Using multiple PDO prepared statements at once - php

Question

This is a request for confirmation/clarification based on a very easily missed comment from 6 years ago in the PHP.net manual for PDO::prepare, which I haven't seen discussed elsewhere (even in the great phpdelusions blog). It's such a powerful possibility if it's true that I feel it deserves a little wider coverage and search highlighting for others (or needs debunking if not).
Here's the comment (by Hayley Watson):
It is possible to prepare in advance several statements against a single connection.
As long as that connection remains open the statements can be executed and fetched from
as often as you like in any order; their "prepare-execute-fetch" steps can be interleaved
in whichever way is best.
So if you're likely to be using several statements often (perhaps within a loop of
transactions), you may like to consider preparing all the statements you'll be using up front.
I have some code that must run like (pseudocode):
foreach (fetchAll row with PDO) {
process row results
if (condition)
update table with processed results
else
delete row no longer needed
}
According to that comment, I could create TWO prepared statements BEFORE the loop, one for the update query and one for the delete query, then execute (only) within the loop. As long as the handles are different and preserved, the connection should cache both, I can use them interchangeably and I'd not have to do any SQL statement parsing INSIDE the loop, which would be very inefficient:
// do statement prepare/execute/fetchAll for main loop, then...
$update_stmt = $PDO->prepare($update_query);
$delete_stmt = $PDO->prepare($delete_query);
foreach (fetchAll row) {
process row results
if (condition)
$update_stmt->execute(some_processed_values);
else
$delete_stmt->execute(some_other_values);
}
Since most questions here only discuss using one prepared statement at a time, and this has excellent implications for code efficiency if widely applied, would anyone like to confirm that this is definitely the case (at least from PHP7)? If so, I guess other neat applications for this form of code could be shared in the solutions.

Answer 1

There's no problem with using multiple prepared statements at the same time and executing them out of order.
You can run code such as the following with intertwined statements and it will work.
$stmt1 = $pdo->prepare('INSERT INTO addplate(Plate) VALUES(?)');
$stmt2 = $pdo->prepare('UPDATE addplate SET Plate=? WHERE Plate=?');
$stmt1->execute(['val1']);
$stmt2->execute(['val2', 'val1']);
$stmt1->execute(['val1']);
$stmt2->execute(['val2', 'val1']);
$stmt1->execute(['val1']);
This can bring you some performance benefit when for some reason you can't avoid N+1 problem. You prepare the inner query once and then execute it inside the loop multiple times.
However, this could be a problem with result-producing queries if you want to run unbuffered query (it's very rarely used). PDO by default executes buffered queries, so you need to switch them off to run into this issue.
$pdo->setAttribute(PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, false);
$stmt1 = $pdo->prepare('SELECT * FROM addplate WHERE Plate=?');
$stmt2 = $pdo->prepare('SELECT * FROM addplate WHERE Plate=?');
$stmt1->execute(['val1']);
var_dump($stmt1->fetch());
$stmt2->execute(['val2']); // <-- Error if stmt1 still has more records
var_dump($stmt2->fetch());
It will produce:
Fatal error: Uncaught PDOException: SQLSTATE[HY000]: General error: 2014 Cannot execute queries while other unbuffered queries are active. Consider using PDOStatement::fetchAll(). Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.

Answer 2

Nice Question! First statement ?
$id = '2';
$username = 'John';
if ($id === 2){
$array = array(':username'=> $username, ':id'=>$id);
$sql = "UPDATE users SET username = ? WHERE id = ?";
}else{
$array = array(':id'=>$id);
$sql = "DELETE FROM users WHERE id = ?";
}
$stmt = $pdo->prepare($sql);
$stmt->execute($array);
Second statement :
$sql = "UPDATE users SET username = ? WHERE id = ?";
$sql = "DELETE FROM users WHERE id = ?";
$stmt1 = $pdo->prepare($sql);
$stmt2 = $pdo->prepare($sql);
if ($id === 2){
$array = array(':username'=> $username, ':id'=>$id);
$stmt1->execute($array);
}else{
$array = array(':id'=>$id);
$stmt2->execute($array);
}
Simple statements:
$stmt = $pdo->prepare("UPDATE users SET username = ? WHERE id = ?")->execute([':username'=> $username, ':id'=>$id]);
$stmt = null;
$stmt = $pdo->prepare("DELETE FROM users WHERE id = ?");
$stmt->execute([':id'=>$id]);
$stmt = null;
First statement run 1 second faster than second statement.
Runing separete statements at the bottom, both update and delete at same time much faster than others.
is that because of if statements ?
I am runing php 7.4.0 and mysql 8.0
Updated if someone wants to try.