Performing operations on cursors in PostgreSQL - Deep Dive

Overview - Performing operations on cursors

What is it?

A cursor in PostgreSQL is a tool that lets you handle query results one row at a time instead of all at once. It helps when working with large data sets or when you want to process rows step-by-step. You open a cursor to start reading, fetch rows as needed, and close it when done. This way, you control how much data you work with at once.

Why it matters

Without cursors, you would have to load all query results into memory at once, which can be slow or impossible for big data. Cursors let you work efficiently with large tables by fetching small parts at a time. This saves memory and lets you write programs that process data gradually, like reading a book page by page instead of all at once.

Where it fits

Before learning cursors, you should understand basic SQL queries and how to write SELECT statements. After cursors, you can learn about stored procedures and loops in PostgreSQL, which often use cursors to handle data stepwise.

Mental Model

Core Idea

A cursor is like a bookmark that lets you read query results row by row instead of all at once.

Think of it like...

Imagine reading a long book. Instead of reading the whole book in one go, you use a bookmark to keep your place and read one page at a time. A cursor works the same way for database rows.

┌─────────────┐
│  Query Result│
│  (many rows) │
└──────┬──────┘
       │
       ▼
┌─────────────┐
│   Cursor    │
│ (bookmark) │
└──────┬──────┘
       │ fetch rows one by one
       ▼
┌─────────────┐
│ Application │
│ processes  │
│   rows     │
└─────────────┘

Build-Up - 7 Steps

1

Foundation

What is a Cursor in PostgreSQL

Concept: Introduces the basic idea of a cursor as a pointer to query results.

A cursor is a database object that allows you to retrieve query results row by row. You declare a cursor for a SELECT query, then open it to start reading. You can fetch rows one at a time or in groups, and finally close the cursor to release resources.

Result

You get a handle (cursor) that lets you read query results stepwise instead of all at once.

Understanding that cursors let you control how much data you process at a time is key to handling large datasets efficiently.

2

Foundation

Declaring and Opening a Cursor

Concept: Shows how to declare and open a cursor in PostgreSQL.

You declare a cursor using the DECLARE statement with a SELECT query. For example: DECLARE my_cursor CURSOR FOR SELECT * FROM employees; Then you open the cursor with OPEN my_cursor; to start reading rows.

Result

A cursor named 'my_cursor' is ready to fetch rows from the employees table.

Knowing how to declare and open a cursor is the first step to using it for controlled data retrieval.

3

Intermediate

Fetching Rows from a Cursor

🤔Before reading on: do you think FETCH returns all rows at once or just one row? Commit to your answer.

Concept: Explains how to retrieve rows from a cursor using FETCH.

After opening a cursor, you use FETCH to get rows. FETCH can retrieve one row or multiple rows: FETCH NEXT FROM my_cursor; FETCH FORWARD 5 FROM my_cursor; Each FETCH moves the cursor forward and returns the requested rows.

Result

You get the next row(s) from the query result, allowing step-by-step processing.

Understanding FETCH lets you control how many rows you process at a time, which is essential for memory management.

4

Intermediate

Moving the Cursor Position

🤔Before reading on: do you think you can move a cursor backward or jump to a specific row? Commit to your answer.

Concept: Introduces cursor movement commands like MOVE and scroll options.

Cursors can move forward, backward, or jump to a position using MOVE: MOVE BACKWARD 1 FROM my_cursor; MOVE ABSOLUTE 10 FROM my_cursor; This lets you re-read or skip rows without closing the cursor.

Result

You can reposition the cursor to read different parts of the result set as needed.

Knowing cursor movement commands gives you flexible control over data navigation.

5

Intermediate

Closing and Releasing a Cursor

Concept: Shows how to properly close a cursor to free resources.

When done, close the cursor: CLOSE my_cursor; This releases memory and locks held by the cursor. Forgetting to close can cause resource leaks.

Result

The cursor is closed and resources are freed.

Properly closing cursors prevents performance issues and resource exhaustion.

6

Advanced

Using Cursors in PL/pgSQL Loops

🤔Before reading on: do you think cursors can be used inside loops to process rows one by one? Commit to your answer.

Concept: Demonstrates using cursors inside PostgreSQL procedural language for row-by-row processing.

In PL/pgSQL, you can declare a cursor and loop through rows: DECLARE my_cursor CURSOR FOR SELECT * FROM employees; OPEN my_cursor; LOOP FETCH my_cursor INTO record_var; EXIT WHEN NOT FOUND; -- process record_var END LOOP; CLOSE my_cursor; This pattern processes each row individually.

Result

You can write procedural code that handles each row from a query stepwise.

Using cursors in loops enables complex row-by-row logic inside the database.

7

Expert

Scrollability and Cursor Sensitivity

🤔Before reading on: do you think cursors always reflect changes made to the underlying data after opening? Commit to your answer.

Concept: Explains cursor types: scrollable, insensitive, and their behavior with data changes.

Cursors can be declared SCROLL to move backward and forward. They can be INSENSITIVE (snapshot of data) or SENSITIVE (reflect changes). For example: DECLARE my_cursor SCROLL INSENSITIVE CURSOR FOR SELECT * FROM employees; Insensitive cursors do not see changes made after opening, while sensitive cursors might. This affects consistency and performance.

Result

You understand how cursor types affect data visibility and navigation.

Knowing cursor sensitivity helps avoid surprises with data changes during cursor use and guides performance tuning.

Under the Hood

Internally, PostgreSQL executes the query when the cursor is opened, but it does not send all rows to the client immediately. Instead, it keeps a pointer to the current position in the result set. FETCH commands move this pointer and retrieve rows on demand. This lazy retrieval saves memory and network bandwidth. Scrollable cursors maintain additional state to allow moving backward or jumping to positions.

Why designed this way?

Cursors were designed to handle large query results efficiently without loading everything into memory. Early databases struggled with big data sets, so cursors let applications process data incrementally. The design balances resource use and flexibility, allowing both forward-only and scrollable access depending on needs.

┌───────────────┐
│ Query Engine  │
│ executes SQL  │
└──────┬────────┘
       │
       ▼
┌───────────────┐
│ Cursor Object │
│ holds position│
│ and state     │
└──────┬────────┘
       │ FETCH / MOVE commands
       ▼
┌───────────────┐
│ Result Rows   │
│ (stored or    │
│ streamed)     │
└───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Do you think a cursor automatically updates to show changes made to the table after it was opened? Commit yes or no.

Common Belief: A cursor always shows the latest data changes made to the table after opening.

Reality: Most cursors show a snapshot of data as it was when opened and do not reflect later changes unless declared sensitive.

Why it matters: Assuming cursors reflect live data can cause bugs where your application processes outdated information.

Quick: Do you think FETCH without opening a cursor works? Commit yes or no.

Common Belief: You can fetch rows from a cursor without explicitly opening it first.

Reality: You must open a cursor before fetching; otherwise, FETCH commands will fail.

Why it matters: Skipping OPEN leads to runtime errors and confusion about cursor state.

Quick: Do you think closing a cursor is optional and resources are freed automatically? Commit yes or no.

Common Belief: Closing a cursor is optional because PostgreSQL frees resources automatically when the session ends.

Reality: While resources are freed at session end, not closing cursors promptly can cause resource leaks and lock contention during long sessions.

Why it matters: Neglecting to close cursors can degrade database performance and cause unexpected locks.

Quick: Do you think cursors always fetch one row at a time? Commit yes or no.

Common Belief: Cursors can only fetch one row per FETCH command.

Reality: Cursors can fetch multiple rows at once by specifying a count in FETCH, improving efficiency.

Why it matters: Limiting to one row fetch can cause unnecessary overhead and slow processing.

Expert Zone

1

Scrollable cursors require more memory and processing because the database must keep track of all rows to allow backward movement.

2

Insensitive cursors provide a stable snapshot but may cause stale data issues; sensitive cursors reflect changes but can be slower and less predictable.

3

Using cursors inside transactions affects visibility and locking; understanding transaction isolation levels is crucial to avoid deadlocks or inconsistent reads.

When NOT to use

Avoid cursors when you can process data with set-based SQL operations, which are faster and simpler. Use cursors only when row-by-row processing is necessary, such as complex procedural logic. For very large datasets, consider server-side batch processing or external tools instead of client-side cursors.

Production Patterns

In production, cursors are often used inside stored procedures to process data in manageable chunks, especially for reporting or data migration tasks. Developers combine cursors with exception handling and transaction control to ensure robust, efficient processing. Scrollable cursors are less common due to overhead but useful in interactive applications.

Connections

Iterators in Programming

Cursors work like iterators that let you access elements one at a time.

Understanding iterators in programming languages helps grasp how cursors provide controlled access to data sequences.

Memory Paging in Operating Systems

Both cursors and memory paging handle data in small chunks to optimize resource use.

Knowing how operating systems load memory pages on demand clarifies why cursors fetch rows incrementally to save memory.

Streaming Data Processing

Cursors enable streaming-like processing of database rows, similar to how streaming frameworks handle data flows.

Recognizing cursors as a form of streaming helps understand their role in efficient, incremental data handling.

Common Pitfalls

#1 Trying to fetch rows from a cursor before opening it.

Wrong approach:

FETCH NEXT FROM my_cursor;

Correct approach:

OPEN my_cursor;
FETCH NEXT FROM my_cursor;

Root cause:

Misunderstanding that a cursor must be opened before fetching rows.

#2 Not closing cursors after use, causing resource leaks.

Wrong approach:

DECLARE my_cursor CURSOR FOR SELECT * FROM employees;
OPEN my_cursor;
FETCH NEXT FROM my_cursor;
-- no CLOSE statement

Correct approach:

DECLARE my_cursor CURSOR FOR SELECT * FROM employees;
OPEN my_cursor;
FETCH NEXT FROM my_cursor;
CLOSE my_cursor;

Root cause:

Forgetting to release resources explicitly leads to performance degradation.

#3 Assuming cursors automatically reflect data changes made after opening.

Wrong approach:

DECLARE my_cursor CURSOR FOR SELECT * FROM employees;
OPEN my_cursor;
-- data changes happen here
FETCH NEXT FROM my_cursor; -- expects updated data

Correct approach:

DECLARE my_cursor SCROLL SENSITIVE CURSOR FOR SELECT * FROM employees;
OPEN my_cursor;
FETCH NEXT FROM my_cursor; -- may see changes

Root cause:

Not understanding cursor sensitivity and snapshot behavior.

Key Takeaways

Cursors let you process query results one row or a few rows at a time, saving memory and improving control.

You must declare, open, fetch from, and close cursors in the correct order to avoid errors and resource leaks.

Cursors can move forward, backward, or jump to positions if declared scrollable, giving flexible navigation.

Understanding cursor sensitivity helps you know whether data changes after opening will be visible during fetching.

Use cursors wisely: prefer set-based SQL for performance, and use cursors when row-by-row processing is necessary.