A very good question as per perspective of information retrieval person, so let's go to the core of question, actually someone from Gmail could probably answer this better, but here are some factors that make mail search harder in some ways:
The total contents of the web is actually smaller than the sum of the sizes of the contents of everyone's gmail. This means it could take more servers to hold all the indexes for mail search than for web search. Yes, users are only searching across their own mail, but servers are still needed to store all the indexes so that every user can do this.
When you search the web, for the most part, you're getting the same results for your query as anyone else would get for that query. This means caching works well for web search. Most search engines have a small "hot index" with the most popular content that can handle the majority of queries which is replicated out to lots of local data-centers, giving low average response time even if the worst case is slow.
Gmail search results are sorted by time and need to be perfect matches, whereas web search results are sorted by relevance and approximations can be made to cut corners.
Must Read: How to earn money from your website? Also, from an economic perspective, Gmail generates relatively low revenue for Google compared to web search, so they have less of a reason to invest in it Thanks and stay blessed!
At a minimum, they need to be able to comply with subpoenas from the government, so whatever employees process those need to be able to access arbitrary Gmail accounts.
I have also heard that if you work on Gmail within Google, you need to sign a document that says that you will not abuse your privileges, but that even if you do end up seeing anyone's email, you agree to not tell anyone outside of Google about it or even the fact that you were able to see it. Also Read: How many Google employees can access your's Gmail data ? Aside from that I think their processes are pretty solid and generally require approval and proof of necessity in order for normal employees to get to the data. Thanks and stay blessed!
The “I’m Feeling Lucky” Button Costs Google $110 Million a Year! Ever notice that ubiquitous “I’m feeling lucky” button on the Google homepage that you've probably never used. What if I told you that this small little piece of grey rectangle, costs Google over 110 million dollars a year! That’s right! What this button effectively does, is that it takes the user directly to the top search result for that query thereby bypassing all the other top 9 results and also the ads that get displayed alongside them. Brin was recently quoted as saying that almost 1% of all Google searches go through the “I’m feeling lucky” button and that costs the company around $110 million in annual revenue.
So why doesn't Google take it off? Well, it just did but in such a sneaky way that you probably didn't even notice. It’s still there on the homepage but thanks to Google Instant, you don’t get to use it anymore! Try it and see for yourself. Thanks and stay blessed!
