PL/pgSQL Cursors Overview
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Questions and Answers

What is a primary benefit of using a PL/pgSQL cursor?

  • To perform automatic updates
  • To execute multiple queries simultaneously
  • To create temporary tables
  • To process a large result set in parts (correct)

A cursor in PL/pgSQL must be opened before it can be used to query rows.

True (A)

What type is used to declare a cursor variable in PostgreSQL?

REFCURSOR

To declare a bounded cursor, use the syntax 'cursor_name cursor for ________'.

<p>query</p> Signup and view all the answers

Match the cursor types with their definitions:

<p>Unbounded cursor = Not associated with any specific query upon declaration Bound cursor = Associated with a specific query upon declaration Scrollable cursor = Allows fetching of rows in any direction Parameterized cursor = Takes parameters to query specific data</p> Signup and view all the answers

Which syntax correctly opens an unbound cursor?

<p>OPEN unbound_cursor_variable FOR query; (B)</p> Signup and view all the answers

You can return a cursor reference from a PL/pgSQL function.

<p>True (A)</p> Signup and view all the answers

What is the keyword used in PL/pgSQL to declare a cursor?

<p>DECLARE</p> Signup and view all the answers

The command 'open my_cursor for ________' is used to open a cursor.

<p>select * from city where country = p_country</p> Signup and view all the answers

What type of cursor allows navigation in both directions?

<p>Scrollable cursor (C)</p> Signup and view all the answers

What does the FETCH statement do?

<p>It retrieves the next row from a cursor. (A)</p> Signup and view all the answers

The MOVE statement allows you to retrieve a row from the cursor.

<p>False (B)</p> Signup and view all the answers

What command is used to open a cursor variable that has already been bound?

<p>open cursor_variable</p> Signup and view all the answers

The valid direction values for a cursor FETCH statement include NEXT, LAST, FIRST, __________, and RELATIVE count.

<p>PRIOR</p> Signup and view all the answers

Match the following cursor operations with their descriptions:

<p>FETCH = Retrieves the next row from a cursor. MOVE = Changes the position of a cursor without retrieving a row. OPEN = Starts a cursor to perform operations. CLOSE = Releases the cursor resources and prevents further access.</p> Signup and view all the answers

Which of the following is a correct way to open a cursor with an argument?

<p>open cur_name(argument:=value); (D)</p> Signup and view all the answers

The command 'open cur_city for execute query using sort_field;' opens a cursor based on a dynamic query.

<p>True (A)</p> Signup and view all the answers

What happens when no more rows are found while fetching from a cursor?

<p>The target_variable is set to NULL.</p> Signup and view all the answers

To move a cursor, the command used is __________.

<p>MOVE</p> Signup and view all the answers

Which direction is NOT valid when using the FETCH statement?

<p>SIDEWAYS (C)</p> Signup and view all the answers

What does the statement 'move relative -1 from cur_films' do?

<p>Moves the cursor to the previous row (B)</p> Signup and view all the answers

The statement 'DELETE WHERE CURRENT OF cursor_variable' updates the current row identified by the cursor.

<p>False (B)</p> Signup and view all the answers

What is the purpose of the CLOSE statement when dealing with cursors?

<p>To release resources and allow the cursor variable to be opened again.</p> Signup and view all the answers

The command to delete a row using the cursor is 'DELETE FROM table_name WHERE current of _____'.

<p>cursor_variable</p> Signup and view all the answers

Match the following cursor commands with their descriptions:

<p>OPEN = Initializes the cursor to start fetching rows FETCH = Retrieves the current row from the cursor CLOSE = Releases the cursor and its resources DELETE = Removes the current row from the table based on the cursor position</p> Signup and view all the answers

What is the output of calling the function 'get_film_titles(2006)'?

<p>Returns titles containing 'ful' and their release years (A)</p> Signup and view all the answers

The command 'loop fetch cur_films into rec_film' retrieves every row until none are left.

<p>True (A)</p> Signup and view all the answers

What is the data type of the variable 'titles' in the function?

<p>text</p> Signup and view all the answers

The function uses a cursor to select film titles where the release year is equal to ______.

<p>p_year</p> Signup and view all the answers

What does the command 'exit when not found' accomplish in the function?

<p>It breaks the loop when no more rows are found (C)</p> Signup and view all the answers

Flashcards

PL/pgSQL Cursor

A PL/pgSQL cursor encapsulates a query, allowing for processing of individual rows. Useful for large result sets to prevent memory overflows and process parts of the data.

REFCURSOR

A special type in PL/pgSQL used to declare a cursor variable.

Unbound Cursor

A cursor that is not initially connected to a specific query.

Bound Cursor

A cursor that is connected to a specific query during declaration.

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OPEN command

Used to initialize a cursor to perform a query, making rows available for processing.

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Declaring Cursor

Creating a variable to hold a cursor object, specifying data types if binding to a query.

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Cursor Variable

A named storage location in PL/pgSQL to hold cursor object.

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Parameterised Cursor

A cursor containing placeholders (parameters) that must be filled in before opening.

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Large Result Sets

Queries that return many rows of data.

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Query

The SQL statement executed by cursor

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Opening unbound cursors

Used to execute a dynamic query; requires specifying the query string and potentially arguments.

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Opening bound cursors syntax

Opens a cursor with predefined parameters; only needs arguments if necessary.

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FETCH statement

Retrieves the next row from a cursor.

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FETCH direction

Controls which row the cursor fetches (e.g., NEXT, LAST, PRIOR).

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MOVE statement

Moves the cursor to a different row without fetching data.

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Cursor Position

The current row being pointed to by a cursor.

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Target variables

Variables used to store data fetched by the FETCH statement.

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Cursor NULL

Indicates that no more rows are available in the cursor.

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Cursor in PL/pgSQL

A cursor in PL/pgSQL allows you to work with individual rows from a query result set, one at a time. This helps to manage large result sets without overwhelming memory, and allows for focused processing.

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How to declare a cursor?

You declare a cursor in PL/pgSQL using the DECLARE statement. This creates a cursor variable and optionally binds it to a query.

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Opening a cursor

The OPEN statement initializes the cursor, executing the associated query and making rows available for processing. Think of it as activating the cursor.

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Fetching data from a cursor

The FETCH statement retrieves a single row from the cursor into a variable.

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Closing a cursor

The CLOSE statement frees up resources and closes the cursor, releasing it for potential reuse.

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Updating or deleting with a cursor

You can use UPDATE WHERE CURRENT OF or DELETE WHERE CURRENT OF statements to update or delete the row currently pointed to by the cursor.

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Example: move cur_films2;

This statement moves the cursor named cur_films to a specific position. In this case, it moves it to cur_films2.

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Example: move last from cur_films;

This statement moves the cursor named cur_films to the last row of the result set.

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Example: move relative -1 from cur_films;

This statement moves the cur_films cursor back one row relative to its current position.

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Study Notes

PL/pgSQL Cursors

  • PL/pgSQL cursors encapsulate queries, processing each row individually.
  • Useful for large result sets to prevent memory overflow errors.
  • Cursors can be used to divide a large result set into smaller parts for processing.
  • A function can also return a cursor reference, allowing the caller to process the result set.

Declaring Cursors

  • Cursors are declared in the declaration section of a block.
  • PostgreSQL uses REFCURSOR to declare cursor variables.
  • Cursors can also be bound to a query using a specific syntax: cursor_name [ [no] scroll] cursor [(name datatype, name data type, ...)] for query;

Opening Cursors

  • Cursors must be opened before use.
  • Unbound cursors: OPEN unbound_cursor_variable [ [ NO ] SCROLL ] FOR query; The query is specified when the cursor is opened.
  • Bound cursors: Open with arguments: open cursor_variable[ (name:=value,name:=value,...)];

Using Cursors with FETCH

  • The FETCH statement retrieves the next row from a cursor and stores it in variables.
  • fetch [ direction { from | in } ] cursor_variable into target_variable;
  • Direction parameters include NEXT, LAST, PRIOR, FIRST, ABSOLUTE, RELATIVE, FORWARD, BACKWARD

Moving the Cursor

  • Use MOVE to change the cursor's position without retrieving the row. The direction accepts the same values as the FETCH statement.

Deleting or Updating Rows

  • After positioning a cursor you can use DELETE WHERE CURRENT OF or UPDATE WHERE CURRENT OF statements to modify rows.

Closing Cursors

  • Use the CLOSE statement to release resources associated with a cursor.
  • close cursor_variable

PL/pgSQL Cursors Example

  • Example code shows how to combine PL/pgSQL functions, cursors, and loops to retrieve film titles and release years from a table.

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Description

This quiz covers the fundamentals of PL/pgSQL cursors, including their declaration, opening, and usage. Learn how cursors are essential for processing large result sets efficiently while managing memory. Test your understanding of cursor operations and syntax through various questions.

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