Searching through lots of applications Google search has instant search feature. While you are typing, it finds the pages. Apple.com search box. StackExchange ask question part finds similar questions while you are typing question title.

I am building a collection of web applications in same system. I will add a general search functionality to search through storage of all web applications. But thinking that tables will have records of thousands of users, and that searching will be done for all applications, that can cost too much server power.

I consider caching, though it is like creating all database tables in a folder or memory with same information. I don't see any benefit from this approach.

How are businesses are building this kind of feature in products professionally? Is there any special technique, or can caching really solve it?

P.S. I am using PHP with MySQL.
If you're searching documents, a text engine like solr could be your friend.  Combine that with ajax for real-time document discovery.