问题
I have a large table (60+) millions of records.
I\'m using PHP script to navigate through this table.
PHP script (with pagination) loads very fast because:
The table engine is InnoDB thus SELECT COUNT() is very slow and mysql_num_rows() is not an option, so i keep the total row count (the number that i use to generate pagination) in a separate table (i update this record total_rows=total_rows-1 and total_rows=total_rows1+1 during DELETE and INSERT).
But the question is what to do with the pagination for search results?
Right now I\'m doing this with 2 steps:
1.
$condition = \" fname=\'rinchik\' \";
$result = \"SELECT * FROM my_large_table WHERE\" . $condition;
Here i got all search results from DataBase.
2. Now i need to count these results to create pagination. I\'m doing this:
$condition; <- we already have this from the step 1
$result_count = \"SELECT COUNT(id) FROM my_large_table WHERE\" . $condition;
And it\'s kinda slow.
Would it be better if i will do it this way (with just one step)?:
$condition = \" fname=\'rinchik\' \";
$result = \"SELECT * FROM my_large_table WHERE\" . $condition;
$result_count = mysql_num_rows($result);
回答1:
Use COUNT, internally the server will process the request differently.
When doing COUNT, the server will only allocate memory to store the result of the count.
When using mysql_num_rows, the server will process the entire result set, allocate memory for all those results, and put the server in fetching mode, which involves a lot of different details, such as locking.
Think of it like the following pseudo scenarios:
SELECT COUNT(*)
Hey Bob, how many people are in the class room?
mysql_num_rows
Hey Bob, send all the people from the classroom over to me, ... I'll count them to get the number of people myself
In summary, when using mysql_num_rows you are transferring all records to the client, and the client will have to calculate the count itself.
回答2:
Use COUNT(id). It only returns the count, With mysql_num_rows($result); php fetch ALL the data from the mysql and count the number of found results.
And finally, don't use mysql_* functions.
Suggested alternatives
Use of this extension is discouraged. Instead, the MySQLi or PDO_MySQL extension should be used. See also MySQL: choosing an API guide and related FAQ for more information. Alternatives to this function include:
mysqli_stmt_num_rows() PDOStatement::rowCount()
回答3:
Tested in inoDB engine and mysql 5.5.
The id has index and I think this is very fast
$q = "SELECT count(`id`) FROM table where 1";
$rows = mysql_query($q);
$count = mysql_fetch_array($rows);
echo $count[0];
if you want more, you have to use one index just on id or what ever you want to select.
Caching is another solution and you can select from 1 set of records in few milliseconds!
来源：https://stackoverflow.com/questions/12864557/select-count-vs-mysql-num-rows