scored":{"science":70, "maths":45}}');As you can see, the JSON values are added within single ... Read More

In order to change the type of a column, the syntax isALTER TABLE table_name ALTER COLUMN column_name TYPE new_data_type USING expression;Please note that altering the type of a column is not recommended generally, especially if your table has a lot of entries already.The USING part of the syntax is optional. It is used when you need to use an expression for converting the existing entries in that column from the current type to the new type. For instance, if you convert a column of type INTEGER to type BIGINTEGER, you need not use any expression for the conversion of existing ... Read More

The syntax to add a new column to an existing table is quite straightforward.ALTER TABLE table_name ADD COLUMN column_name column_type column_constraint;Say you have existing table marks. An example is given below −serial_nonameroll_nomarks_obtainedperc_marksmax_marksdate_of_entry1Yash2642421002021-01-302Isha5617587.52002021-01-30Now, suppose you want to add a column named subject. You can do that using −ALTER TABLE marks ADD COLUMN subject VARCHAR;Now if you query the table again using, SELECT * from marksYou will see the following output  −serial_nonameroll_nomarks_obtainedperc_ marksmax_ marksdate_ of_ entrysubject1Yash2642421002021-01-30[null]2Isha5617587.52002021-01-30[null]Note that the values in the subject column are null because we have just created the column, not populated it. We can populate it using the UPDATE ... Read More

Sometimes, some rogue queries can take too long to execute. If the queries are blocking in nature, i.e., they restrict access to a table while they are executing, then any other query on the same table will be put on hold, and this leads to a pile-up of queries. This can, depending on your DB load, even cause the max connections to be exceeded. Luckily, you can easily kill long queries in pgAdmin.Go to Dashboard in your pgAdmin. At the bottom, in the Server Activity section, under the Sessions Tab, you can see all the Active queries.Now, notice the cross ... Read More

Querying a DB in pgAdmin is quite straightforward. Locate your DB in the Servers dropdown on the left, and extend its dropdown, till you see the Schemas dropdown.Once you click on Schemas, the black button on the top, with the DB symbol and the play arrow will become clickable.Click on that button, and you will see a Query Tab open up. That’s it, you can type your queries for this particular DB in that box, and click the play arrow button to execute the queries.The output will be seen on the bottom, in the ‘Data Output’ section.Read More

In this article, we will learn how to change or relocate the PostgreSQL Database data directory to the new location on Ubuntu 16.04. This database grows more frequently and depends upon the size of the company, as we needed more space and for security reasons we will change the data directory to the other volume or other location.PrerequisitesAn Ubuntu machine with a non-root user with Sudo permission.A PostgreSQL server installed and working.A new volume or location where we want to move the database data location, the new location will be /mnt/data_vol/PostgreSQL as the data_vol is the new volume attached to ... Read More

PostgreSQL is an open source relational database management system (DBMS) developed by a worldwide team of volunteers. PostgreSQL is not controlled by any corporation or other private entity and the source code is available free of charge.PostgreSQL runs on all major operating systems, including Linux, UNIX (AIX, BSD, HP-UX, SGI IRIX, Mac OS X, Solaris, Tru64), and Windows. It supports text, images, sounds, and video, and includes programming interfaces for C / C++, Java, Perl, Python, Ruby, Tcl and Open Database Connectivity (ODBC).Download the latest version of postgresql- from postgresql-jdbc repository.Add downloaded jar file postgresql-(VERSION).jdbc.jar in your class path.ExampleFollowing JDBC program ... Read More