How do we know the browser version used when checking statistics in cp_visitor_stats table?
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When using the table cp_visitor_stats for your statistics you will not see accurate data regarding the browser version used for the visits on your Customer Portal. As specified in the Data Dictionary, the browser_id column is derived from the user agent string and was not intended to provide the current version of the browser used.
In case you need to design a report in a way that retains the browser and its version from the user_agent_string, you might want to use clickstreams table instead.
For more information about the anatomy of a web session and clickstreams table please check the below answers:
Answer ID 5103: Anatomy of a web session as reported in the clickstreams table.
You can read more information how user-agent string is parsed in the link: Understanding user-agent strings.