skip meta tag enrichment when we can

[replay]

when we lookup values of a tag to reply to an auto complete call, we can skip the meta tag enrichment if the given tag we're looking up the values for is a metric tag.

this slightly breaches the rules, because i'm of the opinion that we should inter-mix metric and meta tag values in this case, we only don't inter-mix them for a single metric, but let's do this for the sake of speed.

[Dieterbe]

Can you explain this in a bit more detail please. I don't understand.

[replay]

Sure, I'm happy to:

This change only affects situations where an auto complete request asks for the list of potential values for a tag which is a metric tag and also a meta tag at the same time. This is bad practice, and hopefully nobody does that, but it is possible to do it.
We have a rule in the design doc defining that each metric can only have each tag once (f.e. if metricX has the metric tag tag1=value1 then a meta record which tries to assign it the meta tag tag1=value2 would be ignored).
But this rule is only on a per-metric basis, it is possible to have metricX with a metric tag tag1=value1 and metricY with a meta tag tag1=value2. This means that when an auto complete request comes in for all potential values of the tag tag1, if the associated query expressions match both metricY and metricX, then we should gather all the potential values from the metric tag index and also from the meta tag index and the returned list for tag1 should be value1 and value2. This is what the code does before this PR.
Now, the problem is that the underlying query to also gather all the potential meta tags for the given query expression leads to a significant slow-down if the result that's generated based on the given query expressions is very large (in the millions). Since this additional and significant overhead is only necessary to handle this very unlikely case where a user inter-mixes metric tags with meta tags (which they really shouldn't), we can save that additional over head and skip the meta tag value evaluation if we see that the queried tag is present in the metric tag index. This means if the auto complete request asks for the values of a tag that's in the metric tag index, we don't return values of that tag which only exist in the meta tag index.

I hope that explains. The end goal is to make these auto complete requests faster when drilling down.

[robert-milan]

That makes sense, hopefully this will speed things up a bit in this case.