Office SharePoint Server 2007 supports both full crawls and incremental crawls, which enables you to define schedules to meet your index freshness requirements.

Full Crawls

To begin with, you must start a full crawl of each content source. From that point on, you can schedule a mix of full and incremental crawls.
Further full crawls will re-crawl all existing content regardless of whether the items have changed since the last crawl, along with new content. This ensures that configuration changes since the last crawl (such as iFilter and file mappings, or managed property mappings) are applied to existing content that has not changed.

Incremental Crawls

Incremental crawls are typically much faster than full crawls, because they crawl only new or modified content. However, you can only perform an incremental crawl after a full crawl has taken place. If you start an incremental crawl when no full crawl has taken place, the system automatically performs a full crawl.
The way in which Office SharePoint Server 2007 performs incremental crawls varies between types of protocol handler, because of differences in the information provided by content source systems. For example, Windows SharePoint Services 3.0 maintains a very granular change log that provides detailed information about what has changed since the last crawl. This enables the incremental crawl process to crawl only updated information on a file or list item level; if only a column value or permission for a file in a library has changed, the indexer does not need to re-crawl the document.
File systems, in comparison, maintain only the modified date for a file or folder regardless of what has changed; the indexer will potentially have to re-crawl entire documents to ensure updates are included even if something simple as a file being renamed has occurred.
Windows SharePoint Services 2.0, in further comparison, maintains the modified date at the container level. Therefore, if modifications have been made to list items for example, the entire list must be re-crawled.

When Are Full Crawls Required?

In brief, full crawls are required when source systems report that files have not changed since the last crawl, but you have made changes to Office SharePoint Server 2007 that require the unchanged content to be re-indexed. The most common scenarios that require you to perform full crawls include:

·         Repairing a corrupted index.

·         Changes to crawl rules.

·         Changes to iFilter and file-type mappings.

·         Changes to the managed property search schema.

·         Application of service packs and hot fixes (although you should read the documentations for these fixes to determine whether they require full crawls.)

·         Re-indexing ASPX pages on SharePoint 3.0 or Microsoft Office SharePoint Server 2007 sites. The crawler cannot discover when ASPX pages in Windows SharePoint Services 3.0 or Microsoft Office SharePoint Server 2007 sites have changed. Because of this, incremental crawls do not re-index views or homepages when individual list items are deleted. Therefore you should periodically perform full crawls of SharePoint sites that contain ASPX files to ensure that these pages are re-indexed.

Additionally, Office SharePoint Server 2007 will sometimes perform full crawls even if you start an incremental crawl in some situations, such as:

·         When 100 consecutive incremental crawls fail for some reason.

·         An SSP administrator stopped the previous crawl.

·         A content database was restored.

·         A full crawl of the site has never been done.

·         When the change log retention period of a Windows SharePoint Services 3.0 Web application being crawled has been surpassed.