使用通配符和缩小的mysql查询来拉取详细信息[重复]

Question

This question already has an answer here:

• Which is faster/best? SELECT * or SELECT column1, colum2, column3, etc 47 answers

I'm playing around with a PHP/MySQL. I can't devise what is "better" and why ?

This isn't really a direct technical question but more of a efficiency related question.

I'm trying to pull 1 row from a table, this is the general code:

$query=mysqli_query($con,"SELECT * FROM `tracked_sites` WHERE checkType =1");
while ($row = mysqli_fetch_assoc($query)) {
    echo $row["domain"] . "<br/>";
}

This works and everything is just fine, however I could also do it using a more specific query, instead of using wildcard. I could just specify the domain and it would trim down the results to just that row.

I'm trying to understand what would be the impact (if any) on larger dbs (lets say 10M or 100M items)?

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Answer 1

Reducing the width of the data fetched the database will always improve performance - the DBMS must allocate memory for any data it reads from the disk, it must then transmit that to the client and the client must allocate memory to store it. The more memory is used, he slower a system gets.

However if the difference is only a few kb, then this on its own won't have much impact - but what does have a very big performance impact is when you select data using a covering index.