Counting Rows Efficiently: Choosing the Right Method for Your PHP and PostgreSQL Application

You have a SQL query in your PHP application that uses a LIMIT clause to retrieve a specific number of rows from a PostgreSQL database. However, you also need to know the total number of rows that would be returned without the LIMIT restriction.

There are a few ways to achieve this, and each has its own pros and cons:

Window functions (PostgreSQL 8.4+):

This method is specific to PostgreSQL and uses the power of window functions. By adding count(*) OVER() AS full_count to your query, you can get the total count of rows along with the limited result set.

Example:

SELECT foo, count(*) OVER() AS full_count
FROM bar
WHERE <some condition>
ORDER BY <some col>
LIMIT <pagesize>
OFFSET <offset>;

This query will return two columns: foo with the desired data and full_count with the total number of rows before the LIMIT and OFFSET were applied.

Pros:

• Achieves both actions (limited result and total count) in a single query.
• Easy to understand and implement.

Cons:

• Can be less performant than separate queries, especially for large datasets. This is because the entire result set needs to be processed even if only a limited number of rows are returned.

Subqueries:

This method involves running two separate queries. The first query retrieves the total count, and the second one retrieves the limited data.

-- Get total count
SELECT count(*) AS total_count
FROM bar
WHERE <some condition>;

-- Get limited data
SELECT foo
FROM bar
WHERE <some condition>
ORDER BY <some col>
LIMIT <pagesize>
OFFSET <offset>;

• Generally performant, especially for large datasets.
• Can be used with other databases besides PostgreSQL.

• Requires two separate queries, which can be less code-friendly.

pg_num_rows (PHP-specific):

This method relies on a PHP function called pg_num_rows to retrieve the number of rows affected by the last SQL statement. However, this approach doesn't tell you the total count before LIMIT but the number of rows actually returned after applying the LIMIT.

$result = pg_query($connection, "SELECT foo FROM bar WHERE <some condition> ORDER BY <some col> LIMIT <pagesize> OFFSET <offset>");

$total_returned = pg_num_rows($result);

• Simple to implement in your PHP code.

• Doesn't provide the total count before LIMIT.