How Do Cursor And Fetch Work In Postgresql?

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In PostgreSQL, the combination of the cursor and FETCH statement is used to retrieve data from a select query in a controlled and efficient manner.


A cursor is a database object that can be used to traverse through the rows of a result set one at a time. It provides a way to iterate over the result set without fetching all the data at once, which can be especially useful when dealing with large result sets.


To use a cursor, you first DECLARE it, specifying the SELECT query that will provide the result set. The DECLARE statement creates a named cursor, making it available for subsequent operations.


Once the cursor is declared, you can use the FETCH statement to retrieve rows from the result set. FETCH allows you to retrieve a specific number of rows or fetch them one at a time using a loop, depending on your requirements.


With the help of the FETCH statement, you can control the direction of retrieval (forward or backward) and set the number of rows to be fetched at once. This can be done using arguments like FETCH FORWARD, FETCH BACKWARD, FETCH ABSOLUTE, FETCH RELATIVE, etc.


Fetching rows from the result set using a cursor can be more efficient than performing a single SELECT statement that retrieves all the data, especially when you only need to process a subset of the result set or when working with large datasets. It reduces the memory consumption and provides a way to traverse through the results sequentially.


Once you have finished working with the cursor, you can CLOSE it to release the resources associated with it. Cursors can also be deallocated altogether when they are no longer needed.


In summary, cursors in PostgreSQL provide a way to efficiently retrieve and process data from a result set in a controlled manner, allowing you to work with large datasets more effectively. The FETCH statement is used to retrieve rows from the cursor, enabling you to set the direction, number of rows, and perform iterative operations on the result set.

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What is a snapshot-driven cursor in PostgreSQL?

A snapshot-driven cursor in PostgreSQL is a type of cursor that allows a query to operate on a specific consistent snapshot of the data in the database. When a snapshot-driven cursor is opened, it uses a snapshot of the data at the time when the cursor was created, and subsequent changes to the data made by other transactions do not affect the cursor's result set.


This type of cursor is useful in scenarios where the result set needs to remain consistent even if there are concurrent updates or deletions happening on the database. For example, if a cursor is opened to fetch all users with a specific condition, a snapshot-driven cursor ensures that the result set remains unchanged even if new users are inserted or existing ones are deleted while the cursor is open.


It is important to note that a snapshot-driven cursor consumes resources for the duration of its open state, so it needs to be closed explicitly when it is no longer required. Additionally, multiple concurrent snapshot-driven cursors can be opened on the same data, and they will all operate on their respective consistent snapshots of the data.


How to declare and use a cursor variable in PostgreSQL?

To declare and use a cursor variable in PostgreSQL, you can follow these steps:

  1. Declare a cursor variable using the DECLARE statement. Specify a name for the cursor variable and the data type of the result set. For example: DECLARE cursor_name CURSOR FOR SELECT column1, column2 FROM table;
  2. Open the cursor using the OPEN statement. This will execute the query and make the result set available for fetching. For example: OPEN cursor_name;
  3. Fetch rows from the cursor using the FETCH statement. You can specify the number of rows to fetch at a time and assign the fetched rows to variables. For example: FETCH NEXT FROM cursor_name INTO var1, var2;
  4. Use the fetched values as desired.
  5. Repeat the FETCH statement to retrieve more rows from the cursor until all rows have been fetched. You can use a loop to iterate over the cursor and fetch rows until there are no more rows left.
  6. Close the cursor using the CLOSE statement to release the resources associated with it. For example: CLOSE cursor_name;


Here's an example that demonstrates the above steps:

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DECLARE my_cursor CURSOR FOR SELECT name, age FROM employees;
OPEN my_cursor;

LOOP
    FETCH NEXT FROM my_cursor INTO emp_name, emp_age;
    EXIT WHEN NOT FOUND;
    
    -- Use the fetched values
    -- Do something with emp_name and emp_age
    
END LOOP;

CLOSE my_cursor;


Note: Cursor variables provide a way to process result sets in a procedural manner. However, it's generally recommended to use SQL queries directly instead of cursors whenever possible, as it tends to be more efficient.


How to create a forward-only cursor in PostgreSQL?

In PostgreSQL, you can create a forward-only cursor by performing the following steps:

  1. Declare a cursor using the DECLARE statement. Specify the cursor name and the FORWARD ONLY option. DECLARE cursor_name CURSOR FORWARD ONLY FOR SELECT column1, column2 FROM table_name; Replace cursor_name with the desired name for the cursor, column1 and column2 with the columns you want to fetch, and table_name with the name of the table you want to query.
  2. Open the cursor using the OPEN statement. OPEN cursor_name;
  3. Fetch rows from the cursor using the FETCH statement. FETCH NEXT FROM cursor_name; This fetches the next row from the cursor.
  4. Continue fetching rows from the cursor until there are no more rows to fetch. LOOP FETCH NEXT FROM cursor_name; EXIT WHEN NOT FOUND; -- Process the fetched row END LOOP; The EXIT WHEN NOT FOUND condition checks if the FETCH statement didn't fetch any rows and exits the loop if true.
  5. Close the cursor using the CLOSE statement. CLOSE cursor_name;
  6. Optionally, deallocate the cursor using the DEALLOCATE statement. DEALLOCATE cursor_name; This releases the resources associated with the cursor.


By using a forward-only cursor, you ensure that rows are fetched and processed sequentially without the ability to backtrack or re-fetch previously fetched rows. This can be useful in scenarios where only one-time forward traversal is required, resulting in improved performance and reduced resource consumption.


How to declare and use a cursor in PostgreSQL?

To declare and use a cursor in PostgreSQL, you can follow these steps:

  1. Declare a cursor: DECLARE cursor_name CURSOR FOR SELECT column1, column2 FROM table_name WHERE condition; Replace cursor_name with the desired name for your cursor, column1 and column2 with the column names you want to fetch, table_name with the name of the table, and condition with any desired condition for filtering the rows.
  2. Open the cursor: OPEN cursor_name;
  3. Fetch rows from the cursor: FETCH NEXT FROM cursor_name; This will fetch the next row from the cursor. You can repeat this command to fetch subsequent rows.
  4. Process the fetched row: -- Example of processing values from the fetched row FETCH NEXT FROM cursor_name INTO var1, var2; Replace var1 and var2 with the names of variables where you want to store the fetched column values.
  5. Close the cursor: CLOSE cursor_name;
  6. Deallocate the cursor: DEALLOCATE cursor_name; This step is optional, as the cursor is automatically deallocated when the transaction ends.


Remember to include the necessary error handling and exception management in your code to handle any possible errors that may occur during cursor usage.


Note: Cursors in PostgreSQL are only available within a transaction block.


What is a scrollable cursor in PostgreSQL?

A scrollable cursor in PostgreSQL is a database feature that allows a user to navigate through a set of query results in a forward, backward, or random order. It provides the ability to move the cursor to a desired row position within the result set, retrieve data from that position, and then move the cursor again, making it useful for scenarios where random access to data is required.


By default, PostgreSQL cursors are forward-only, meaning they can only traverse through the query results in a forward direction. However, with the scrollable cursor feature, users can modify the cursor behavior to navigate through the result set in different ways.


Scrollable cursors in PostgreSQL support four types of movement:

  1. FETCH NEXT: Moves the cursor to the next row in the result set.
  2. FETCH PRIOR: Moves the cursor to the previous row in the result set.
  3. FETCH FIRST: Moves the cursor to the first row in the result set.
  4. FETCH LAST: Moves the cursor to the last row in the result set.


To create a scrollable cursor in PostgreSQL, the DECLARE statement is used with the SCROLL keyword. Once declared, the cursor can be scrolled through using the appropriate FETCH command.


Scrollable cursors provide flexibility and convenience for working with query results, especially when dealing with large datasets or complex data manipulation requirements.


What is a cursor in PostgreSQL?

In PostgreSQL, a cursor is a database object that allows users to retrieve data from a result set in a controlled and efficient manner. It works similarly to a pointer or iterator, providing a way to traverse through the rows of a query result.


A cursor is particularly useful when dealing with large result sets, as it enables the retrieval of a small portion of data at a time, rather than fetching the entire result set in one go. This reduces memory consumption and improves performance.


Cursors can be defined, opened, fetched, and closed within a transaction block. The basic process involves declaring a cursor, opening it to establish a connection to a specific query result, fetching data rows one by one, and finally closing the cursor. Cursors can also be scrolled to move backward or forward in the result set.


PostgreSQL supports several types of cursors, including static, dynamic, and scrollable cursors, each with its own advantages and use cases.

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