计算表中行的最有效方法是什么？

Question

Is it more efficient to use:

$sql = 'SELECT COUNT(*) AS count FROM users';
$odbcResult = OdbcExec($sql);
@odbc_fetch_row($odbcResult);
$count = @odbc_result($odbcResult, 'count');

or to use:

$sql = 'SELECT * FROM users';
$odbcResult = OdbcExec($sql);    
$count = odbc_num_rows($odbcResult);

Answer 1

The former. The server is doing the work. It may already know the answer without counting,

The later requires all the rows to be returned to your program over the network (and into memory).

Answer 2

$sql = 'SELECT COUNT(id) AS count FROM users';
$odbcResult = OdbcExec($sql);
@odbc_fetch_row($odbcResult);
$count = @odbc_result($odbcResult, 'count');

or some other index field besides id. Its a little faster than COUNT(*), but the count method is the way to go. If you need to do something with the results, method 2 is faster, but only needing count this is the one you want.

-edit- Added keyword of index before field. True comment below, made the assumption that there was an id index column (should have an index somewhere at any rate)

Answer 3

使用x> = 6的10 ^ x元素填充表格并查看所需的时间。

Answer 4

The first method is definitely faster. But COUNT(id) being faster than COUNT(*) - questionable. They usually are exactly the same, and there are cases when COUNT(id) is actually slower (see: http://www.mysqlperformanceblog.com/2007/04/10/count-vs-countcol/ )