[Expressions] var_set should be allowed to set multiple variables in parallel

[wylieconlon]

Based on the approach described in #68457 where we want to set variables to contain the results of esaggs, it is important to run the data fetching in parallel. Therefore we need to be able to set multiple variables, for example:

var_set name="table1" value={esaggs ...} name="table2" value={esaggs ...}

I think this is a small change to the var_set code, by changing both name and value to be multi-value instead of single-value.

@ppisljar @streamich @flash1293

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[ppisljar]

as var_set just passes the context thru i don't think thats a requirement, you can do:

var_set name="table1" value={esaggs...} | var_set name="table2" value={esaggs...}

but i agree that this should not be hard to add

[flash1293]

@ppisljar But wouldn't the first var_set wait till the value sub-expression has completed? In this case that would mean that the second esaggs call would wait for the first esaggs call - with a multi set both can run in parallel.

[ppisljar]

even with multiset i am not sure they would run in parallel, as the function (var_set in this case) does not get expression strings passed in but those expressions already evaluated. It depends how exactly interpreter does that argument resolution, but you are right it should at least be possible to make it work in parallel.

[flash1293]

It might be possible to not evaluate the value sub expressions automatically by the interpreter, so the implementation of var_set can kick them off in parallel.