Non-selective query against large object type (more than 200000 rows)
You're organization has Case Merge Premium installed and experience the following error when attempting to update cases from a Visualforce page in the Service Cloud Console.
An error occurred while querying for cases. Non-selective query against large object type (more than 200000 rows). Consider an indexed filter or contact salesforce.com about custom indexing. Even if a field is indexed a filter might still not be selective when: 1. The filter value includes null (for instance binding with a list that contains null), 2. Data skew exists whereby the number of matching rows is very large (for instance, filtering for a particular foreign key value that occurs many times).
This error may quickly flash on your screen when the Visualforce page reloads, before being brought back to the main case record.
Each time a case is created or updated, Case Merge Premium will run a query to search for duplicate cases.
This error likely indicates that based on the duplicate criteria you have specified in the setup wizard, the query is looking through over 200,000 cases and was not selective enough based on Salesforce's standards.
As the next step in troubleshooting this, please review your duplicate criteria to ensure that it is using at least 2 parts of criteria, one of which is excluding closed cases.
For additional information on query selectivity, please review the following Salesforce knowledge article.