Here's a quick tip for people using the (simply awesome) Apache Solr Search Integration module.
The Apache Solr module depends on Drupal's core Search module. This means the Search module will be enabled too when the Solr module is installed. As soon as the core Search module is enabled it starts to index all your nodes and stuff too. This not only takes time to run, but also fills up your database (search_dataset, search_index, ... tables).
Most of the time when you are installing the Apache Solr Search module, you don't need Drupal's core search form and replace the search form with the Apache Solr one using the Apache Solr module setting "Make Apache Solr Search the default:". You have now disabled the core Search module's form, but you have not disabled the indexing. To disable the indexing and save some CPU cycles and database space, just go to your site's search settings page (admin/settings/search) and set the "Number of items to index per cron run" to 0.