Google boss Eric Schmidt said the search engine could one day answer hypothetical questions

The web giant currently anonymises a user's search history after 18 months.

There are concerns among some privacy advocates that Google could know too much about a user's web history.

But Marissa Mayer, vice-president of search, said the firm would look at letting users opt in to having their search data held for longer.

Google currently offers an opt-in personalised search facility, which learns how and what users search for in order to improve the accuracy of results.

Personalised search

Speaking at a press event in Paris, Ms Mayer said: "Personalised search tracks and shows you in your search history the clicks and trends of your searches.

"Based on what we see as your searching pattern it ultimately can enhance your result.

"There's a simple way to turn it on and off. We will only use the data that a user gives us to target personalised search."

Google anonymises that information after 18 months and so the search engine has to re-learn the patterns of user behaviour.

Our goal is to have a Google search as fast as a light beam to and from our data centres from your location

Marissa Mayer, Google

She said: "We have declared that we keep our records of searches for 18 months. We think that this was a good compromise and also something which benefited our users.

"Eighteen months is sufficient to do a good job of personalising so we think that personalised search will continue and will be successful.

"At the same time it protects our users' privacy as we anonymise the logs after 18 months."

But she said Google would consider offering an enhanced form of the service.

"For users who opt in for us to have data longer, as long as they explicitly consent, there is the possibility we can do that," she said.

Predict intent

The more a search engine can learn about a user's surfing habits, the better it can predict their intent.

With more web history data, Google could offer users a "hyper-personal" experience, with results based on potentially years' worth of pattern analysis of a user's search history.

Speaking about the long-term aspirations for Google, Eric Schmidt, the firm's chief executive, said one day the search engine could potentially answer questions such as "What shall I do tomorrow?" and "Which college should I go to?".

"Google is not at all done with your information problems. There are many, many examples of where it would be nice if Google had more of an ability to understand time and choices.

"It will be some years before we can at least partially answer those questions. But the eventual outcome is... that Google can answer a more hypothetical question."

He added: "The important principle, and I want to say this over and over again, is that this is opt-in, user choice."

Biggest hurdles

One of the biggest hurdles facing any search engine is trying to understand what a user is specifically searching for when using terms which could be interpreted in different ways and the context of the search itself.

Google says it searches tens of billions of pages, three times more than its nearest rival.

The number of pages it searches has grown by more than a factor of a 1,000 in the last eight years, said Ms Mayer.

She said: "In the early days of the internet you could actually offer search results as a list, and organise them by hand.

"But as you include more and more information, relevance and ranking gets harder."

Ms Mayer said that speed of search results was becoming an increasingly important factor.

"We have continually tried to improve our speed and that speed has yielded more and more searches.

"Our goal is to have a Google search as fast as a light beam to and from our data centres from your location."