PostgreSQL - LIMIT clause

In PostgreSQL, the LIMIT clause is used to constrain the number of rows returned by a query. This is particularly useful for pagination scenarios, where you might want to retrieve a subset of records.

Syntax:

SELECT column1, column2, ...
FROM table_name
LIMIT number;

Examples:

Retrieve only 10 rows:
If you have a table called employees and you want to fetch only the first 10 rows:
```
SELECT * FROM employees
LIMIT 10;
```
Pagination using OFFSET:
Along with the LIMIT clause, you can use the OFFSET clause to skip a number of rows before returning the result set.
For example, to skip the first 30 records and then fetch the next 10:
```
SELECT * FROM employees
LIMIT 10 OFFSET 30;
```
This is equivalent to the following, where the FETCH FIRST clause is an SQL-standard way of limiting results:
```
SELECT * FROM employees
OFFSET 30 ROWS
FETCH FIRST 10 ROWS ONLY;
```

Important Considerations:

Performance: While LIMIT can speed up retrieval times (since fewer rows are sent over the network), using OFFSET can still be slow for very large datasets. This is because the database still has to fetch and discard the rows specified by the OFFSET before returning the remaining rows.
Ordering: When using LIMIT without an ORDER BY clause, the returned rows are dependent on the natural order of rows in the database, which might be arbitrary and not guaranteed to be consistent across queries. Therefore, it's often a good idea to use an ORDER BY clause with LIMIT to ensure a deterministic and meaningful order of the results.
Consistency: If data in the table is being added or removed while you're paginating, using LIMIT and OFFSET might result in inconsistent views of the data. One solution to this is to use a consistent ordering and filtering mechanism based on unique keys or timestamps.

In summary, the LIMIT clause in PostgreSQL is a powerful tool for constraining the number of rows in a result set, especially useful for applications that require pagination. However, careful considerations around performance and consistency are necessary when working with large datasets or rapidly changing data.

How to use LIMIT clause in PostgreSQL:
- The LIMIT clause restricts the number of rows returned in a query.
```
SELECT * FROM products
LIMIT 10;
```
LIMIT with OFFSET in PostgreSQL:
- Use OFFSET to skip a specified number of rows before starting to return rows.
```
SELECT * FROM products
LIMIT 5 OFFSET 10;
```
Paging results with LIMIT and OFFSET in PostgreSQL:
- Implement paging with LIMIT and OFFSET for result pagination.
```
-- Page 1
SELECT * FROM products
LIMIT 10 OFFSET 0;

-- Page 2
SELECT * FROM products
LIMIT 10 OFFSET 10;
```
LIMIT vs. FETCH in PostgreSQL:
- LIMIT and FETCH are interchangeable in most cases.
```
SELECT * FROM orders
FETCH FIRST 5 ROWS ONLY;
```
Using LIMIT with ORDER BY in PostgreSQL:
- Combine LIMIT with ORDER BY for retrieving a specific number of ordered rows.
```
SELECT * FROM employees
ORDER BY salary DESC
LIMIT 5;
```

Dynamic LIMIT values in PostgreSQL:

Use variables or parameters for dynamic LIMIT values.

DO $$ 
DECLARE
   limit_val INT := 10;
BEGIN
   EXECUTE 'SELECT * FROM products LIMIT ' || limit_val;
END $$;

LIMIT in subqueries in PostgreSQL:
- Apply LIMIT in subqueries for controlling the number of rows retrieved.
```
SELECT * FROM orders
WHERE customer_id IN (SELECT customer_id FROM customers LIMIT 5);
```
Using LIMIT with FETCH FIRST in PostgreSQL:
- Use FETCH FIRST as an alternative syntax to LIMIT.
```
SELECT * FROM employees
FETCH FIRST 5 ROWS ONLY;
```

LIMIT and window functions in PostgreSQL:

Use window functions with LIMIT for more advanced result restrictions.

SELECT employee_id, salary, 
       ROW_NUMBER() OVER (ORDER BY salary DESC) AS row_num
FROM employees
WHERE row_num <= 5;

LIMIT and JOIN operations in PostgreSQL:

Integrate LIMIT with JOIN operations for controlling the number of joined rows.

SELECT customers.customer_id, customers.customer_name, 
       orders.order_id
FROM customers
LEFT JOIN orders ON customers.customer_id = orders.customer_id
LIMIT 10;

LIMIT and GROUP BY in PostgreSQL:

Use LIMIT with GROUP BY for limiting grouped results.

SELECT department_id, AVG(salary) AS avg_salary
FROM employees
GROUP BY department_id
LIMIT 5;

Limiting the number of rows returned in PostgreSQL:
- Use LIMIT to restrict the overall result set size.
```
SELECT * FROM large_table
LIMIT 1000;
```
Combining LIMIT with other clauses in PostgreSQL:
- Combine LIMIT with other clauses like WHERE or ORDER BY for tailored results.
```
SELECT * FROM products
WHERE category_id = 1
ORDER BY price DESC
LIMIT 5;
```
LIMIT with aggregate functions in PostgreSQL:
- Use LIMIT with aggregate functions for summarizing a specific number of rows.
```
SELECT department_name, COUNT(employee_id) AS employee_count
FROM employees
GROUP BY department_name
LIMIT 5;
```
Using LIMIT with UNION and INTERSECT in PostgreSQL:
- Apply LIMIT when using UNION or INTERSECT to restrict the overall result set size.
```
(SELECT * FROM products WHERE category_id = 1 LIMIT 5)
UNION
(SELECT * FROM products WHERE category_id = 2 LIMIT 5);
```