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I had seen a post by Wordpress where they mention that they support unlimited 'tags' (as opposed to categories). Since php is driving Wordpress, would you mind explaining how/why they can make such a statement?

Suppose I create a wordpress site with thousands of posts and let's say 100,000 tags. Can you explain in php/mysql terms why retrieving posts for a tag on a shared hosting site (hostgator), can work?

Bonus points if you can explain why it won't work for Wordpress categories

Edit: Here is the support article on wordpress.com - http://en.support.wordpress.com/posts/categories-vs-tags/

and the statement:

Is there a limit to the number of tags I can have? Yes, the sky. In other words, no.

Edit 2: What I am really looking if some insight into the php/sql server process which might make scalability possible or not. I am not looking for using hardware to scale the system or for solutions like clustering or enhancing the database in anyway.

I am interested in things like: "querying a table with 100K entries can be quite responsive with php/mysql on a shared host" or if you think not "these queries will not work after 100k entries in the posts table"

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Please link to said post. It is not quite possible to answer the claim without seeing it. Technically tags and categories both are just taxonomies, stored in same way. – Rarst Sep 19 '13 at 12:29

First of all, WordPress.com is not the same as self-hosted WordPress, anything there can be assumed not relevant to a self-hosted install for one reason or another.

Obviously the hardware you have will impact its ability to scale, I'm sure you could find hosting that didn't provide the memory to even get through the install process.

In terms of the code, there are no artificial limits placed on the number of posts, tags, categories, etc., you can create.

Your example of thousands of posts and 100,000 tags is certainly possible on properly tuned hardware, but they're making no claims that your shared hosting will magically be without limits.

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See my edit to the question. are you saying that on a shared host thousands of posts with 100K tags won't/may not work? I am trying to figure out if it will work on a shared host or not. – nilanjan Sep 20 '13 at 11:47
It's not a yes or no question, it depends on many factors. Some types of queries will work fine, some may not. If you really want to know how a given server will perform, load it up with data and test it. – Milo Sep 20 '13 at 14:49

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