Exception:   

System.QueryException: Non-selective query against large object type (more than 200000 rows). Consider an indexed filter or contact salesforce.com about custom indexing.

Root Cause:

This exception is caused when the SOQL query is against on object with more than 200000 records and the query is not efficient. The query takes a longer time to execute and hence the exception.

Ex: Select Name,Email from Contact 

Ex: Select Name,Email from Contact where Status__c != NULL (Status__c is a custom non indexed field)

Resolution:

Option 1:

Make the query selective. Use Indexed fields when making the query selective, for example Id, CreatedDate, LastModifiedDate  etc are indexed.

Ex: Select Name,Email from Contact where LastModifiedDate  >=  LAST_N_DAYS:30

To find the indexed fields, use the object manager under setup

 Option 2:

If you need a custom field to be indexed, Salesforce support could add a custom index for you. Create a support case with Salesforce to get your custom index created. 

When the custom index is created, make your query selective using the custom indexed field.

You can then schedule this apex class either using the standard scheduler , or just from the developer console using a simple script.

ClassName gp = new ClassName();
String cronStr = '0 0 * * * ?';
System.schedule('Job which runs at every 0th minute of hour', cronStr, gp);


ClassName gp = new ClassName();
String cronStr = '0 30 * * * ?';
System.schedule('Job which runs at every 30th minute of hour', cronStr, gp);