Does full reindex done automatically ?plz explain
A "FULL" reindex is not done automatically. You can manually trigger a FULL reindex which will look for changes and update the index. What you have running automatically is bg KMUpdate which is checking every so often for changes and updating the index so that you do not need to do a full reindex. Problems is when KMupdate is not running or you had occasional issue with it and you need to do a full reindex to bring everything up to date again instantly.