ogle to both make efficient use of its resources and keep its index reasonably current.
2. Google’s Indexer
Googlebot gives the indexer the full text of the pages it finds. These pages are stored in Google’s index database. This index is sorted alphabetically by search term, with each index entry storing a list of documents in which the term appears and the location within the text where it occurs. This data structure allows rapid access to documents that contain user query terms.
To improve search performance, Google ignores (doesn’t index) common words called stop words (such as the, is, on, or, of, how, why, as well as certain single digits and single letters). Stop words are so common that they do little to narrow a search, and therefore they can safely be discarded. The indexer also ignores some punctuation and multiple spaces, as well as converting all letters to lowercase, to improve Google’s performance.
3. Google’s Query Processor
The query processor has several parts, including the user interface (search box), the “engine” that evaluates queries and matches them to relevant documents, and the results formatter.
PageRank is Google’s system for ranking web pages. A page with a higher PageRank is deemed more important and is more likely to be listed above a page with a lower PageRank.
Google considers over a hundred factors in computing a PageRank and determining which documents are most relevant to a query, including the popularity of the page, the position and size of the search terms within the page, and the proximity of the search terms to one another on the page. A patent application discusses other factors that Google considers when ranking a page. Visit SEOmoz.org’s report for an interpretation of the concepts and the practical applications contained in Google’s patent application.
Google also applies machine-learning techniques to improve its performance automatically by learning relationships and associations within the stored data. For example, the spelling-correcting system uses such techniques to figure out likely alternative spellings. Google closely guards the formulas it uses to calculate relevance; they’re tweaked to improve quality and performance, and to outwit the latest devious techniques used by spammers.
Indexing the full text of the web allows Google to go beyond simply matching single search terms. Google gives more priority to pages that have search terms near each other and in the same order as the query. Google can also match multi-word phrases and sentences. Since Google indexes HTML code in addition to the text on the page, users can restrict searches on the basis of where query words appear, e.g., in the title, in the URL, in the body, and in links to the page, options offered by Google’s Advanced Search Form and Using Search Operators (Advanced Operators).
Let’s see how Google processes a query.
Copyright © 2003 Google Inc. Used with permission.
For more information on how Google works, take a look at the following articles.
- Google’s page on Google’s Technology, www.google.com/technology/.
- How does Google collect and rank results?, www.google.com/newsletter/librarian/librarian_2005_12/article1.html.
- Google’s PageRank Algorithm and How it Works, www.iprcom.com/papers/pagerank/
- Google’s PageRank Explained and How to Make the Most of It, www.webworkshop.net/pagerank.html
The results page is filled with information and links, most of which relate to your query.
- Google Logo: Click on the Google logo to go to Google’s home page.
Statistics Bar: Describes your search, includes the number of results on the current results page and an estimate of the total number of results, as well as the time your search took. For the sake of efficiency, Google estimates the number of results; it would take considerably longer to compute the exact number. This estimate is unreliable.
Every underlined term in the statistics bar is linked to its dictionary definition. Queries that are linked to just one definition are followed by a definition link.
- Tips: Sometimes Google displays a tip in a box ju