creating unique user hash for autoincrement field - php

Question

So in this app, we have a user id which is simple auto-increment primary key. Since we do not want to expose this at the client side, we are going to use a simple hash (encryption is not important, only obfuscation).
So when a user is added to the table we do uniqid(). user_id. This will guarantee that the user hash is random enough and always unique.
The question I have is, while inserting the record, we do not know the user id at that point (cannot assume max(user_id) + 1) since there might be inserts getting committed. So we are doing an insert then getting the last_insert_idthen using that for theuser_id`, which adds an additional db query. So is there a better way to do this?

Answer 1

A few things before the actual answer: with latest version of MySQL which uses InnoDB as default storage engine - you always want an integer pk (or the famous auto_increment). Reasons are mostly performance. For more information, you can research on how InnoDB clusters records using PK and why it's so important. With that out of the way, let's consider our options for creating a unique surrogate key.
Option 1
You calculate it yourself, using PHP and information you obtained back from MySQL (the last_insert_id()), then you update the database back.
Pros: easy to understand by even novice programmers, produces short surrogate key.
Cons: extremely bad for concurrent access, you'll probably get clashes, and you never want to use PHP to calculate unique indices required by the database.
You don't want that option
Option 2
Supply the uniqid() to your query, create an AFTER INSERT trigger that will concatenate uniqid() with the auto_increment.
Pros: easy to understand, produces short surrogate key.
Cons: requires you to create the trigger, implements magic that's not visible from the code directly which will definitely confuse a developer that inherits the project at some point - and from experience I would bet that bad things will happen
Option 3
Use universally unique identifiers or UUIDs (also known as GUIDs). Simply supply your query with surrogate_key = UUID() and MySQL does the rest.
Pros: always unique, no magic required, easy to understand.
Cons: none, unless the fact that it occupies 36 chars bothers you.
You want the option 3.

Answer 2

Since we do not want to expose this at the client side
Simply don't.
In a well-designed database, users never need to see a primary-key value. In fact, a user need never know the primary key even exists.
From your question it seems you actually replace your normal auto-increment ID column with a surrogate id (If not skip to the last paragraph).
Try creating a column with another unique surrogate ID and use that on your frontend. And you can keep your normal primary ids for relationships etc.'
Remember one of the basic must rules for primary keys:
The primary key must be compact and contain the fewest possible attributes.
Also integer serials have the advantage of being simple to use and implement. They also, depending on the specific implementation of the serialization method, have the advantage of being quickly derivable, as most databases just store the serial number in a fixed location. Meaning in stead of max(id)+1 the db has it already stored and makes auto-increment fast.
So we are doing an insert then getting the last_insert_id then using
that for theuser_id`, which adds an additional db query.
last_insert_id Isn't actually a query and is a saved variable in your db connection when you performed a insert query.
If you already have a second column for your surrogate ID ignore all the above:
So we are doing an insert then getting the last_insert_id then using
that for theuser_id`, which adds an additional db query. So is there a
better way to do this?
No, you can only retrieve that uniqid by doing a query.
$res = mysql_query('SELECT LAST_INSERT_ID()');
$row = mysql_fetch_array($res);
$lastsurrogateid = $row['surrogate_id'];
Anything else is making it more complicated than necessary.