Understanding the differences between a polling Perspective Tag History binding and system.tag.queryTagHistory

[paul-griffith]

I went into some detail on how the 'Cache & Share' mechanism works here:

Cache & Share vs Cache Query Results

Named query caching is a distinct layer from the 'Cache & Share' option available in some Perspective binding types. Named query caching works by keying off the named query path plus parameters, with a fixed expiration time. So, let's have sessions A and B. Both have a query binding, not cache & share, set to run a named query with a cache time of 5 seconds. If A issues a query, and it returns a value, then within the next five seconds any identical query (including all parameter values) iss…

The main benefit for the historian is when you're dealing with the exact same query being issued by multiple sessions at once.
Without cache and share, that's multiple hits to the database to return the exact same data.
With cache and share, that's optimized away, trading off some memory for the DB hit.

It is not a general purpose value cache bucket (like Vision has...), something Perspective lacks. That is, cache & share does nothing for a power chart that's querying "the last five minutes", and then a second later "the last five minutes" again, with 4:59 of overlap between the two queries.