Unexpected double inserts with MyISAM - php

Question

I have a like table for customers like products in my website.
The problem is, I use this table:
CREATE TABLE IF NOT EXISTS `likes` (
`id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`user` varchar(40) NOT NULL,
`post` int(11) UNSIGNED NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 AUTO_INCREMENT=1 ;
user who like, post is product id.
like button send a ajax request to this php:
session_start();
$user = $_SESSION["user"];
$pinsid=$_POST['id'];
$stmt = $mysqli_link->prepare("SELECT * FROM likes WHERE post=? AND user=?");
$stmt->bind_param('is', $pinsid, $user);
$stmt->execute();
$result = $stmt->get_result();
$stmt->close();
$chknum = $result->num_rows;
if($chknum==0){
$stmt = $mysqli_link->prepare("INSERT INTO likes (user, post) VALUES (?,?)");
$stmt->bind_param('si', $user, $pinsid);
$stmt->execute();
$stmt->close();
$response = 'success';
}
echo json_encode($response);
My problem is, I have double inserts in like from the same person. eg:
1 josh 5
2 josh 5
but it only happens if MySQL engine is set as InnoDB, if I change it to MyISAM I have only 1 insert.
What is happening? What should I do to make it work properly?

Answer 1

but it only happens if MySQL engine is set as InnoDB, if I change it to MyISAM I have only 1 insert.
What is happening? What should I do to make it work properly?
The MyISAM engine uses table level locking, which means that if an operation is executing on a table, all other operations wait executing till that oparation is finished.
InnoDb is transactional and uses row-level locking, since you're not using transactions nothing is locked.
As mentioned in the comments and answers the simplest solution is to create an unique constraint on user and post, in youre case you can use both as primary key because the auto-increment column has no added value.
To create a unique constraint:
ALTER TABLE likes ADD UNIQUE KEY uk_user_post (user,post);
As for your question:
but it can slow down my inserts?
If we speak solely about the insert operation at the table, yes it does slow down because each index has to be rebuild after an insert,update or delete operation. How much it slows down depends on the size of the index(es) and the number of rows in the table.
However in your current table structure you have no indexes at all on user and post, and in your application you perform a select with a lookup on both colums, which will result in a full table scan.
With the unique index (user,post) you can skip the select because when the unique constraint is violate you'll get an SQL error.
Also user and post are foreign keys so the should be indexed anyway.
The unique index (user,post) covers the user FK, so you will also need an index on post separatly

Answer 2

One way of doing this would be to set up a unique key for user and post in the likes table (see https://dev.mysql.com/doc/refman/5.0/en/constraint-primary-key.html).
If that was in place, the database would ensure that there are no duplicates of user and post. However, for data which are already in the table, it could be problematic if there are already duplicates