I've already tested the sql_last_value with an integer and datetime type of fields in my jdbc plugin within the logstash input and it's working perfectly.
I'm just curios, is there any possibility of using a varchar/string type of field, in order to update the sql_last_value time to time in case if there aren't any datetime or int field in my table.
Any help could be appreciated.
The answer is no. As a proof you can look at the source code for the jdbc input plugin.
The declaration of the tracking_column_type field highlights the following:
# Type of tracking column. Currently only "numeric" and "timestamp"
config :tracking_column_type, :validate => ['numeric', 'timestamp'], :default => 'numeric'
This means that this field will only be allowed to contain two values, namely timestamp and numeric.