What is indexing?

Indexing refers to the process of search engines crawling the internet to discover webpages and storing that information within an organized database called an index.

How Search Engines Index — and Why it’s Important for SEO

Search engines serve users by providing “answers” to their queries. To do this, search engines crawl the internet to discover keyword information (and a host of other data) attached to websites and pages. These results are stored and organized into a database called an “index” for quick retrieval. Content that has been indexed is ranked to compete and to be displayed in the search engine results page (SERP) for relevant queries. In short, if you want your content to be found, it needs to be indexed for the opportunity to be seen.

How Do Search Engines Index Websites

The process that search engines use to populate the SERPs can be defined by three primary functions: observation, organization, and categorization. The technical terms for this process are crawling, indexing, and ranking. It is important to understand crawling and ranking to completely understand the term indexing.

Search Engine Crawling

To provide the most exceptional answers to a user query, a search engine has to know what information exists across the internet. This initial discovery is done by robots known as crawlers, or spiders, that fetch a web page and then continue on their journey by hopping from link to link — foraging for information. To ensure that a page or site will populate in a SERP, it is important for the content to make it into the index. For this to be done, the content must be found and crawled. A webmaster may choose to take initiative and invite crawlers to crawl a site by providing a sitemap to Google and asking them to submit it to their index.

Search Engine Indexing

The information gathered by crawlers is organized and stored into an index, a massive database that organizes (indexes) all of the discovered content that has been interpreted as containing valuable search results. It isn’t enough to simply get a page crawled for indexing; the information on the page must be valuable enough to be added to the index. The content must be accurate and up to date, have quality, topical authority, and be original. After a crawler finds a page, the search engine analyzes the page’s content. If the content is found to be valuable, comparative, and competitive, the search engine will add the page to the index. 

Search Engine Ranking

Search engine ranking is the quality control valve that feeds pages to the SERPs from the index. To ensure that the results produced by a query are relevant, the search engine uses an algorithm or formula to retrieve pages in a meaningful way and to provide quality results.

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Optimizing Indexing for a Website

There are a few methods for inviting a search engine to crawl a page in order to be indexed more quickly. This can be especially beneficial when measuring on-page improvements or publishing time-sensitive content. 

  • XML Sitemaps —  Though not the most immediate way to become indexed, creating a sitemap that includes all the pages on the site, the date of last modification, and how important pages are in relation to one another, directs how the bots crawl and influences the page’s indexing
  • Robots Meta Directions/ Robot Meta Tags — Meta tags, similar to robot.txt, offer a firm directive on how to crawl and index a page’s content. 
  • Fetch as Google — Using the old version of Google Search Console, you can submit the URL you want to be indexed by using the “Fetch as Google” navigation, and then clicking “Request Indexing”.
  • Submit URL — Google, Yahoo, and Bing can all be requested to crawl an individual page through submission.
  • Hosting Content On a Search Engine Platform — Hosting content directly on a platform run by a search engine, such as Google’s Firebase, may increase the timeliness of being indexed. 

Removal From the Index

Making it into the index is not the only feat in keeping content visible — it is also important to keep up the technical housecleaning. A URL could be removed if a 301 was not set up during a redirect, or a page was deleted and 404’d. Not adhering to the Google's Webmaster’s Guidelines may result in a penalty and removal from the index. Applying a “noindex” meta tag can be used to instruct the bots not to crawl the page for indexing, or using a visitation password to access the page may result in removal or non-submission to the index.

A Deeper Understanding of How Indexing Works

Tokenization is often employed by reducing words to their core meaning to enhance the speed at which data is retrieved, as well as reducing the number of resources needed to store the data. A cached version of a page may also be used, storing highly compressed text-only documents with HTML and metadata.

The framework of an index is designed to meet the needs of:

  • Merge Factors - Discovering if the data is updated, old, or new, and deciding how the information should be entered into the index.
  • Storage techniques - Deciding on if the data should be compressed or filtered for a more manageable size.
  • Index Size - How much storage space is needed by the computer to host the index.
  • Lookup Speed - The speed of finding an entry or word in the data structure, as well as how quickly it can be updated or removed.
  • Maintenance - How often/much an index must be maintained over time. Performance may sometimes hinge on maintenance and updates.
  • Fault Tolerance - The importance of reliability.

Indexes use a variety of structures to categorize web pages including:

  • Suffix Tree/Suffix Array- Named after its tree-like shape, this type of indexing is built by storing the suffixes of words and supports linear time lookup.
  • Inverted Index/ Reverse Index- Organizes data from full-text searches for word placement and occurrence.
  • Citation Index- Stores hyperlinks and citations between pages.
  • Ngram Index - Utilizes text mining through stored sequences of data.