PDO::rowCount vs. COUNT(*) in PHP Performance Analysis

When working with SQL databases using PDO in PHP, a common task is to count the number of rows matching a specific criterion. This can be achieved using either PDO::rowCount() or COUNT(*) in the SQL query. However, the question arises as to which method provides better performance.

PDO::rowCount() vs. COUNT()

PDO::rowCount() returns the number of rows affected by the last SQL statement, which may not always be the same as the number of rows returned. On the other hand, COUNT() specifically retrieves the count of rows that satisfy the query criteria.

Internally, the MySQL server processes COUNT() and PDO::rowCount() differently. COUNT() allocates memory only for the count result, while PDO::rowCount() allocates memory and prepares for fetching the entire result set, which involves additional overhead.

In general, using COUNT() in the SQL query is more efficient than using PDO::rowCount() because it optimizes the processing in MySQL.

COUNT(*) vs. COUNT(id)

When using COUNT(*) and COUNT(id), the former is typically preferred for performance reasons.

COUNT(*) retrieves the count of all rows, including rows with NULL values. COUNT(id) only counts rows where the id column has a non-NULL value.

MySQL has an optimization that allows COUNT() to be evaluated more efficiently than COUNT(id). This is because the wildcard () indicates that it does not need to retrieve any actual data from the table, only the count of rows.

Conclusion

For optimal performance when counting rows using PDO in PHP, it is recommended to use COUNT() in the SQL query instead of PDO::rowCount(). Additionally, when selecting between COUNT() and COUNT(id), COUNT(*) is generally preferred due to MySQL's optimization.