SELECT COUNT() vs mysql_num_rows(): Which is Better for Counting Large Tables?

Counting Large Tables: SELECT COUNT() vs mysql_num_rows()

When navigating through a table with millions of records, it is crucial to consider the performance implications of counting the rows. Two common options are SELECT COUNT() and mysql_num_rows(), each with its own advantages and drawbacks.

SELECT COUNT()

SELECT COUNT() is specifically designed for counting rows and allocates memory only for storing the result. It operates efficiently by using an internal server process that avoids retrieving the actual data. This makes it ideal for situations where only the row count is needed.

mysql_num_rows()

mysql_num_rows(), on the other hand, processes the entire result set. It allocates memory for all the returned records and places the server in a "fetching mode" that involves locking and other resource-intensive operations. This approach can be slower, especially for large result sets.

Pagination for Search Results

For pagination based on search results, using SELECT COUNT() is generally preferable. By incorporating the search condition into the SELECT COUNT() statement, you can obtain the row count without the need for a separate query. This approach reduces the number of queries and improves overall performance.

For instance, instead of using two separate queries as in your example:

1. $condition = " fname='rinchik' ";
2. $result_count = "SELECT COUNT(id) FROM my_large_table WHERE" . $condition;

You can modify your code to:

$condition = " fname='rinchik' ";
$result = "SELECT * FROM my_large_table WHERE" . $condition;
$result_count = mysql_num_rows($result);

Conclusion

While mysql_num_rows() can provide the row count, using SELECT COUNT() is a more efficient approach for counting large tables or retrieving the row count for search results. It avoids unnecessary data retrieval and significantly improves performance.

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